The Hewins Cladding Book

The Cladding Book

Hewins carries a long-standing reputation as a specialist oak supplier to the joinery and construction industry, and over the last 25 years, has become the supplier of choice for oak throughout the UK.

This new range of timber cladding accompanies our well-renowned Oak Book and provides the ultimate solution when specifying and selecting timber façades.

Introduction

Begin your journey learning about one of the most popular exterior façade choices.

Species

Discover our vast range of species, each with their own characteristics, benefits and properties.

Profiles

Our wide range of profiles cater to any design aesthetic, from traditional to contemporary.

Surfaces & Coatings

Explore the

between Maintenance and

and the advantages of each.

Additional Information

The information, advice and photos of this publication are indicative only. Timber is a natural product and variation in colour, grain, specification etc. will inevitably occur. Information is subject to withdrawal/change without prior notice. All measurements are in mm unless otherwise stated. Copyright © 2024 Hewins Timber Ltd. All rights reserved.

Enhance your build with timber.

Timber cladding is becoming an increasingly popular choice for both residential and commercial construction, its natural beauty and versatile characteristics adding both style and elegance to the structure.

Whether your project is of a contemporary or traditional type, choosing to specify timber provides warmth and character, accentuating the aesthetic of any design, whilst the natural origins of timber mean that each board comes with a unique grain pattern, resulting in a truly bespoke look.

In addition to its visual appeal, wooden cladding can help improve the energy efficiency of a building as wood is a natural insulator. It helps to regulate the temperature inside the structure and reduce the need for artificial heating and cooling, thereby reducing energy costs.

In this guide, you will find a full range of species and profiles to suit your specific design requirements, as well the various surface and finishing options that we offer. Also included is information on fire retardant treatments, durability and movement in timber, and some installation tips, to assist with making the right decisions on your project.

Our team look forward to supporting you on your next cladding scheme!

As one of the most ecofriendly building materials available, wood is an exemplary example of a renewable resource, and is the perfect choice for the environmentally conscious client.

All our cladding is sourced from sustainably managed forests and the production process for wooden cladding has a significantly lower carbon footprint compared to other building materials.

When the product reaches the end of its life, recycling methods for timber are amongst the simplest and most effective.

Hewins has held FSC® and PEFC certification since 2006, and the majority of the species shown in this book are available from FSC® and PEFC approved sources.

Species

Air Dried Oak European

The unique aesthetic of oak enhances the appeal and status of both traditional and contemporary builds, and is the classic choice for creating a highly resilient natural shield against the elements. Carefully machined from our well-seasoned stock to maximise stability and consistency, oak will naturally weather to a silver/grey.

■ Grade overview

■ Moisture content

Largely clear and straight grain cuttings, containing knots and traces of heart. Some sap permitted on the back face. European grade QF2/3. Air dried timber may contain light splitting across the surfaces as a result of the natural seasoning process.

14-22%

■ Density 650-850kg/m 3 (more info on p.68)

■ Durability (BSEN350)

Class 2: Durable (more info on p.66)

■ Movement class Medium (more info on p.67)

■ Available lengths 1.8-3.0m+

■ Price range High

■ Recommended fixings Stainess steel

Available Profiles

Surface Options

Coating Options

HC200 HC300/1

HC400

HC401 HC402 HC500

HC600/1

Planed Fine Sawn Brushed Maintenance Transitional Uncoated

HC900

Fresh Sawn Oak European

The distinctive grain of oak adds beauty and prestige to both period and modern properties alike - providing a very durable barrier against the elements. Sawn fresh from the log, the boards have a fine sawn finish and will shrink slightly as the timber naturally dries and settles in situ, weathering to a silver/grey colour.

■ Grade overview

Largely clear sections with some tolerance of knots and other natural defects. Some heart trace and the occasional dead knot is allowed. Overall, a light character appearance.

■ Moisture content 40-60%

■ Density 850-1000kg/m 3 (more info on p.68)

■ Durability (BSEN350)

Class 2: Durable (more info on p.66)

■ Movement class Medium (more info on p.67)

■ Available lengths 2.5-4.2m

■ Price range Medium

■ Recommended fixings Stainess steel

Available Profiles

Surface Options

Coating Options

Uncoated

HC100/1

Fine Sawn Charred

HCWWB

Larch British

British Larch provides a good quality and cost-effective cladding. It is lively with knots and features and will naturally weather to an attractive silver/grey colour, more consistent than other species.

This cladding will last for 20–30 years in average external conditions.

■ Grade overview

Varying in colour from pale orange/brown to pink-reddish brown. Ungraded production, typically quite knotty.

■ Moisture content 16-22%

■ Density 500-650kg/m 3 (more info on p.68)

■ Durability (BSEN350) Class 3: Moderately durable (more info on p.66)

■ Movement class Large (more info on p.67)

■ Available lengths Up to 6.0m; mostly 3.0-4.8m

■ Price range Low

Available Profiles

Surface Options

Coating Options

HC102

HC400

HC401

HC402

HC501

HC600/1

Planed Fine Sawn

Brushed

Uncoated

Charred

HCWWB

Western Red Cedar Canadian

A very stable natural timber, having very little movement or cupping tendencies. This high quality timber is slow growning, resulting in a very straight grain with very few knots. It comes in a whole range of colours, varying from orange hues right through to a reddish-brown, giving greater depth across boards and a clear, flawless finish. One of the highest grade cladding species available, this dense timber includes a lot of brown oil, giving it exceptional durability, but this will leach out during the early months while weathering.

■ Grade overview

■

Produced from grade No. 2 clear and better allowing 15% of No. 4 clear grade mix. Long clean lengths with some knots.

