Expert Advice
Frank DiStefano

The Importance of Decking Ventilation

Over the years my colleague Jeff Harvey and I have written many articles on the fundamental need for adequate ventilation to sustain a long life for timber decks.

We still come across decks that are damaged due to a lack of ventilation.

The principal aim of ventilation is to achieve a sub-floor and deck micro-climate that is similar to the outside climate. For this to be achieved, fresh air must have an entry point and a corresponding exit point, creating a corridor for fresh air that keeps the subfloor and deck space relatively dry. What happens if this is not achieved?

The Result of Poor Decking Ventilation

Image: Expanded decking buckling and lifting 

When a structure does not receive enough ventilation, it becomes humid and allows mould to grow and the timber to decay. Humidity is the main driver in the rise of the moisture content in timber flooring and decking products.

As wood is hygroscopic, timber decking and flooring will absorb available moisture from the ground. Lack of ventilation allows only the top of the boards to dry and this difference in moisture content between the top of the boards will result in cupping.

In a recent deck inspection, Jeff and I discovered a 135×19 Spotted Gum deck, built only 6 years ago, had cupped, and splintered in three main areas. Why? Here are the reasons.

The Enclosed Area

The small area at the entrance of the house was completely enclosed by the base boards providing little to no ventilation to the sub deck area. This area was surrounded by a garden, making it the perfect situation for moisture build-up.

Lining Boards to First Floor Balcony

Decking to first floor balconies are often well ventilated due to the nature of the construction, however, in this case, the structure was lined with lining boards. This prevented the opportunity for air to flow to the underside of the decking.

The Sub-Deck Space

The largest area of decking affected was surrounded by the dwelling and a swimming pool. The natural ground level at this point provides room for head clearance offering plenty of opportunity for ventilation. The issues? It failed to provide adequate cross flow ventilation.

Fresh air must have an entry point and an exit point to provide a cross flow effect. In this case, the only access for ventilation was through one open side of the deck.

Installation of Decking and Flooring

While inspecting this deck for damage, we noted that the builder had correctly applied some of the things we recommend during construction.

Our measurements showed the builders had allowed a 6-millimetre gap, which is what our guidelines for these boards suggest. Also, a decking membrane was applied between the treated pine joists and the decking. This is to keep the moisture content of the decking joists low and to ensure fixing screws retain their holding power and cannot move easily.

Most Australian decking species will encounter issues if not correctly installed. We highly recommend you follow the installation guidelines to avoid post-installation problems.

Other Decking Issues to Consider

  • High fences adjacent to and enclosing the deck. Consider seeking advice about mechanical ventilation
  • Lack of passive vents in subfloors. Flooring regulations dictate the size and spacings. Plinth boards surrounding floor and deck subfloors should have sufficient gap spacing to allow airflow under the deck
  • Vents placed in the wrong position. Vents placed hard up against sub-floor bearers limit airflow. They should be installed below the bearer.

Ensuring your subfloor have good cross flow ventilation, can be the difference between boards that need to be replaced often or a timber deck that will stay looking new for a lifetime.

If you would like further advice, visit www.bowens.com.au/installation-guidelines/ for our flooring and decking installation guidelines.

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