Bowens - The Builders Choice

Building Advice
Frank DiStefano

Q: I installed a deck in May this year and had to return soon after to fix it as the decking boards had expanded. The decking is 135×19 Spotted Gum and I believe should not have moved as much as it has. The decking is an extension to the rear of an older home that covered a lawn area. B Nelson, Hampton.

A: After viewing the many photos, I can tell you it’s not the timbers fault. There is no ventilation opportunity in this deck and this is the disastrous result. All the sides of the decking have been enclosed with more decking as baseboards with similar narrow gaps. The decking is also very low to the ground, approx. 200mm from the underside of the decking. There is expansion of the decking boards, tenting of boards, broken fixings, cupping and mould growth on the underside.

Rainy days quickly increase the moisture content in this limited subdeck. When ventilation is restricted the subdeck space cannot dry and leads to decking expansion as it takes up moisture. Subdeck ventilation is having a good cross flow of air beneath the decking that runs from one side to the other always offering fresh air.

A 3mm gap spacing for 135×19 mm decking is insufficient. The gaps between decking boards are there to allow for the natural expansion of the timber from rainy periods. Our Decking Guidelines call for wide decking boards up to 140mm to have a gap spacing of a minimum of 6mm.

For decking up to 90mm wide a minimum 4mm gap spacing is required. If 135×19 decking is low to the ground or will experience poor ventilation then opening up the gap spacing between boards to 8mm is recommended.

These decking boards have expanded an additional 2-3mm each. This rate of expansion, indicates that the moisture content of the decking is extremely high. It’s not uncommon to find moisture content readings of over 20% in these situations.

The problem is compounded when the decking actually starts to hold water on the surface when the gaps have closed up. Even though the surface is able to dry, the underside will remain wet for longer from trapped moisture. This is what causes cupping in decking.

Removing soil beneath the decking does create more space but not ventilation. It also can create a low spot for water to pool in. Drainage must also be installed to direct water away from beneath the decking.

It’s not uncommon to find moulds starting to grow when water is trapped under the deck. If the deck is low to the ground and has limited ventilation then I would suggest the following to help limit any future problems.

  • The decking boards should be reduced to a narrower 70mm or 90mm width to reduce the expansion rate.
  • Use a durable and stable decking species like Merbau or consider Composite decking.
  • The decking should be pre coated on all sides including the bottom before installing.
  • The gap spacing should be increased.
  • A 200um black plastic can be used to cover the ground beneath the decking to prevent water pooling. The ground must slope away from the home.
  • Use a waterproof membrane between the joist and decking.
  • Use minimum 10g x 65mm decking screws.
  • Maintain finishing protective coatings as required.

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