■ Density 330-390kg/m 3 (more info on p.68)

■ Durability (BSEN350)

Class 2: Durable (more info on p.66)

■ Movement class Small (more info on p.67)

■ Available lengths Up to 4.8m

■ Price range High

■ Recommended fixings Stainless steel or specialist coated fixtures

Available Profiles

Surface Options

Coating Options

Uncoated

Planed

Fine Sawn

Brushed

Charred

HC200

HC300/1

HC400

HC401

HC402

HC500

HC600/1

HC900

Western Red Cedar British

A more economical choice to its foreign cousin, British Western Red Cedar is faster growing and boasts a more consistent, pale-red colour with varied grain and character for a more natural look. It is less dense and contains more knots than Canadian, but weathers more evenly and has a considerably lower carbon impact product due to it being locally grown. Likewise, it is a durable product that lasts for many years externally.

■ Grade overview

Tends to have smaller tighter knots than species such as Larch, but is more knotty than Canadian. Ungraded production with knots up to 60mm+.

■ Moisture content 16-25%

■ Density 330-390kg/m 3 (more info on p.68)

■ Durability (BSEN350) Class 3: Moderately durable (more info on p.66)

■ Movement class Small (more info on p.67)

■ Available lengths Up to 4.8m; 3.0-4.8m

■ Price range Medium

■ Recommended fixings Stainless steel or specialist coated fixtures

Available Profiles

Surface Options

Coating Options Uncoated

Planed

Fine

Sawn

Brushed

HC102

HC400

HC401

HC402

HC501

HC600/1

Douglas Fir British/European

Douglas Fir maintains its appearance and shape over time, even when exposed to changes in temperature and humidity. With a warm, rich color and distinctive grain pattern, it adds texture and interest to a surface and has good resistance against decay, rot and insect infestations. The colour may differ according to part of the tree cut, with variations of pink, brown, and cream coming through. Notably characteristic, these variations are often incredibly desirable and are what draw clients to the species.

■ Grade overview

■ Moisture content

Often a very characteristic timber, with a strong grain pattern and usually quite a few knots (some dead), adding to the overall appearance.

20-25%

■ Density 480-580kg/m 3 (more info on p.68)

■ Durability (BSEN350)

Class 3: Moderately durable (more info on p.66)

■ Movement class Large (more info on p.67)

■ Available lengths

Up to 6.0m

■ Price range Low

■ Recommended fixings Stainless steel or specialist coated fixtures

Available Profiles

Surface Options

Coating Options

Uncoated

Planed

Fine Sawn

Brushed

Charred

HC102

HC400

HC401

HC402

HC501

HC600/1

HCWWB

Thermowood Heat treated timber

Thermowood is a sustainable and durable timber created through a heat treatment process that involves subjecting timber to high temperatures and steam in a controlled chamber, altering its cellular and physical properties. As a result, the timber boasts improved stability, moisture resistance, and longevity, making it an eco-friendly choice for various outdoor and indoor applications. Commonly known as Redwood D, this is a readily available and economically priced species.

■ Grade overview

Centre-cut – live-knot ‘A/B’ quality. Fine texture with live, inter-grown knots. Slowly grown with tight straight grain.

■ Moisture content 6-9%

■ Density 350-480kg/m 3 (more info on p.68)

■ Durability (BSEN350)

Class 2: Durable (more info on p.66)

■ Movement class Small (more info on p.67)

■ Available lengths 4.2m-5.1m

■ Price range Low

■ Recommended fixings Stainless steel or specialist coated fixtures

Available Profiles

Surface Options Coating Options

Planed Fine Sawn

Brushed Charred Maintenance Transitional Uncoated

HC200

HC300/1

HC400

HC401

HC402

HC500

HC600/1

HC900

Accoya® Modified timber

Accoya is an innovative and eco-friendly wood product produced by passing Radiata Pine through acetylation, a modification process. This enhances the wood's performance by increasing its durability, stability, longevity and resistance to decay. Accoya is available in a number of finishes, making it ideal for various applications, whilst offering a sustainable, long-lasting solution with reduced environmental impact. Non-toxic and long lasting, it has a warranty of 50 years above ground and 25 years on the ground or in freshwater.

■ Grade overview

Primarily clear with a few small knots. Bark, resin streaks/ pockets and medium birds eye/fleck can be present. The production process can result in small, short splits.

■

■

Available Profiles

Surface Options

Coating Options

Uncoated

Planed

Fine Sawn

Charred

HC200

HC300/1

HC400

HC401

HC402

HC500

HC600/1 HC900

Other species For diverse applications

In addition to our core range, we can also supply additional species, providing a wider range of textures and tones to cater to diverse architectural applications.

Sweet Chestnut

Resembles Oak in appearance, but tends to offer greater stability and is lighter in weight. Durable and hard-wearing, it provides a beautiful finish, adding character and charm. The golden color is similar to oak, but the grain is more pronounced, occasionally accompanied by darker mineral streaks. Most commonly supplied as finger-jointed.

Iroko

Iroko is an extremely durable timber with a dense grain, making it resistant to decay. This sustainablysourced African hardwood is known for its durability and has a rich golden tone.

Canadian Douglas Fir

A great-looking and highly versatile softwood species with natural durability, good dimensional stability, and good working qualities, commonly used as a replacement to Siberian Larch. Rich, warm wood tones and marked 'flame-like' growth ring figures give it an attractive appearance.

Thermally Modified American Red Oak

A premium hardwood species with an attractive reddish-brown colouring. Distinctive, delicate rays create a unique and characteristic look, delivering real wood aesthetics and unrivalled charm. The species gives walls an elegant and timeless finish that will retain their noble appearance as the wood ages gracefully.

