Composite decking clips and fixings: how many you need and how to fit them

Composite decking is fixed with hidden clips, not screws through the face of the board. The clips sit in the groove along each board edge, hold the board down and set the gap to the next one. Get the clip count right and the deck goes together in an afternoon with no visible fixings; get it wrong and you are back online mid-build. This guide explains the three clip types, how many you need for any deck, and the screws to use with them.

The short answer

One clip per board per joist. Count your boards, multiply by the number of joists they cross, add 10%. A 3.6m by 3m deck (20 boards over 11 joists at 350mm centres) needs about 220 clips and 220 screws: two packs of 100 clips at £16.50 plus a spare pack, and the same in screws at £12. About £70 for the whole deck. Never screw through the face of a composite board.

The three clips

• Starter clips. Screwed to the outer joist along the first edge of the deck. They hold the outside edge of the first board where there is no neighbouring board to clip to. One per joist along the starting edge; the same again along the finishing edge if you use them there.
• T-clips (intermediate clips). The main fixing. Each one sits between two boards, engaging the groove on both, and screws down to the joist. One per joist for every board after the first.
• Finishing clips. Steel clips for the last board, where you cannot slide a T-clip in from the side. Alternatively, screw the final board at an angle through the groove every 350mm, without over-tightening.

Our packs are T-clips with matching screws; starter and finishing clips come in the same pack. Every clip is stainless or coated steel so it will not rust-stain the boards.

How many for your deck

Three numbers: how many boards, how many joists each board crosses, and a spare allowance.

1. Boards. Deck width across the boards divided by 150mm (board plus clip gap). A 3m wide deck is 20 boards.
2. Joists crossed. Board run divided by 350mm, plus one. A 3.6m board crosses 11 joists.
3. Clips. Boards times joists: 20 by 11 is 220. Add 10% for the border, the ends and the ones that vanish in the grass.

Deck	Boards	Joists	Clips	Packs (clips and screws)
3.6m by 2.4m	16	11	176 + spares	2 of each
3.6m by 3m	20	11	220 + spares	3 of each
4.8m by 3.6m	24	15	360 + spares	4 of each
4.8m by 4.8m	32	15	480 + spares	6 of each

A picture-frame border adds a double joist on each side and its own clips; add one board's worth of clips per side.

Screws

Use the screws supplied for the clips, one per clip. They are sized for the clip and for a composite or plastic joist without splitting it. Into a timber joist they are fine as they are. Into a plastic joist, pre-drill a 3mm pilot hole; plastic does not compress the way timber does and a screw driven without a pilot can crack it. Countersink the starter clip screws so the first board sits flush.

Fascia boards, L-trim and step edges are screwed through the face because there is no groove to clip into. Pre-drill a 4mm countersunk hole, use a coloured composite screw if you want it to disappear, and do not sink the head below the surface; the board should be able to expand around the screw.

Fitting sequence in brief

1. Starter clips along the outer joist, one per joist, screws countersunk.
2. First board clipped in against the starters.
3. T-clip loosely screwed to each joist, next board offered up, lifted slightly at the edge and pushed home, then the clips tightened once the board is on all of them.
4. Repeat. Check the board is touching every clip along its length before tightening; a board that is up on one clip will creak.
5. Last two boards: measure with a 6mm gap between them, fit finishing clips, drop the final board in, slide the remaining T-clips down and screw.

Expansion gaps at the board ends (3mm, 6mm or 9mm depending on the temperature that day) are set by you, not by the clips. The installation guide has the figures.

Mistakes we see

• Screwing through the face. The board cannot move, so it buckles at the first hot week, and the hole lets water into the hollow core.
• Two boards sharing one clip at a butt joint. Each board end needs its own joist and its own clip; use a double joist at every end.
• Over-tightening. The clip should hold the board down, not crush the groove. Snug, then stop.
• Running out. Order a spare pack. It is £16.50 and it is never wasted.

Boards, clips, screws and trims all ship together, usually within 2 to 4 working days. Reply with your deck size and we will confirm the clip count before you order.