How much composite decking do I need?

How do I measure the area for a composite deck?

Measure the finished deck rather than the frame under it. For a rectangle, multiply the length by the width. For an L shape, split it into two rectangles, measure each and add them. Treat steps, a raised section or a picture frame border as separate jobs, because they cut the boards into short pieces and waste more.

Write down which way the boards will run before you work out the sums. Boards run along the length of a row, and rows stack across the other dimension. That choice decides how many boards you cut and how many joints you have, and it also decides which way the joists run, because the joists always go across the boards.

How many boards make a square metre?

About 1.8. The 3.6m composite decking board is 3,600mm long, 150mm wide and 25mm thick. The laying guide leaves 5 to 6mm between boards, so each row takes up about 156mm of width. One board then covers 3.6m x 0.156m, which is roughly 0.56m². Divide the deck area by 0.56 for the number of boards.

That figure counts the gap, which is why it is smaller than 3.6m x 0.15m. Work from 0.56m² per board.

How do I count rows?

Rows across the deck equal the width plus one gap, divided by 156mm, rounded up. The last row is usually ripped narrower to fit. Aim for a last row of reasonable width, which is stronger and looks right. If the sum leaves a sliver, nudge the frame width or plan a slightly wider last board.

Boards for a deck one board long (3.6m), by width

Deck width	Rows	Boards	With 5% added	Clips at about 23 per m²
1.5m	10	10	11	125
2.4m	16	16	17	199
3.0m	20	20	21	249
4.2m	27	27	29	348

Clips are rounded up to a whole number. Clips come as a quantity of 100 or 200, so round up again when you order. The fixings guide, composite decking fixings and clips, shows what each quantity covers.

How much waste do I allow?

The laying guide says 5% for a boards-square lay and 10 to 15% for a diagonal lay or a cut-up shape. Go higher again if the deck has steps, curves or several corners. Waste comes from offcuts that are too short to use, from boards damaged in handling, and from boards you rip to fit the last row.

A deck longer than 3.6m needs joints, which use up board length. A 4m run needs two boards per row and leaves a short piece, which can be reused in a later row if you plan the joints for it.

How do the joists affect the order?

Joists run across the boards, so they set where every clip goes. The laying guide gives 400mm centres for boards laid square, 300mm for a diagonal lay, and a frame designed properly for hot tubs or other heavy loads. The number of joists is the length of the deck divided by the spacing, plus one. Double the joist wherever two board ends meet, so each end has its own fixing. Leave at least 25mm of clear air under the boards, open at both ends, and build in a fall of 5 to 10mm per metre away from the house.

The boards, clips and trim are in composite decking, and we can talk you through the frame at the counter.

What changes with a diagonal lay?

The frame, the waste and the fixings all go up. The laying guide gives joists at 300mm centres for a 45 degree lay instead of 400mm, so you buy more joists and each board crosses more of them, which pushes the clip count towards the top of the 11 to 13 range. Allow 10 to 15% for waste, because every board is cut at an angle and the offcuts are triangles. A diagonal lay also needs trim at every open edge, since the cut ends sit along the perimeter.

What is a worked example for a 4.8m x 3.1m deck?

A deck 4.8m long by 3.1m wide against the back of a house, with the boards running along the 4.8m length. Joists run across, at 400mm centres.

Worked example: a 4.8m x 3.1m deck

Step	Working	Result
Area	4.8 x 3.1	14.88m²
Rows	(3,100 + 6) divided by 156, rounded up	20 rows, last ripped narrower
Board per row	One 3.6m board plus a piece of about 1.2m	Joint on the joist at 3.6m
Boards for the long pieces	One per row	20
Boards for the short pieces	Three 1.2m pieces from each board, 20 needed	7
Boards before waste	20 + 7 (area check: 14.88 divided by 0.56)	27
Boards to order	27 plus 5%, rounded up	29
Joists	(4.8 divided by 0.4) + 1, plus a doubled one at 3.6m	14, each 3.1m
Clips	14.88 x 23 (29 boards x 11 to 13 gives 319 to 377)	343, order 400
Corner trim	Two 3.1m ends plus the 4.8m front, 11.0m in all	4 lengths of 3.6m

The trim comes out at four lengths because each 3.1m end takes one length with 0.5m left over, and the 4.8m front takes a full length plus a 1.2m piece from a fourth. If three sides are open, as here, mitre the two corners. Ask us if the deck wraps a corner or steps down, because the counts change.

What else should I order with the boards?

Clips for every board, trim for the open edges, and anything you need to bond the trim. The clips are a separate product, and the corner trim comes in charcoal or grey. For bonding trim, the catalogue has Everbuild Multi Stick and Soudal Fix All Flexi. Choose board colours from Anthracite Grey, Light grey or Black on the board page. The laying steps are in how to lay composite decking.

Collect free from the Liverpool showroom at Unit 7, The Boxworks, Heysham Road, L30 6UR, Monday to Friday 7:30am to 4:30pm and Saturday 7:30am to 1pm. Delivery is priced by postcode and shown on each product page and at checkout. Bring your plan and we will check the count with you.