Thermally Modified Ash

Typically seen as the most durable of all thermospecies, Thermo-Ash is stable and relatively knotfree, with a strong grain, and deep, dark chocolate colouring. It offers a locally grown alternative to imported hardwoods, with the performance and price and similar to Canadian Western Red Cedar.

Thermally Modified Ayous

A versatile, sustainable and high-performance product, popular as a economical alternative to Western Red Cedar. The African hardwood undergoes a drying, heating and cooling process to alter its appearance and increase its durability to a 1–2 Class rating. Boards are clear grade and essentially knot free, with a luxurious, mid-brown tone.

Thermally Modified Radiata Pine

Radiata Pine that has undergone heat modification to enhance the timber’s stability and reduces resin content, leading to improved performance. Available as a sustainable alternative to Canadian Western Red Cedar and Siberian Larch, it is free from chemical preservatives and solvents, ensuring a sustainable solution.

Profiles

Feather Edge

HC100/HC101/HC102

HMade up of tapered boards that overlap each other, with the thicker end at the bottom and the thinner end at the top, creating a natural slope. This design allows water to run off the cladding and prevents it from penetrating the wall. Commonly used in traditional and contemporary architecture for its rustic and natural appearance. Also known as weatherboard cladding. 25-50mm overlap required depending on the width and movement class of the species being used.

■ Standard surface option: Sawn

TGV HC200

HFeatures a tongue and groove joint system, where the boards fit together neatly, with a protruding tongue on one board that fits into a groove on the adjacent board. This design creates a strong and secure joint and is often used for both traditional and contemporary architecture due to its clean, uniform appearance, and versatility.

■ Standard surface option: Planed

*Face width does not allow for expansion gap [see page 71]

Secret Fix T&G

HC300/HC301

VHFeatures a T&G joint system with a unique, hidden fixing method. A profiled board with a groove cut into the back fits over a protruding tongue on the support batten, concealing the fixing, creating a clean and seamless finish. The hidden fixing method creates a clean, uniform appearance that is ideal for contemporary buildings or to update the look of a traditional building. Available with either a square detail (HC300) for vertical installation, or a splayed detail (HC301) for horizontal installation.

■ Standard surface option: PAR

HC300

HC301

*Face width does not allow for expansion gap [see page 71]

Splayed Halflap HC400

HHalf the thickness of each board is removed along one edge, creating a rebate. The other edge of the board is then machined to a matching profile, creating a tongue that fits snugly into the rebate of the adjacent board. Provides a good overlap between boards, creating a virtually weather-resistant barrier, preventing rainwater penetration. The rustic and traditional appearance of half lap cladding makes it a popular choice for the more traditional and period-style buildings. The halflap profile is a great choice for homegrown timbers, providing a practical and weatherproofing joint.

■ Standard surface option: PAR

*Face width does not allow for expansion gap [see page 71]

Vertical Chamfer Halflap

HC401

VFeatures a V-shaped groove cut along the edges of each board to create a series of lines that run the length of the cladding, giving the surface a unique and distinctive look. The chamfered edges of the boards create a shadow line effect, which enhances the texture and depth of the cladding. The unique design of the cladding adds character and interest to any building, making it a popular choice for feature walls or accent areas.

■ Standard surface option: PAR

HC401

Vertical Square Halflap HC402

VA square profile where each board has a flat face with a square edge that runs the length of the board, creating a clean and uniform appearance. The boards are installed vertically, creating a series of vertical lines that run the length of the cladding. The simple, geometric design of the cladding creates a modern, minimalist look that is ideal for contemporary buildings and the uniformity of the boards creates a cohesive look that is ideal for large-scale projects.

■ Standard surface option: PAR

*Face width does not allow for expansion gap [see page 71]

Shiplap HC500/HC501

HSimilar to TGV cladding in that it also features a secret fix tongue and groove joint system (HC500), the edges of Shiplap boards overlap each other, creating a distinctive shadow line effect. The shape of the profile assists with guiding water away from the joint, and the clean lines and rustic appearance of shiplap cladding make it a popular choice for both traditional and modern architecture.

■ Standard surface option: Planed

HC501

HC500

Rainshield HC600/HC601

HAlso known as Splayed cladding, Rainshield is a rhombus shape, installed with a 5-10mm decorative gap between the board. The boards sit adjacent to one another, without being interlocked or connected, allowing air to circulate the boards and therefore helping to prevent the build-up of mildew and mould behind the cladding. Essentially an outer ‘skin’ with a ventilated cavity, the cladding must only be installed in front of watertight surfaces. Available with either a 30o or 45o splay.

■ Standard surface option: Planed

British

Canadian

British

British

Douglas

Douglas

Thermowood

Thermowood

Accoya

HC600

HC601

Slatted HC900

HThis profile is becoming an increasingly popular choice for contemporary builds, with the defined gaps on each board forming a sleek, subtle shadow. Perfect for narrower and smaller areas of cladding, the thin, batten-like appearance gives a powerful aesthetic with sharp, attractive lines that evolve with the movement of the sun. The shadow gap on this profile appears deeper than the more common tongue and groove shadow gap. The profile can be fixed with finishing screws through the grooves, resulting in a hidden fixing.

■ Standard surface option: Planed

HC900

*Face width includes a 4mm expansion gap.

Waney Edge HCWWB

HTypically used on barn conversions and rural applications, the varied and wide widths of waney edge boards provide a traditional, rustic look, whilst maintaining a very durable barrier against the elements. Sawn fresh from the log, the boards have a fine sawn finish and will shrink slightly as the timber naturally dries and settles in situ. Dimensions are indicative as board width will vary.

