Landscape and exteriors: specification Timber for use in garden and landscape construction

Timber can be used in almost any landscape or garden; however, without suitable selection and treatment many types of timber, when used in external environments and in contact with water, will have a limited service life. While timber is usually categorised into hardwood or softwood, not all are suitable for use in external construction and cladding applications. There are hardwoods that are poorly suited to external applications, such as birch, and softwoods with a natural resistance that perform well in external situations, such as Western red cedar. More practical classifications for the purposes of construction are applicable: see ‘Use classes’ below, and ‘Durability classes’ overleaf.

The majority of timber used externally, available in the UK, is derived from softwood. Hardwood is mostly limited to green oak (green identifies the timber as being unseasoned) and is usually used for simple structures such as raised beds and pergolas. Cost and reduced availability limit the use of hardwoods to specialist and statement items. Without the correct specification, the timber may be seriously compromised and unlikely to reach the intended service life. Designers and specifiers must understand what the intended application is, and therefore which use and durability classes will apply.

Use classes (outlined in BS EN 84172)

Timber is a natural product and, depending on its exposure to the elements, is prone to biological and physical degradation, including bacteria, fungal decay, wood-boring insects, marine borers, ultraviolet (UV) light, weathering and erosion, and movement.

BS EN 335 Durability of wood and wood-based products. Use classes: definitions, applications to solid wood and wood-based products lists five use classes of timber exposure:3 1. Internal – no risk of contact with water (roof timbers) 2. Internal – risk of contact with water (roof tile battens) 3. External – above ground/exposed (fence panels) 4. External – in contact with the ground/in freshwater (fence posts) 5. External – permanent contact with seawater (groynes). >>

The classification of the designed structure should be determined with some degree of common sense, for example timber decking boards fall into use class 3, but the accumulation of debris and soil may create areas of high water retention lifting the deck substructure into use class 4.

Durability classes

While use class defines the intended environment timber will be used in, durability classification supports the specification of an appropriate species to achieve a given service life.

While some timbers may naturally not achieve the higher classifications they can, with appropriately applied preservatives or processes, have their durability improved considerably. Some methods allow suppliers to offer periods of guarantee for applications in ground of ten years, 20 years or longer.5 The heartwood of some timbers has a natural resilience to decay either in ground or in contact with water and in some circumstances additional treatment may not be required.

1 Very durable

2 Durable

25+ years

15–25yrs

Moderately durable 10–15yrs

Slightly durable 5–10yrs

Not durable 0–5yrs

Note: taken from BS EN 3504 / see Further reading for a useful chart *Timber is from heartwood

Timber should therefore be specified by its use class and durability class, which will allow the selection of both an appropriate wood species and application of additional preservatives, if required. Without treatment, timber may have less than satisfactory durability for structural applications, especially in use class 4 or 5 environments. Not all timber used externally requires treatment but even those in high durability classes can benefit from imparted protection (such as resistance to UV and rain penetration) and extending the life in service. All treatments benefit from repeated application and allow movement cracks and shakes (splits along the grain as timber ages) to be addressed as they emerge.

Untreated timber will naturally weather to a grey patina. The speed of weathering depends on several factors, including species, but within a year most timbers will have a matured, lead-coloured surface. It can be difficult for an untrained eye to distinguish between oak and cedar, for example. There are proprietary treatments that purport to accelerate the weathering process,6 which are useful for creating the impression of maturity and consistency across all surfaces where natural exposure may inconsistent.

An alternative to natural, single species timber is the use of engineered or modified timbers. Engineered timbers consist of derivative wood products that are laminated or fixed into composite units such as plywood or glued laminated timber beams making them structurally superior to single species timbers and often suitable for external use, when protected from direct environmental exposure (for example, by an overhang).

