Laying decking

Choose the decking material according to the conditions: pressure-treated softwood, hardwood or composite. The choice of material determines durability, shrinkage and fixing requirements. An outdoor deck is in service class 3 (AK3), where the moisture content can be persistently high.

Ensure a stable, load-bearing and free-draining base under the joists so the deck does not settle and water can get away. Standing water under the deck keeps the timber damp and shortens its life.

Lay bearers/joists with a slight fall, so rainwater runs off, and with the bearer spacing (c/c) the material requires — thin or soft boards require closer bearers. The bearer spacing follows the supplier's instructions; do not lay it by estimate.

Outdoors (AK3) the fixings must be corrosion-resistant — stainless grade (e.g. 1.4404) is recommended/required, particularly together with pressure-treated timber, whose salts attack ordinary galvanised steel. Follow the bracket/screw supplier's material instructions for the service class.

Ordinary galvanised fixings rust in pressure-treated timber — use stainless steel outdoors.

Lay the boards with a joint gap that suits the material's shrinkage and moisture movement — timber moves with the seasons, so a deck that is too tight can buckle, and one that is too open can become uncomfortable. The specific gap follows the supplier's instructions for the product and its moisture content when laid.

Pre-drill, especially in hardwood and near board ends, to avoid the timber splitting at the screw. Place the screws uniformly so the deck has a calm, regular look.

Make sure there is air under the deck so the timber can dry from both sides. Good ventilation keeps the moisture content down and reduces the risk of rot and fungus (risk above approx. 18–20 % MC with persistent damp).