■ Standard surface option: Sawn

2 Piece External Corner

HM500

■ Works well when a larger, more defined external corner is required.

■ Suitable for both horizontal and vertical cladding.

■ Can be fitted flush with the face of the cladding (trim) or,

■ Can be fitted on top of the cladding to cover the ends/edges (capping).

■ Most suited to hardwoods.

2 Piece Internal Corner

HM501

■ Works well when a larger, more defined internal corner is required.

■ Suitable for both horizontal and vertical cladding.

■ Can be fitted flush with the face of the cladding (trim) or,

■ Can be fitted on top of the cladding to cover the ends/edges (capping).

■ Most suited to hardwoods.

Solid External Corner

HM502

■ Machined from a single piece.

■ Usually fitted on top of the cladding to cover the ends or edges of the boards.

■ Can be used with both horizontal and vertical cladding.

■ Most suited to softwoods.

Square Corner Trim

HM503

■ Can be designed to sit ‘proud’ of the boards to create an over-hang, or sit ‘shy’ to create a small recess.

■ Not advisable to line up the block with the boards exactly as any amount of movement will be noticeable.

■ Stop boards short of a solid corner to create a shadow gap.

■ Recommended for Feather Edge and Waney Edge cladding.

Shakes & Shingles

Oak & Cedar

VHTimber shakes or shingles are a natural, durable solution for wall or roof cladding, providing an attractive aesthetic. Made from either Oak or Western Red Cedar, they offer excellent weather resistance, insulation and soundproofing properties, and are lightweight and easy to install. The shape and size can be customised for a truly bespoke look.

Wood shingles are sawn on both sides and are thinner at the butt end when compared to a wood shake. Wood shakes are typically sawn on one side and hand split on the other side, making them thicker than wood shingles. Both are wedge shaped and are affixed individually to a roof/wall deck.

European Oak Shakes

Western Red Cedar Shingles

In terms of appearance, shakes provide a thick textured appearance and give a roof beautiful depth and dimension, due to the varying grain patterns, ridges, and grooves that occur during the splitting process. They have a rugged appearance, with each piece looking a bit different than the rest. On the other hand, shingles are much more uniform in appearance, with a smooth, flat appearance. They lean towards a more contemporary look that is complementary of many different home styles and color schemes.

There are many guides available on how to install shingles and shakes. The density or overlap of one shingle or shake over the other is different between walls and roofs, and the amount of overlap on roofs is also determined by the slope or pitch of the roof. As a rule of thumb, when used vertically for wall cladding, you need to allow double overlap, where approximately half of each shingle or shake is visible. For roofs, depending on the pitch, you may need to allow up to triple overlap, where a third or less of each shingle or shake is visible.

Bespoke Requirements To suit your needs

You imagine, we create! We can machine to any profile up to 300mm wide, subject to availability, and we also have a range of additional air dried cladding profiles that we can produce from our Joinery grade board stocks.

Whatever your needs, we have the knowledge, skill and capacity to bring your ideas to life!

Surfaces & Coatings

Surface Textures Visual & tactile

Differing textures can be applied to timber cladding to offer a distinct visual and tactile experience, and allowing for a diverse range design possibilities.

The choice of texture depends on the desired aesthetic, the architectural style, and the overall design objectives of the project, and we offer three types, each with their own unique appearance and characteristics.

Planed

Planed texture refers to a smooth, uniform surface that is achieved by machine-planing the wood, creating an even and sleek finish. This texture showcases the natural colour and grain of the timber, allowing for a more refined and contemporary appearance. Planed textures are often chosen for modern or minimalist designs, offering a clean and polished aesthetic that contributes to a sleek and sophisticated architectural style.

Fine Sawn

Fine Sawn texture is the result of a specific milling process where the wood is cut parallel to the grain, creating a rough, textured surface. This process leaves behind prominent, linear bandsaw markings, giving the timber a distinct, sawn appearance. Fine Sawn textures provide a rough, natural look, adding a sense of authenticity and raw appeal to the cladding. This texture is often favoured for its ability to evoke a sense of traditional craftsmanship and natural warmth in architectural designs. This finish can enhance the durability performance of a coating.

Brushed

This texture is achieved by brushing the wood's surface with a wire brush to remove the soft fibres, leaving the harder grains exposed. The result is a textured surface with a pronounced grain pattern and a weathered, rustic look. Brushed texture provides a natural and rugged appearance, working espcially well on softwoods, where the timber's wider grain and character is beautifully enhanced. It is popular for creating a more tactile and visually appealing finish, particularly suited for a traditional or rustic aesthetic.

Planed

Fine Sawn

Brushed

Charred Yakisugi technique

Charred timber cladding, an ancient Japanese technique known as Yakisugi, involves charring the surface of wood to create a unique, visually striking, and durable exterior finish.

An increasingly popular choice of finish in contemporary architecture, the process of charring timber not only enhances its natural beauty but also significantly improves its resistance to fire, weathering, rot, and pests. The charring effectively creates a layer of carbon that acts as a shield, making the wood more durable and weather-resistant, therefore extending its lifespan.

The finish can be applied to various types of wood, but is most commonly used on larch and pine, and provides a dramatic aesthetic appeal to any building. Different levels of charring are available, depending on the desired look, to provide contrasting hues and deep, rich tones that create a sense of depth and visual interest, seamlessly blending with both natural and urban landscapes.

As a result, this age-old technique has stood the test of time and reemerged as a modern choice of innovative design and sustainable construction practice.

