Designing durable timber structures

The effects of moisture on timber can be challenging. Andrew Lawrence discusses how wood differs from steel and concrete and, therefore, needs a different approach to design.

The effects of water can affect the strength of the wood. It can also cause it to shrink and swell, which then affects the detailing of connections. Finally, but most importantly, moisture can cause wood to decay. This article focuses on ways of preventing decay through design.

Along with BM TRADA’s timber inspection team, I have, throughout my career, witnessed degradation of timber in buildings and other structures, but I really should emphasise that this is not a problem with the material itself, rather a lack of understanding about its properties. This is because many of those switching to using timber in construction are more familiar with design detailing intended for concrete or steel. So, the problem is more to do with a lack of understanding about timber as a construction material than with the material itself.

Understanding how to avoid timber decay

My own personal understanding of how to avoid timber decay has come from the projects I’ve undertaken.

For example, Durham University wanted to convert an old barn into a student bar and student computer room. It was my job, as a very young engineer, to decide what condition that barn roof was in. It was a trussed timber roof and, from what I could see on the surface, it looked structurally sound. However, the ends of the roof trusses were hidden within the brickwork, so I thought it best to check what condition they were in. Having exposed the first timber, I discovered it was so rotten that I was able to push my entire hand right through it. So, on my very first timber project, I discovered a key fact. If exposed to the air, timber is ventilated and able to dry. However, as soon as you enclose it in other materials such as brick, if it becomes wet it will stay wet and be likely to rot. >>

The good news is that wood needs water to rot. So, all we must do is keep it dry and it can last for centuries.

Historical lessons in the natural durability of timber

This is demonstrated perfectly by Greensted Church, in Essex. Believed to be the oldest timber building in Europe, the walls of the nave are built using a traditional Saxon method from large split oak tree trunks, protected from rain by deep eaves. Built just before the Norman invasion, this building has survived nearly a thousand years.

So, if designed using the correct detailing, timber is an incredibly durable material and correct detailing means keeping it dry –by lifting it up off the ground, overhanging the roof to shelter the walls, and selecting the timber for its natural durability.

The designers of Greensted Church knew that oak is full of tannic acid. If you slice through an oak log, the heartwood in the middle of the log is darker because the cells are full of tannins and other extractives. Natural waste products from photosynthesis, these tannins are also toxic to fungus, making this timber naturally resistant to rot. However, while tannin is an excellent natural preservative, it will eventually wash out.

International lessons in timber preservation

To better understand how to protect timber from decay using good design, I was lucky to receive an award from The Institution of Structural Engineers (IStructE) to travel to see how timber bridges had been built in the USA, Norway and Germany.

I visited many traditional timber bridges, including the famed covered bridge in Iowa that is highly recognisable from the film The Bridges of Madison County. I came to realise that, like many bridges across North America and Central Europe, this iconic bridge is covered not to keep besotted tourists dry, but to keep that timber structure inside dry and prevent it from rotting. The wooden cladding probably leaks, of course, but the entire structure is so well ventilated it can survive a bit of water coming through.

A very similar principle is used in timber framed buildings, where tiles may occasionally slip and allow a small amount of water in, but as long as the roof space remains well ventilated all should be well. This is a point that is often forgotten when loft rooms are built within a roof space.

Treating softwood timbers for increased durability

During the 19th Century, people were looking for ways to build that no longer relied upon less-available, naturally durable timbers or expensive roofs. Instead, they sought ways to make plentiful softwoods from the Baltic regions more durable by using chemical protection.

In London, John Howard Kyan discovered a process for preserving wood which involved dipping logs in a solution of mercury (II) chloride (mercury bichloride). This ‘kyanising’ process was, of course, highly toxic but extremely effective. Produced by mixing hydrochloric acid with a hot, concentrated mercury (I) nitrate, the solution was a common over-the-counter disinfectant at the time, used to treat everything from measles to ulcers.

Around the same time, another treatment was developed from a common antiseptic cream derived from the distillation of various tars and pyrolysis of organic matter. Known as the ‘preserver of flesh’, before being recognised as carcinogenic, the mixture was used for a variety of applications from stabilising smoked meat, to burning off malignant skin tissue and preventing necrosis in dentistry. Pronounced κρέας (kreas) ‘meat’, and σωτήρ (sōtēr) ‘preserver’ in Greek, this treatment later become known as creosote. It is still used to help protect timber bridges in North America. Here in the UK, we use it to protect ground contact external timbers such as telegraph poles. Creosote can bring the service life of these timber structures to around 50 years or so.

In Norway, the road administration wanted to achieve 100 years of service life for their bridges. So, they developed a combination of four complementary measures.

1. First, before the individual planks were glued together they were treated with a careful combination of copper, chrome and arsenic in a pressure vessel. Although this is now restricted, it was a common preservative at the time. However, like all pressure treatments it only penetrates the sapwood even of permeable timbers, and therefore it needs to be combined with other measures.

2. Secondly, once the planks had been glued together into finished glulam, they were pressure treated with creosote. This gave them a water-resistant shell that prevented the wood from getting wet or drying inconsistently and fissuring.

3. Thirdly, copper caps were used to protect flat surfaces from standing water.

4. Lastly, they were very careful with connection details which would include steel plates in a slot that was open at the bottom allowing water to drain away quickly.

This four-pronged approach is relatively successful but is challenged in the 21st Century because the creosote is not chemically fixed into the wood. This means it will slowly leach into the environment, impacting the health of humans, wildlife and the biosphere. The use of copper chrome arsenic is also now heavily restricted, partly because of the challenges of disposal.

Softwood timber protection by design

Germany was the first country to move away from the use of wood preservatives. They came up with a simple twist on traditional covered bridges, which was to use the deck as an overhanging roof to keep the timber dry. Using this approach, they used completely untreated softwood glulam as an underbridge support structure. However, an issue arose with this design when water leaked through the joints of the deck. This could have led to serious structural damage within a matter of months if the leak had not been repaired. But this was a very useful learning experience.

A single line of defence is rarely enough.

Later German bridge design uses what we call a passive leak detection system, for example a pre-cast concrete deck with gutters and drainage pipes that catch and divert the water coming through the gaps between the planks. The system lets the bridge maintenance team know when there’s a leak and where it is, allowing them to quickly resolve and repair the problem >>