UK Decking Board Sizes & Profiles

The most common softwood decking board size is 32 x 150 mm (finished approximately 28 x 144 mm) in 3.0 m, 3.6 m and 4.8 m lengths. The 32 x 125 mm size is also widely available. These are typically pressure-treated (tanalised) softwood with a grooved or smooth profile on each face.

Leave 5-8 mm gaps between softwood decking boards for drainage and expansion. For composite decking, follow the manufacturer guidance — typically 5-6 mm side-to-side gaps and 3-5 mm end-to-end expansion gaps. Insufficient gaps cause water pooling and rot in timber decks, or buckling in composite decks.

Composite decking (wood-plastic composite or WPC) requires no annual treatment, does not splinter, resists rot and insect damage, and typically carries a 10-25 year warranty. It is more expensive initially (typically 3-5 times the cost of softwood) but lower maintenance over its lifetime. Common brands in the UK include Trex, Millboard, TimberTech and Eva-Last.

For 32 mm softwood decking boards, maximum joist centres are 400 mm. For 28 mm boards (finished thickness), 350 mm centres are recommended. Composite decking varies by manufacturer — typically 300-400 mm centres. At a 45° diagonal lay pattern, reduce joist centres by 25% (e.g. 300 mm instead of 400 mm).