How to lay composite decking

How do you lay composite decking?

Start from a straight edge and work across the frame, fixing each board to every joist. Layout decides everything after the first board, so use a string line. A UK installer starts at the house or garden wall and works outwards, setting a small packer between the first board and the wall for a breathing gap, then removing it once the board is clipped down. He pulls a string line along the boards every third board or so, so a small bow is corrected early rather than growing across the deck.

What should the frame look like?

Close joists, airflow, a slight fall and no direct contact with a flat surface. The makers’ numbers differ:

Double up joists wherever two boards meet, because each board end needs support. For the build itself, see the frame guide. One UK builder adds noggins between joists to stop twisting, treats cut ends of timber with preservative and lays a weed membrane underneath so weeds don’t push through the gaps.

What gaps should I leave?

Clip systems usually set the side gap, and some makers don’t state one. End gaps and gaps to walls depend on the maker and the temperature.

Ecoscape and Eurocell also publish tables of gap against board length and installation temperature. Boards laid in the cold will expand into the gap in summer, so a winter installation needs more room. One installer says hot boards are longer: keep them in the shade or wet them down before fixing on a hot day.

How do you fit hidden fasteners?

Fix a starter clip along a chalk line, hook each board into the clip, push it home with a rubber mallet and screw the clip to the joist. EnviroBuild’s range has metal starter fasteners for the first and last boards (one box of 100 per 25 m²), 3 mm stainless clips and 6 mm nylon clips; the 6 mm clip lets you release a single board with a screwdriver. Composite Prime puts one Universal Clip and screw on every joist. Eva-Last says grooved boards take clips for a standard 5 mm gap and square-edge boards need spacers.

Don’t overtighten the clip screws. One installer says to stop at the first firm resistance, because overdriving cracks the clip, and to check that every screw stands upright, since a leaning screw means a clip was knocked.

For face fixing, Millboard’s rules are the most specific we found: two screws at each joist crossing, three at board ends, 20 to 25 mm from the end and 20 mm from the side, with at least 20 mm of end support. The screws guide covers grades and sizes.

How do you cut composite decking?

With a sharp, fine-toothed blade. Millboard says use a carbon-tipped blade and place every cut over a joist. Composite Prime says saw with the face up using a new fine-toothed blade. One UK installer leaves boards long past the frame edge and trims them in one pass with a plunge saw against a guide for a clean straight edge, and cuts round downpipes and posts with a jigsaw after scribing the shape. Trim the factory edge too: another installer says factory edges are rarely dead straight and the error adds up.

Seal or coat visible cut ends where the maker says to; Millboard requires touch-up coating on cut edging profiles. Hollow boards need end caps on exposed ends to stop moisture entering the chambers, according to EnviroBuild.

How do you join boards?

Butt joints go over double joists, with a gap between the two board ends. Stagger them like brickwork. Composite Prime joins only over double joists and allows a board overhang of 25 mm beyond the last joist, against Guardian’s 10 mm and Millboard’s 50 mm. A US installer’s tip: don’t make joint sections too short, so each short length is held by enough joists.

Do you need a professional?

Not for a low, simple deck. Composite Prime recommends a professional installer for elevated decks above 600 mm. In every case the warranty depends on following the maker’s guide, which is a good reason to ask any installer which guide they use.