Charred and Oiled

Seared and Brushed

Charred, Brushed and Oiled

Maintenance Coatings Preserve & enhance

Whilst it is completely acceptable to leave your cladding with a natural finish, some choose to add a coating to the timber. Coatings typically serve two purposes, either to enhance the appearance of the timber cladding, or to increase its durability, helping to protect the wood from moisture and sunlight, and therefore extend its lifespan and maintain its visual appeal.

Increase durability

One of the primary benefits of using maintenance coatings on timber is to create a protective barrier that repels water and prevents it from seeping into the wood. This is crucial for preventing rot, warping, and other forms of water damage that can compromise the structural integrity of the cladding. Additionally, coatings can also shield against UV rays, reducing the impact of sunlight and delaying the wood from fading or developing a greyish patina over time.

Coatings that are being used to increase durability should ideally be microporous. At some point during the timber’s life, moisture will eventually penetrate into the cladding, and will therefore need a route out, or the coating will trap moisture, causing the timber to rot quicker. Oils, lacquers and silicon-based products, such as SiOO:X, will all service this purpose, meaning durability can be increased regardless of what aesthetic finish is desired.

Enhance appearance

For those wanting to change the appearance of their timber cladding after installation, semi-transparent or opaque coatings can be used to give the surface a unique colour and personality whilst protecting and enhancing the natural wood grain. This not only adds to the visual appeal of the cladding but also helps in highlighting the inherent character of the wood, making it an even more striking feature of the building's exterior. Depending on the amount of pigment included in the coating, it will also provide UV protection, and therefore less maintenance will be required needed (i.e. the darker the colouring, the higher protection the coating will offer).

Maintain colour

Often people want to enjoy the original natural beauty of the timber, and prevent any discolouration caused through weathering. This can be achieved by either using a clear oil, or a translucent coating that has been coloured to a similar shade of the original timber. Both options give the cladding a consistent hue, however this is very maintenance-intensive as the coating will need to be frequently reapplied for the finish to be preserved.

When selecting a coating for timber cladding, it is essential to consider the specific type of wood and the environmental conditions it will face. Some products offer better water resistance, while others provide superior UV protection. We are on hand to assist you with choosing a suitable coating that is specifically formulated for exterior use and compatible with the type of timber being treated.

Moisture content of timber needs to be at a specific level for certain finishes to perform. Coating timbers with a MC of over 20% is not recommended by most manufacturers.

Transitional Coatings Consistent colouring

When left uncoated and exposed to the elements, all timber will eventually weather to a silver-grey colour. How quickly and evenly timber weathers is dependent on a variety of factors, including location, aspect and building design, for

example, cladding which is sheltered from the elements will take longer to weather, and may result in an uneven or patchy appearance over time. This can take years to even out and in some cases the timber may never weather evenly.

A Transitional (or 'pre-weathered') coating eliminates this problem, providing a consistent, weathered tone within a fraction of the time that it would take to occur naturally. This treatment eliminates differential weathering and offers a quick transition from ‘new-look’ timber, to a natural surface that appears to have been installed for several years.

Depending on the species, the weathering process can cause the surface of the timber to appear ‘patchy’, with tannin build-up and leaching causing dark watermarks. These can be unsightly for those that are looking for a more refined, less-traditional aesthetic, so a dark transitional coating can be used to conceal the marks.

A final note

There are many coatings available for timber cladding, but ultimately, the choice depends on the desired aesthetic outcome, the level of protection required, and the maintenance preferences of the property owner. Whilst some highlight the natural beauty of the wood and require more frequent maintenance, others provide a wider range of colour options and offer a higher level of protection with longer maintenance intervals.

To reduce the level of maintenance, most manufacturers recommend the finish is applied to a fine sawn finish. It is also important to note that coatings are best applied to timbers that are more stable, or those that have been treated to minimise their movement, to reduce the risk of the finish peeling or cracking as the timber naturally moves in situ. Accoya or Thermowood typically absorb finishes well as a result, however, coatings on Western Red Cedar – a typically stable timber – often break down due to lignin found on its surface causing the finish to fail quickly.

There are many factors to consider; we are happy to give further advice as required.

Additional Information

Fire Retardancy Regulations and classifications

In this day and age, when timber cladding is experiencing a resurgence in popularity, fire retardancy and the applicable regulations for your project need to be considered and allowed for.

Is wood cladding a fire risk?

Technically speaking, large areas of wood are difficult to ignite quickly because as the exterior surface chars, a protective layer is formed that shields the core and reduces the rate of fire spread. This means that choosing timber for external wall cladding should not exhibit an extreme fire risk unless the building in question falls into one of the categories stipulated overleaf and thus a specific fire rating is required.

Fire regulations for external cladding

In the UK, building regulations state that the materials used for the external wall construction or cladding should not be a medium for the spread of fire. Approved Document B requires that consideration is given to the choice of materials as well as their arrangement used for external wall construction, attachments to the wall, or cladding, to reduce the risk of fire spread. This is intended to help reduce the risk of vertical fire spread and spread to adjacent buildings.

Specific fire ratings are required depending on building height, use, and distance from adjacent properties. Given that most wood has a fire rating of Euro Class D;

• For buildings under 18 metres: The fire rating of external cladding should be Euro Class B or better if it is within 1m of the boundary. If it is more than 1m from the boundary, no rating is required.

• For relevant buildings over 18 metres: The fire rating of external cladding should be Euro Class A, whatever distance it is from the boundary. Euro Class A materials are also known as non-combustible, providing little or no contribution to fire.

