When building an outdoor decking area, it’s common to think that problems may arise because of the type of timber being used. However, it is usually in the setup or lack of planning that often is the problem.
1. Deck Gap
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, boards of 130 millimetres in width will expand by about half a millimetre for every 1% increase in moisture content that the boards will take up during winter. Usually the increase (in moisture content) is about 4% to 6%, so the increase in board width can be about 2 to 3 millimetres.
As a rule, have a gap allowance when planning your deck; board width 130 millimetres at least 6 to 7 millimetres, boards up to 90 millimetres in width allow 5 millimetres gap.
2. Board Preparation
Apply an oil-based preservative to 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.
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. The usual consequence of a lack of or no ventilation is damp sub-deck micro climate, causing the bottom of the boards to gain moisture and unsightly cupping.
4. Install A Membrane Over the Joists
Another important facet of decking construction overlooked. This process not only helps to stop dry rot, but also means the wood remains dry and the joists will hold fixing nails/screws in place. Although treated pine will not rot, being a softwood 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.
Drill a pilot or countersink hole, then drill the holes to facilitate the decking screws. This will help to harness the stress the decking will experience when there are the usual seasonal movement changes in the overall deck. These 2 operations should prevent the boards from cracking or splitting. If using a decking nail ensure the board ends are pre drilled as well.
6. Type of Fixing Pin That Should Be Used
Always use a properly designed decking nail or screw. If using a nail, it should be at least 65mm long for a 19mm thick decking product. The screw gauge should be at least 10g or for a wide board, 14g. The fixing pins should be hot dipped galvanised or stainless steel particularly if the deck is close to salt water.