Bowens - The Builders Choice

Building Advice
Frank DiStefano

Fight Off Moisture With Ventilation

The relationship between moisture and timber can cause a variety of issues in the building and construction industry, and this is magnified during rainy periods. However, it is often a result of poor ventilation under decking and flooring. Regular readers will know this is not the first time we have raised the issue of ventilation for both flooring and decking.  So why is it so important?

Why should I provide ventilation?

Ventilation is allowing for an adequate crossflow of air to an enclosed space. Importantly it allows the subdeck space to dry as air enters and exits across the ground. When high humidity subdeck spaces can only dry from the top surface this will cause cupping of the boards as the underside of the decking remains wetter than the top surface.

Some modern deck designs can push the performance of timber decking boards beyond their limits. These are mostly wider board decks with minimal gap spacing and are often very low to the ground with poor drainage. These decks are further starved of ventilation when the deck sides and ends are fully enclosed with baseboards.

What are the BCA requirements?

The Building Code of Australia (BCA, vol. 2) requires the subfloor area between a suspended floor and the ground to be ventilated. This requirement is the same for decks. Homes constructed on stumps, bearers and joists immediately produce a subfloor space that allows for ventilation, including the attached deck areas which offer the same opportunity for ventilation. However, this is not always the case for homes built on concrete slabs.

For subfloor ventilation, these are the general requirements;

  • The minimum ground clearance should be 400mm
  • If termite resistant timbers are used the minimum clearance can be 150mm from the underside of the lowest horizontal timber in the sub floor
  • Ventilation openings need to provide a minimum 6000 mm2 per metre of wall. This is achieved with subfloor vents or baseboards with gap spacings
  • If an impervious membrane is laid to cover the subfloor ground it can be reduced to 3000 mm2
  • Sealing off and enclosing the perimeter of decks with no ventilation is not acceptable.

What about the timber boards?

All timber will expand and contract in relation to its environment. Most timber species have a similar rate of movement though wider 135mm boards will expand more than narrower 86mm boards. Catering for this movement is good building practice. The reason we leave a gap between decking boards is to allow for the seasonal expansion while still allowing for water to freely drain between the boards. Our decking installation guidelines recommends a minimum 4mm gap for 90mm boards and 6mm minimum for boards up to 140mm.

When the gap is insufficient the boards can swell to the point where they are touching with no remaining gap. This compounds the problems when water pools on the surface, increases the timber moisture content and further restricts ventilation. This pressure between the boards can cause decking screws to shear and boards to tent upwards. Longer-term issues may involve mould and decay from the underside of the boards.

Consider the design of the deck

Timber decks designed and constructed low to the ground can often result in poor performance from the lack of ventilation beneath the decking. The underside of the boards easily absorb moisture from the ground that can’t dry. It is important in these designs to consider factors like having the decking pre-coated on all sides to minimise moisture uptake. Grade the ground so the fall is away from the deck area and surrounding buildings. Include additional drainage to divert water or conceal the ground beneath the deck by laying down a plastic membrane covered with gravel or sand. This helps to keep the soil moisture from affecting the underside of the boards.

Consider these major factors when designing a deck or installing flooring, so you can enjoy a finished product that will withstand the test of time.

If you enjoyed this article, you might like to read How to Build a Timber Deck that Lasts for a step-by-step guide to building a deck that will look good year-round.

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