Alternatively, timber can be modified so that the cell structure is no longer capable of absorbing water or harbouring microbial activity. Treatment can be through thermal processes (such timbers are often identified as ‘thermo’ woods) or through acetylation (for example, Accoya). These treatments greatly improve the durability class of timbers that naturally might have a poor life in service.

Oak clad bench stained from gravel pigmentation due to rainfall. Photo: Green Zone Design Ltd Seasoned oak pergola beams assembled with galvanised steel brackets and A4 bolts. Photo: Green Zone Design Ltd

Fire

The use of combustible materials in construction has recently come under increased scrutiny; in particular on elevated structures (such as podiums and balconies). The use of timber in such structures is beyond the scope of this article. Advice and approval should be sought when designing for such locations.

Certification

Timber is a resource that requires constant renewal. Certification is not new, but it can easily be overlooked. The premiums once associated with timber certified by the Forest Stewardship Council® (FSC®), and the Programme for Endorsement of Forest Certification (PEFCTM) no longer apply. It is reasonable, ethical and practical to specify timber from certified sources. Some sources of timber, local plantations for instance, may not carry formal certification; where such a resource is available, it is diligent and worthwhile investigating both their provision and management practices. >>

Cumaru seating, part of the legacy plan for the London 2012 Olympic site. Photo: Claire Borley Photography / LDA Design

Simple design principles can be applied that will extend the life in service of timber components:

• protect end grain (rounded, chamfered or preferably capped, end-grain sealant) • avoid unnecessary cross-cuts or notching • use seasoned timber (this reduces splits and cracks) • use heartwood rather than sapwood • prefabricate as much of the structure as possible prior to the application of any treatments • ensure on-site cuts and holes are sealed using end-grain sealant • avoid holes close to the end of beams • decking boards should be placed heartwood side down • use austenitic stainless steel connections (A2/A2 for screws and fasteners) • allow for ventilation and air movement around timbers • avoid timber being in direct ground contact with damp paving, brick or concrete.

Whatever the structure, the design should accommodate periodic inspection, maintenance and replacement, without inconvenient or complex dismantling. The ultimate failure of timber should be anticipated, along with the recommended period and nature of servicing and treatment reapplication. n

1. Reynolds, T. and Suttie, E., (both BRE), and Coggins. C., (Wood Protection Association), External Timber Structures (DG503), BRE, 2007 2. BS EN 8417:2011+A1:2014 Preservation of wood. Code of practice, BSI 3. BS EN 335:2013 Durability of wood and wood-based products. Use classes: definitions, application to solid wood and wood-based products, BSI 4. BS EN 350 Durability of wood and wood-based products.

Testing and classification of the durability to biological agents of wood and wood-based materials, BSI 5. Wood Protection Association (WPA) code of practice,

January 2021: www.thewpa.org.uk/preservative-treatments 6. Sansin WoodForce™ Accel

• The Timber Decking and Cladding Association (TDCA) www.tdca.org.uk • Timber Trade Federation (TTF) www.ttf.co.uk • Wood Protection Association (WPA) www.thewpa.org.uk

• BS 8417:2011+A1:2014 Preservation of wood. Code of practice, BSI • BS EN 350:2016 Durability of wood and wood-based products. Testing and classification of the durability to biological agents of wood and wood-based materials, BSI • Choose & Use: Timber for garden projects, TRADA

Technology, 2012 • WIS 1-31 Timber for landscape architecture, BM TRADA, 2020 • WIS 2/3-1 Finishes for external timber, BM TRADA, 2020 • WIS 2/3-60 Specifying timber exposed to weathering,

BM TRADA, 2021 • WIS 2/3-63 Modified wood products, BM TRADA, 2021 • WIS 2/3-71 Specifying externally exposed structural timber,

BM TRADA, 2019 • WIS 4-28 Durability by design, BM TRADA, 2019 • Wood Protection Association (WPA) code of practice

January 2021: www.thewpa.org.uk/preservative-treatments • www.trada.co.uk/media/12056/timber-durability-chart.pdf