Before Your Timber Decking Fails

Decking Installation Tips

I still see problems with how timber decking is installed in my job as the company’s trouble shooter and I feel compelled to raise the subject again in this month’s Bulletin.

A major factor that has influenced this situation is the trend for wide decking boards. A natural consequence associated with the greater use of wide decking is a higher rate of board movement, that is, shrinkage/ expansion, depending on the time of year. It is also fashionable to use timbers of Australian origin – the likes of Spotted Gum, lronbark, Silvertop, or Ash. Unfortunately, compared to Merbau, they have relatively high shrinkage/ expansion rates.

This means the installation fundamentals, as I state below, are more important for the long-term performance of the timber decking product, than a product with a low shrinkage rate such as Merbau.

Timber Decking Gaps

You should plan to leave a sufficient gap that will allow the boards to move, particularly in winter, so they will not come together. A consequence of this occurrence is severe cupping. Keep the following in mind when planning your deck;

• Boards of 130mm in width will expand by about half a millimetre for every 1% increase in moisture content during winter. Usually the growth in moisture content is about 4% to 6%, so the increase in board width can be about 2mm to 3mm
• The adjacent board is experiencing the same conditions, hence the need for a gap to allow for this movement
• Species that have a relatively high content of the lighter coloured sapwood, particularly Spotted Gum, will have boards that will expand more than others.

As a rule, I suggest a 7mm gap for 130mm wide boards and a 5mm gap for boards up to 90mm wide.

Decking Board Preparation

I believe an oil-based preservative (I suggest Cutek®) should be applied on all surfaces prior to installation, including the trimmed ends. This process acts as a sealer and helps slow down moisture loss or gain. Applying the preservative to the cut ends is particularly important because that is where most moisture is gained or lost in the decking product.

A second coat should be applied after installation to the exposed surfaces. At that stage a coloured tint can be added if desired. This process gives the decking more stability and should be undertaken even if a tinted appearance is not required. Cutek® for instance, will slowly diffuse into the wood and “grey off” in time and will be not seen on the surface of the boards. A common mistake I see is the first coat, usually a decking stain, is applied to the exposed boards after installation. This means the underneath surfaces can lose or gain moisture at a quicker rate than the top surfaces, accelerating the possibility of board distortion.

Deck Ventilation

Timber decking is no different to flooring – it must have fresh air underneath the deck. Many people mistakenly think the gaps between the boards will service the deck. It does not work that way. Allowance should be made to ensure air can easily service the sub-deck cavity. A problem I often see is the deck perimeters are a concrete slab or a swimming pool. This means a cross flow of ventilation is not possible and the possibility of mechanical ventilation should be investigated. The usual consequence of no or lack of ventilation is a damp sub-deck microclimate, causing the bottom of the boards to gain moisture and unsightly cupping.

Install a Membrane Over the Joists

Another important facet of timber decking construction overlooked is the membrane installed over the joists. This process not only helps to stop dry rot, but also means the wood remains dry and the joists will hold nails and screws in place. Although treated pine will not rot, being a softwood, it will become too soft and consequently make it too easy for the fixing pins to move when the decking experiences climatic changes and naturally expands or contracts. These days custom U-shaped products are on the market, or just use plain malthoid.

Type of Fixing Pins that Should be Used

Always use a properly designed timber decking nail or screw. If using a nail, it should be at least 65mm long for a 19mm thick decking product. Unfortunately, I believe many of the decking screws on the market to be an inadequate gauge, particularly when the joist is a treated softwood or a laminated LVL product. In my opinion the screw gauge should be at least 10g, or for 14g for a wide board. The fixing pins should be hot dipped galvanised or stainless steel, particularly if the deck is close to salt water.

If you have any queries, please do not hesitate to contact me via email at [email protected].