Here, ‘relevant’ is taken to mean a building with a storey at least 18 metres above ground level which contains one or more dwellings, an institution, or a room for residential purposes.

This means untreated cladding isn’t suitable for buildings under 18 metres and within 1m of a boundary, or buildings over 18 metres any distance from a boundary. To become suitable for the external wall construction or cladding of a building that is under 18 metres and within 1 metre of a boundary, the fire rating of wood would need to be boosted to Euro Class B through some form of treatment.

Can timber cladding be fireproofed?

By applying flame retardant treatment, it is possible to significantly enhance the fire rating of timber cladding, thereby expanding its suitability for a wide range of building applications.

It is worth noting that because wood is inherently a combustible material, it cannot have its fire rating improved to class A, meaning it is unsuitable for constructing residential, institutional, or multi-occupancy buildings exceeding a height of 18 meters. Nevertheless, excluding these particular cases, wood remains a reliable and adaptable construction material, offering safety and versatility.

Through treatment, timber cladding with a D rating can be elevated to a B rating, enabling buildings with specific fire safety needs to utilise timber while benefiting from its environmental advantages and aesthetic appeal.

We work with recognised partners to offer a variety of treatment solutions to ensure your cladding meets the required rating. Further information is detailed on the following pages.

Types of Treatment Impregnation, surface & dipping

Flame retardant treatments reduce the surface spread of flame, heat, and smoke. This provides valuable extra time for evacuation and fire suppression in the event of an emergency.

The three most common methods of applying flame retardant are:-

• Impregnation in high pressure autoclaves

• Surface applied under factory-controlled conditions

• On site application, or ‘dipping’

Pressure Impregnated Fire Treatment

This treatment is a combination of high-pressure impregnation in a treatment cylinder, followed by heat curing under controlled conditions. With the highpressure impregnation pushing the treatment into, and around, the packs of timber within the cylinder, there is a consistent uptake within the packs, ensuring the treatment lasts for the life of the building, with no maintenance required after installation. Undergoing this process can create an element of wastage so this needs to be accounted for.

Factory Spray & Brush applied Fire Treatment

This method is where protection is applied through a factory controlled system to EuroClass B through an audited (e.g. ISO 9001 & 14001) Spray & Brush Line. It is a more labour-intensive process than pressure impregnation, with every board going through the line ending up with a consistent application of fire treatment to all faces. However, as with the pressure impregnated treatment, it is designed to last for the life of the building, and no maintenance is required after installation.

On site application, or ‘Dipping’

This is where packs of timber are immersed into a fire treatment solution bath, before removing and allowing them to drip dry. If necessary, the process can be repeated. There are two main disadvantages of this process:-

• It lacks the advantages of a high-pressure closed cylinder system that drives the solution throughout the pack and into each individual timber

• It doesn’t have the precise, monitored approach found in a Spray & Brushline

On-site application, for any form of coating, makes things very difficult to control things such as humidity, contamination, application consistency and curing time. Because of this, the application of flame-retardant products on the construction site, either by brush or spray, is not approved by the Wood Protection Association as quality control is almost impossible to assure.

Classification At a glance

The reaction to fire performance of timber cladding is defined under EN 13501-1, and construction products are assigned a Euroclass rating under this system. The Euroclass system comprises ratings from A - F, and describes characteristics such as flame spread, ignitability, heat release, smoke production, burning droplets etc. The ratings system and examples of each category are:-

Class Definition Description Examples

A1 Noncombustible No contribution to fire

A2 Limited combustibility

Very limited contribution to fire

B Combustible Limited contribution to fire

C Combustible Minor contribution to fire

D Combustible Medium contribution to fire

E Combustible High contribution to fire

F Unclassified Not tested

Glass / Stone / Concrete

Gypsum Boards with thin coverings (eg paper faced)

Some flame retardant treated wood products

Some flame retardant treated wood products

Most wood based panels over 390kg/m³ in density

Some wood panel systems / Plastic based insulation / Composites

N/A

These classes are further divided to provide information on a product’s tendency to produce smoke and flaming droplets / particles [see over].

Smoke Generation

Smoke generation is measured for Reaction to Fire classes A2 to D. Smoke emission classifications are not provided for products with an E or F overall rating. There are three smoke intensity levels

s1 Emissions absent or very little

s2 Emissions with average volume intensity

s3 Emissions with high volume intensity

Fire Generation

Burning droplets/particles can inflict skin burns and cause further spread of fire. Burning droplets/particles are measured for Reaction to Fire classes A2 to E. E-rated products receive a d2 flaming droplet classification. F-rated products receive none. There are three classes of burning droplets:

d0 No burning droplets

d1 Slow dripping droplets

d2 High/Intense dripping droplets

As an example, a product with the resulting highest classification would have the standard: B/S1/d0. This translates to:

B - Combustible materials – Very Limited contribution to fire

s1 - Smoke emissions absent or very little

d0 - No burning droplets

Sentrin FRX by PTG Treatments

Sentrin FRX is fire retardant treated timber which is suitable for use in external applications directly exposed to the weather. It is a WPA Type LR treatment and the wood is capable of achieving Euroclass B. Typical examples of uses include external cladding, decking and boundary timbers.

Sentrin FRX treated timber can meet the requirements of national Building Regulations where Euroclass B or C are required. These fire performance properties do not compromise critical engineering properties such as strength, durability, corrosivity and hygroscopicity.

Treatment is carried out under controlled factory conditions. The process is ISO 9001 accredited and productions centres hold Factory Production Control (FPC) certification and are WPA Benchmark FR approved. A computer controlled treatment process ensures the correct retention of fire retardant to achieve the required classification.

