Designing for longevity: getting better life out of your wood

Naturally eroded end grain of a mahogany log... ain't she beautiful?

Two unavoidable factors that will impact upon wood in an exterior application are sun and rain. Why is it so important to consider these factors from the outset, and what can you do to minimise their impact?

We love the sun for generating solar energy, warming our bodies, growing trees and drying clothes. We love the rain for feeding food crops, watering plants and filling up dams for drinking. Both the sun and the rain are essential life giving forces on our planet. But when it comes to timber, the sun and the rain can be a powerful factor in shortening the potential lifespan of our timber structure.

Wood that is kept dry and away from sunlight in an indoor application will last indefinitely. But what happens when wood is exposed to the sun’s rays and moisture from the rain? In simple terms, the surface of the wood starts to break down and decay, and the wood begins to split, rot, warp, lose its rich colouring and literally fade to grey.

Decay and degradation in timber is caused by a variety of factors: attack from fungi and bacteria, attack from the sun’s UV rays and from moisture, and biological attack including termites and borers. We call these ‘biological hazards’. Timber is usually rated against its ability to withstand these biological hazards, and it is wise to explore timber durability and strength gradings before you start building to choose the best timber for the right application to maximise the life of your structure (see end section for references). One of if not the best timber to use in Indonesia for external applications is Ulin (Kalimantan ironwood -Eusideroxylon zwageri) as it is highly resistant and durable.

Solar degradation or weathering occurs when UV light from the sun causes chemical changes in the wood cells and breaks down the lignum and cellulose in timber making it brittle, whilst also dying out and shrinking the surface of timber in comparison to its inner section, causing the timber to split. The best way to protect against this is by simply ensuring that you use an exterior grade timber with an interlocking grain to reduce splitting, and a dense oily consistency to resist drying out. Another essential consideration is to build out solar attack by designing your structure so that east and west timber walls are shaded from setting sun and that all other timber elements are shaded from above. Adequate eaves can greatly increase the life span of your timber.

Rot occurs when timber is continuously exposed to excess water causing slow decay, with rot, mould and fungus slowly but forcibly degrading the timber. To protect against rot, make sure timber is kept free from direct ground contact. Use an exterior grade timber in exterior applications as per above. And make sure end timber is covered from above. Most water intrusion occurs at the ends of sections of timber or in joints where water can pond. The best builders paint inside their joints prior to assembly as well as designing joints so water can naturally drain from them.

A common building technique in Indonesia is to cut large, flat mortise joints into post and beam construction (see pic below). This is a big fat no-no in the wrong application such as a two story structure as besides weakening the timber, the joints are rarely snug-fitting hence water can gather in the mortises and causes decay.

 Two story Ulin post and beam construction showing mortise join which weakens the wood structure - a big fat no-no

Two story Ulin post and beam construction showing mortise join which weakens the wood structure - a big fat no-no

It is much better to attach beams to posts using galvanized screws and bolts, using two per join on a diagonal (much stronger than one and better earthquake resistance - see pics above). Another builders trick, is to cut the corners off the ends of timber sections to reduce the direct end grain surface. This is why you often see posts chamfered around the top, or rafters cut off at an angle towards the end.

 Showing beam bolted to post using screws, the correct way to attach beams

Showing beam bolted to post using screws, the correct way to attach beams

For both rot and weathering prevention, you can apply a finish such as a paint, oil or a stain. I won’t go into great detail here as timber finishing is a huge topic that will be covered in an upcoming edition. In brief, the best protection against weathering is really pigment as it effectively blocks the ultra violet rays from the sun. This is why when you remove the paint from a 100 year old timber house, the timber underneath looks new. However, it is not particularly practical for wood lovers as the downside is you can’t see the beautiful properties of the wood. Another option is a permeable finish such as wood oil. The benefits of using wood oil is that unlike a polyurethane finish, it penetrates into the wood cells and expands and contracts as the wood swells and shrinks in response to changes in the weather. The downside is it doesn’t provide lasting protection and you will need to re-apply the oil every year or so. We recommend a high-quality natural permeable oil-based sealer such as Tung oil which can be bought in Bali at Little Tree (http://www.littletreebali.com/).

Remember it’s the combination of water and timber that cause most trouble, so design your house so the timber stays nice and dry and preferably out of the sun.

More info on timber durability and strength grading:

http://www.timber.net.au/index.php/outdoor-timber-naturally-durable.html
http://www.timber.net.au/images/downloads/exterior/timber_users_guide_01.pdf
http://www.woodsolutions.nl/test/beeld/beeld-engels/PDF-woodspecies/AS%205604-2005%20Timber%20-%20Natural%20durability%20ratings.pdf

Thanks to Dave Hodgkin and Sam Shultz for input into this article.