Building and Pest Inspection - Decks Talk

Building and Pest Inspection - Keep timber crear of the ground

Deck On ground

Our frequent observations as building inspectors has prompted us to write this blog on timber decks.

If you currently have or are building a timber deck on or near the ground (less than 400mm above the ground) there are a number of important things to consider if you wish to avoid a short deck life caused by timber pest damage.

Where your deck contains any timber in the posts, bearers, joists or decking boards, the minimum level of treatment is H5 for structural members and H4 for non-structural members where the member is in ground contact or within 150mm of the ground. Most timber sold for deck construction is treated to H3 which is only suitable for weather exposed locations that are well ventilated and well clear of the ground. Where good ventilation, a moisture barrier and a termite barrier has installed a minimum of 150mm may be used otherwise a 400mm ground clearance is required.

The reason for this requirement is that timber pests such as fungal decay and termites will occur in timber that is constantly damp. Timber that is in contact with the ground will absorb moisture quickly. Furthermore, timber that is not in contact but close to the ground will also absorb moisture although the moisture content diminishes as the distance of the timber member from the ground increases. This phenomenon is referred to as a moisture gradient and is why a minimum ground clearance is important.

Unfortunately, there are many cases where this requirement is ignored by both builders and home owners who build decks themselves. Basically, H3 treated should only be used in a location that is more than 150mm from the ground and completely dries out after rain.

At ABIS our inspectors see incorrectly constructed decks ever day whilst conducting Pre Purchase Building and Pest Inspections so the problem is very common.

Here are three basic choices you can make to avoid problems and greatly increase the life of your decks:-

  1. Build the deck using materials that do not contain timber elements.
  2. Lower the ground level beneath the deck to maintain a minimum ground to timber clearance of 150mm and install a moisture and termite barrier.
  3. Build the deck using H4 and H5 treated timber products.
  4. A combination of the above.

If you are lowering the ground beneath a deck, make sure the area is adequately drained.

When building a deck that abuts the house, ensure that there is a minimum gap of 40mm between the deck members and the house exterior wall or house slab edge.