Composite decking

Composite boards move with temperature. A 3.6 metre board can grow several millimetres between a frosty morning and a hot afternoon, so end gaps are set by the manufacturer, commonly 5mm to 8mm at 20 degrees and wider in cold weather. Hidden clips set the side gap and let each board slide, whereas face screwing composite down tight is how buckling begins. Butt joints want a doubled joist or a full width bearer beneath, never a single joist shared by two board ends.

Sub-frames for composite are tighter than for timber. Joist centres drop to 300mm for most solid boards and 250mm where boards run at 45 degrees, because composite flexes more than softwood and sags between widely spaced supports. Aluminium sub-frames cost more and move less; treated timber is the usual choice and wants the same clearance and membrane isolation as any deck frame. Manufacturer warranties are conditional on that spacing, and a frame built at 400mm centres voids a great many of them.

Ground preparation is unchanged by the board above it. Vegetation and topsoil come off, a weed membrane goes down, gravel spreads over it and pads or posts get set level. Water has to get away from beneath the deck, so a frame in a hollow needs the ground graded or a land drain out to a border. Capping is the remaining decision: capped boards carry a bonded polymer shell resisting stains and fading, cost £20 to £25 per square metre more, and uncapped composite in British weather looks tired inside five years.