Burnblock® by WJ Group

Burnblock® is an patented, eco-friendly type of protection that is 100% natural, non-toxic and bio-degradable! It effectively prevents oxygen from reaching the treated object, and without oxygen there is no fire! A highly effective and well documented fire retardant, it is an invisible treatment that when applied correctly leaves no greasing, no staining and no trace.

Once absorbed by the treated material, Burnblock® leaves behind a surface that can char and release water upon the application of heat, absorbing a portion of the heat applied which helps prevent the further spread of fire. Therefore, materials treated with Burnblock® are protected from combustion and spread of flame is reduced.

Burnblock® Fire Retardant Powder is Cradle to Cradle Certified™ at ‘Gold Level’ which makes the product is the very first Fire Retardant in the Cradle to Cradle Certified™ Products Program. Part of the sustainability certification includes a Platinum status for Material Health – the highest level of certification possible.

WJ Group provide a Declaration of Performance, in line with their certification for each of their fire-retardant treatments. To ensure these standards are adhered to and factory controls remain in place, third-party audits are carried out at the fire-retardant facility to ensure the consistency of performance of the manufactured products.

We have partnered with these two industry-leading organisations to offer pressure impregnated treatments to our clients.

Durability A key performance factor

Durability refers to the time the timber will last, usually measured in years, and is a key performance factor involved in the design and specification of timber cladding. The main contributor to durability is the ability of a species to resist decay either naturally or through preservatives.

The durability of a timber depends on how it performs in a particular environment e.g. a commonly known non-durable species such as European Beech will not last many years if left outside, especially if in ground contact, but it may last centuries as a table in your kitchen or as your bookshelves.

Many hardwood species of timber have natural durability and can be used outdoors untreated. The downside to this is that the cost of these naturally durable hardwood species is often high, and supply sometimes limited. Some softwoods are durable to an extent, however, most of them will need additional preservative treatment before they can be used outdoors or in environments prone to condensation or humidity. On the plus side, these softwoods are less costly and more widely available. The modified timbers such as Thermowood, are both more sustainable and durable, and are only slightly more expensive, therefore becoming a more popular and widely available option.

BS-EN-350 is what is used to class species of timber by their natural durability and show the lifespan of the wood. The class is based on the ability of the heartwood of that species to resist fungal decay and has to be considered relative to where the timber will be used (use class).

The standard test to determine the durability of timber involves taking a stake of the heartwood, placing it into the ground, and monitoring it over time. Sapwood in timber used for untreated cladding is not considered durable and is excluded from the test.

There are five classifications of natural durability:-

Class 1: Very durable | Life span of 25+ years

Class 1 represents the highest durability of natural timber. This means that in normal outdoor above-ground conditions, it should last at least 40 years. In the ground, it should last at least 25 years. Class 1 timbers are best for projects that involve exposure to the elements, e.g. wet/rainy conditions. The drier the timber is, the longer it lasts.

Class 2: Durable | Life span of 15-25 years

Class 2 timber types have a life expectancy above ground of 15 to 40 years. Inground, that number sits lower at 15 to 25 years. Class 2 timbers are still extremely durable timbers and are renowned for their long lifespan and beautiful features.

Class 3: Moderately durable | Life span of 10-15 years

Class 3 timber has an average durability, lasting between 7 and 15 years in normal outdoor above-ground conditions, and 5 to 15 years underground. Class 3 timber is great for indoor and ornamental features and furniture.

Class 4: Slightly durable | Life space of 5-10 years

Class 4 timber has the lowest durability, and will likely last between 0 to 7 years in above-ground outdoor conditions, while in-ground conditions will likely deteriorate the wood within 5 years. Class 4 timbers, like most softwoods, are still useful as structural and building materials, and serve a special purpose in their lightweight and easy-to-work nature.

Class 5: Not durable | Life span of 0-5 years

Class 5 timber has very little, if any, durability and includes species such as Sitka spruce, Alder, Silver Birch, European Horse Chestnut and Sycamore.

The lifespan figures in this Class system refer to the lifespan in external, unprotected environments; if timber is kept in a protected, internal environment, then the lifespan can actually be 50+ years. These classes only consider the heartwood of the species and any sapwood will always fall under class 5, regardless from which species it comes.

Moisture movement What to expect

Temperature and humidity conditions in the UK cause a high degree of moisture content variation in timber cladding. As the cell cavities of the wood fibres lose or gain moisture, the wood shrinks or expands causing movement in the timber.

The point that moisture in timber matches that of its surrounding environment is called the Equilibrium Moisture Content. For properly installed cladding in the UK this point is around 16% moisture content. The figure fluctuates up and down, so on hot days it can be down at 12% and on a very wet day the figure can climb around 20%.

Moisture content is one of the most important aspects of joinery specification and BS1186 gives four levels of moisture content which relate to the different end use conditions for joinery. It recommends the following average moisture content:

• External Joinery: 13% to 19%

• Internal Joinery: Unheated Building: 13% to 17%

• Internal Joinery: Room temperature 12°C-21°C: 10% to 14%

• Internal Joinery: Room Temperature in excess of 21°C: 8% to 12%

BS 1186-3 recommends that exterior cladding timber dries to a moisture content of between 13-19% before installation, although this will vary between species.

To help describe and specify movement, TRADA devised a simple movement classification: Across the grain dimension change within a moisture content range of 5-30%:-

• Large: 1% for every 3% change in moisture content

• Medium: 1% for every 4% change in moisture content

• Small: 1% for every 5% change in moisture content

See page 71 for guidance on how to allow for movement when installing cladding,

Density Understand why it matters

Density is an important factor that significantly impacts the performance and longevity of timber cladding, so it is an essential consideration for architects, builders, and homeowners.

