How to lay composite decking

What goes under composite decking?

A level, ventilated substructure of treated timber or aluminium joists sitting on solid bearers. The frame does the work; the boards are only a wearing surface.

Leave at least 25mm of clear air under the boards, open at both ends so it can breathe. Trapped damp is what kills a deck, and it takes the frame long before it takes the boards. On soft ground use paving slabs or concrete pads under the bearers. Composite is heavier than timber decking, so do not skimp on the frame to save money.

What joist centres does composite decking need?

400mm maximum, centre to centre, for boards laid square to the joists. Drop to 300mm for a 45 degree diagonal lay.

A diagonal board spans further between supports, which is why it needs tighter centres. Go to 300mm or less under a hot tub, a raised planter or anything else heavy, and have the frame designed properly if the load is real. Double up the joist wherever two board ends meet, so each end gets its own fixing.

How big do the expansion gaps need to be?

5 to 6mm between boards side to side, 6mm at butt ends on a 3.6m board, and 10mm minimum to any wall or fixed object.

Composite moves with temperature. Boards laid tight in February will buckle in July. If you are working in cold weather, err on the wider end of every gap. Set the fall while you are framing: 5 to 10mm per metre away from the house sheds standing water. Check the maker’s sheet, because the figure varies between boards.

Composite decking spacings and quantities

Item	Figure	Note
Joist centres, boards laid square	400mm maximum	Centre to centre
Joist centres, boards laid at 45 degrees	300mm maximum	A diagonal board spans further
Joist centres under heavy load	300mm or less	Hot tubs need a designed frame
Gap between boards, side to side	5 to 6mm	The clip sets this for you
Gap at a butt joint	6mm per 3.6m board	Wider if laying in cold weather
Gap to a wall or fixed object	10mm minimum	All four sides
Ventilation under the boards	25mm minimum, open both ends	Trapped damp destroys the frame
Fall away from the building	5 to 10mm per metre	Built into the frame, not the boards
Overhang past the last joist	25mm maximum	Unsupported ends flex
Clips needed	1 per board at every joist	Plus starter clips along the first edge
Board length	3.6m	Plan joints before ordering
Wastage to order	5 per cent square, 10 to 15 per cent diagonal	Higher again on a cut-up shape

Hidden clips or face fixing?

Hidden clips through the field of the deck, face fixing for the first and last boards and anywhere a clip cannot reach.

A clip sits in the groove on the board edge and screws down into the joist, setting the side gap as it goes. Nothing shows and there is no screw head for water to sit around. You need one clip per board at every joist it crosses, plus starter clips along the first edge, so work the count from your joist plan. Face fixing means pre-drilling and countersinking every hole, at least 5mm in from the board edge, then screwing into the joist. It is slower and you can see it, but it is the right call at the perimeter.

How do I cut composite decking?

A fine-tooth circular saw blade cuts it cleanly. A 40-tooth negative-rake blade is ideal; a general timber blade chips the edge.

Support the board both sides of the cut so it cannot pinch the blade. Cut face up with a circular saw, face down with a hand saw. The dust is a nuisance, so wear a mask and goggles and work downwind. Cut ends are unfinished and read paler than the factory edge, which is another reason to hide them under trim.

How do I finish the edges?

Corner trim over the exposed cut ends and the outside face of the perimeter board. It comes 3.6m long in 40 x 40mm.

The trim covers the hollow board ends and the frame timber in one pass. Pin or screw it through the face into the joist end or a batten behind, using stainless colour-head fixings so nothing rusts down the board. Mitre outside corners at 45 degrees. Leave the same expansion gap behind the trim that you left everywhere else.

How much decking do I order?

Deck area divided by board coverage, plus 5 per cent for a square lay or 10 to 15 per cent for a diagonal or a cut-up shape.

Boards are 3.6m long. Work out how many full lengths cover each row rather than working from area alone. A 4m deck needs two boards per row and wastes 3.2m unless you plan the joints onto doubled joists. Check the covering width on the product page and count the rows. If the deck is an odd shape, has steps or wraps a corner, bring the measurements in and we will work it through with you.