Design For Durability

Design Durability

What factors should I be aware of when planning a new project using timber?

Exterior timber if specified and used in the correct way can last for more than 100 years. It is very important that the appropriate timber is incorporated in a suitable application. A number of factors should be taken into account when designing with timber outdoors.

1. Durability

All timber species have different durability against decay, weather, and termites. Durability 1 is the highest and 4 the lowest. In high exposure in ground conditions only a class 1 timber should be used. Above ground exposed class 2 timbers are okay. A class 4 timber should not be used outside unless fully protected from the weather. An example of this is Builders in the 80’s used a lot of Oregon externally which is only a durability class 4 timber. This incorrect specification is now causing home owners a lot of expensive replacement due to decay.

2. Orientation

In Australia the worst orientation for weather is North / Northwest. The high UV and wind and rain from this direction cause the extreme weathering and degrade of timber. Generally it is advised to offer some cover for timber if it must be used in this elevation. This could be via wide eaves, pergolas, veranda or some similar structure to offer protection from the rain and sun. Alternatively a better result will be achieved if your design can incorporate timber on a east or southerly elevation and using an alternate material on north/northwest side.

3. Finish

All exterior timber need some maintenance whether this is a initial once off waterproofing treatment that allows timber to age gracefully to a driftwood grey (low maintenance) or a penetrating oil that will help maintain the timbers natural colour but needs to be applied every few years. In exposed locations, we always recommend a penetrating finish helps to protect timber from premature aging. Once again timber on north/northwest faces will require much more regular maintenance. Also exposed decks or horizontal surfaces require more maintenance than vertical cladding as decks are exposed to more UV and swelling and shrinking from the rain and sun. So in the design phase its best to limit the amount of timber on fully exposed elevations especially if your client is concerned about ongoing maintenance.

4. Design and specification

Only use class 1 timber in ground contact. Class 2 timbers can be used for decking, cladding, and screenboards. When designing with timber outside make sure all structures are engineered to drain well. Water sitting on timber for extended length of time will cause issues. Other issues that should be addressed include:

  • Timber Joins on exposed walls should be minimised.
  • Flashings around windows should allow water to run away
  • Expansion spacing on decking and cladding are essential to allow the natural movement that will occur in timber between summer and winter

If proper planning on how and why timber is to be incorporated in a building project is undertaken at the design stage, it can help significantly prolong the service life of the timber, minimise ongoing maintenance and eliminate any costly refurbishment works down the track. If you need more advice on using timber outdoors, Radial timber has the experts and we are more than happy to provide professional guidance at the design stage.

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