High-density wood is generally more resistant to moisture absorption, insect infestations, and fungal decay, and is less likely to warp, crack, or split when exposed to changing weather conditions.

Typically, the higher the density of a timber, the greater its structural integrity, which not only makes it capable of withstanding external forces such as wind/ impact/pressure but also allows for better fixing and installation, resulting in a longer-lasting and more reliable cladding system overall.

Higher density wood typically has finer grain patterns and more consistent colouring, which enhances the visual appeal of the cladding and it can also provide better insulation against temperature fluctuations, potentially leading to improved energy efficiency within the building.

Fixing Tips Best practises & techniques

The chosen profile will largely influence how the individual cladding boards fit together, and consequently impact the installation of the cladding.

We strongly recommend that the installation of our cladding products is undertaken by qualified professionals, but below are a few key points you should bear in mind for optimal results.

Acclimatisation

Timber is a natural material and as such will respond to the environment. All timber will swell and shrink as moisture is absorbed and lost from the timber. This is a natural process as the timber seeks to acclimatise and reach equilibrium moisture content with the surrounding environment. This depends on the location of the build. For example, timber used in London will respond to the environment differently to timber used in the Scottish Highlands.

Timber cladding may require time to acclimatise on-site before install. Remove wrappers and store the timber in a dry, cool place with good ventilation through the package to allow it to acclimatise. Spread the boards out on stickers (small timber strips) and cover loosely to reduce the exposure to moisture and allow air flow around all boards, greatly reducing the risk of discolouration to the timber.

Membranes

A timber rain-screen can allow some moisture through joints, and in such an installation an appropriate breather membrane is recommended to prevent moisture ingress into the structure of the wall. Choose a UV Facade membrane for open jointed cladding, and a normal BBA approved breathable membrane for closed rainscreen cladding. It is also recommended to install insect mesh around all open cavities.

Support battens

Cladding boards are usually fixed to preservative-treated softwood cladding battens. These will normally be spaced 400-600mm apart, and oriented perpendicular to the cladding boards: horizontal cladding boards on vertical battens; vertical cladding boards on horizontal battens. Depending on the board profile and installation method, vertical counter battens may be installed first to provide drainage and ventilation. Horizontal battens should have their top edge angled to shed water.

Ventilation

The space behind the cladding created by the cladding battens should be drained and ventilated. This serves three purposes:

• If the cladding is fixed to a timber frame building, the drainage and ventilation help to ensure long-term durability of the structure.

• The ventilation space allows cladding boards to dry more rapidly after wetting.

• The ventilation space helps to equalise the moisture content of the inner and outer faces, and reduces the risk of the boards cupping.

Flashing details at the base of the cladding and around windows and doors should be designed to shed water away from the building while maintaining the required ventilation. Check the requirements for fire-resistant cavity barriers with local building control and consider using third-party approved, ventilated cavity barriers where needed.

Dealing with oak?

Scan below for specific tips about fixing oak cladding and boarding

Allowing for movement

Boards will expand and contract in width (but not length) through the different seasons of the year as moisture levels change. In wet conditions they will expand, and in dry conditions they with shrink. It is essential to allow for this natural change in width of the boards by allowing a gap between the boards when installing the cladding.

For species with large movement, allow a 4-6mm gap; for species with medium movement, allow a 2-4mm gap; for species with small movement, allow a 1-2mm gap. Thermally modified timbers are very stable and therefore no gap is necessary. Note, face widths shown on the profile drawings in this guide do not allow for an expansion gap - the width of the gap needs to be added to the face width when calculating requirements.

You should also allow between 6-10mm for drainage gaps on corner profiles, ends, and sides.

Fixing the boards

Each cladding board should be individually fixed. Softwood cladding boards are normally nailed onto timber support battens with stainless steel nails. It may be possible to use galvanised nails if the boards are to be painted, but we would always recommend using Stainless Steel grade 304 fixings. Cheaper galvanised or coated options will eventually rust and mark the cladding, which can ruin the appearance and is difficult to rectify.

If using nails, we recommend using annular ring shanks. We do not recommend using brad nails or pins as they are typically not strong enough.

Western Red Cedar, Douglas Fir and hardwood species such as Oak must be fixed with stainless steel screws. Pre-dilling pilot holes is recommended to reduce splitting around fixing points, and slightly oversized to allow expansion and shrinkage of the board. Cladding boards that are 100mm or wider should have two screws per batton, except for secret fix tongue and groove profiles and 150mm wide featheredge. Screws should penetrate into the batton 2 to 2.5 times the thickness of the cladding.

Detailing

The design of the cladding should avoid moisture traps and projections which will allow splash back onto the cladding. Cutting the ends of vertical cladding board to an angle will allow moisture to drip from the ends of the boards more easily. Sealing any cuts on the board, as well as the end grain, is recommended practise to limit moisture ingress.

Image Credits: Covers + pages 4, 16, 55, 62: Hopkins Estates, Page 5/6: Martine314, Page 7: Lukonic, Chloe Bennetts, Page 10: Henry Preston, Page 12, 27, 70: Bridgewater Carpentry, Page 14: Golden Mede, Page 18: Silva Timber, Page 20: Whitney Sawmills, Page 22: Lunawood, Page 24: Accoya, Page 41: Coyle Timber & Build & Plumb, Page 42: Marley, Page 50: Capricorn Timber, Page 52: Remmers, Page 53, 54: SiOOX, Page 57; Panifile, Page 60: WJ Group, Page 63: PTG Treatments, Page 68: Sarah Rowlands.

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