BRANZ 'Guideline December 2012' - ground clearance for deck bearers
A common question is how close can H3.2 deck bearer be to the ground when installed over timber piles?
NZS 3604 Timber-framed buildings requires that the top of a timber or concrete pile must be at least 150 mm above the ground, which determines the clearance between the bottom of the bearer and the ground to be a minimum of 150 mm.
Where bearers are installed closer to the ground using an alternative foundation support detail such as concrete pads and stainless steel brackets, BRANZ considers it prudent to specify a higher level of treatment – H4 where the bearers are close to being clear of the ground or H5 if there is a risk of ground contact. In this situation, fixings and brackets attached to the bearers and piles are regarded as exposed and must be stainless steel.
The decking and joist fixings are required to be stainless steel when in Exposure Zone D and all zones when decking or framing material is treated with AZQ or CuAz treatment chemicals. Galvanised fixings to joists and decking are permitted in Exposure Zones B and C.
The moisture content of the decking at time of installation will determine the required gap between boards. If the decking has been supplied dry, the boards will swell and will require larger gaps. Also, wide boards will move more than narrow boards. The typical gap between 90 mm wide decking boards supplied wet is 3 – 4 mm. If possible, have decking on site a week or so before it is required to allow it to acclimatise to the as-installed conditions.