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Composite Decking Calculator — Boards, Joists & Clips
Calculate composite decking boards, joists, clips, starter/end clips and fascia for your deck.
Length parallel to boards
Width across boards
Per linear metre of board
Total exposed edge length
10% typical
Between board edges (5mm typical)
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How We Calculate This
This calculator determines composite decking board quantities, joist counts, fixing clips and fascia trim based on your deck dimensions.
Board quantity
Boards across = ceil(Deck Width ÷ (Board Width + Gap))
Internal joists = ceil(Deck Length ÷ Joist Spacing) + 1
Hidden clips = Deck Area × clips/m² + 10%
The joist figure counts the internal (intermediate) joists running across the deck. It does not include the outer perimeter rim frame or any noggins/blocking at board butt-joints — add those rim members separately. For a full timber subframe with rim joists, use our timber Decking Calculator.
Accessories
Starter clips fix the first board, and end clips the last board, to each joist — so you need one starter clip and one end clip per joist (UK manufacturer guidance: one clip for each and every joist). Expansion gaps of 5mm between board edges are maintained by the hidden clip system.
L-shaped decks
In L-shaped mode the deck is two rectangles: Leg A is the main deck and Leg B carries on past one end of Leg A, covering part of the width and sitting flush with one edge, with the boards running the same way across the whole deck. Each leg uses the formulas above, then the orders are merged. Joists: the junction line runs across the boards, so it is a joist line, and Leg B's subframe butts into Leg A's end joist there. That joist is shared and counted once, so total joists = Leg A joists + Leg B joists minus 1, and it is ordered at the wider leg's span so the junction is always covered. Board metres and hidden clips are calculated per leg, with wastage rounded up per leg, and clips are not reduced at the junction because board ends on each side of the joint still need fixing to the shared joist. Starter clips: the first board row runs the full straight edge of the L, so one starter clip per merged joist. End clips: the L has two finishing edges, one on each leg, and both of those rows cross the shared junction joist, so end clips = Leg A joists + Leg B joists. If you build the legs as fully separate frames, add one joist back.
Frequently Asked Questions
The number of clips depends on joist spacing and board width. At 350mm joist spacing with 140mm boards, you need approximately 20 clips per m². Each board-joist intersection requires one clip. Plus starter clips along the first board and end clips for the last board. We recommend ordering 10% extra.
Most composite decking manufacturers recommend 300-400mm joist centres. Use 300mm for heavy-traffic areas, hot tubs or diagonal board layouts. Standard domestic use is 350mm. Never exceed the manufacturer's maximum spacing — composite boards can flex more than timber under load.
Yes, composite decking expands and contracts with temperature changes. Leave a 5mm gap between board edges (maintained by the clip system) and 10-15mm at each end of the boards against walls or fixed structures. In hot weather, boards can expand by up to 3mm per linear metre.
While possible, composite or aluminium joists are recommended for longevity. Timber joists (treated C16/C24) will deteriorate before composite boards which last 25+ years. If using timber, ensure it's pressure-treated to UC4 hazard class. Aluminium joists are the premium option for a truly maintenance-free deck. For a full timber deck instead, see our Decking Calculator.
Fascia boards cover the exposed ends and edges of the deck frame, giving a neat finished appearance. They also protect the cut ends of boards from moisture ingress. Fascia is measured by the perimeter of exposed deck edges. Corner trims and L-profiles are available for neat joins.
Yes. Choose L-shaped as the deck shape and split the L into two rectangles: Leg A is the main deck and Leg B carries on past one end of Leg A, covering part of the width, with the boards running the same way across both legs. Enter each leg's length (along the boards) and width (across the boards), measured up to the junction without overlapping. Each leg is calculated as its own rectangle, then the orders are merged: the joist line at the junction is shared because Leg B's subframe butts into Leg A's end joist, so one joist is deducted from the count and the shared joist is ordered at the wider leg's span. Board metres and hidden clips are worked out per leg with wastage rounded up per leg, and clips are not reduced at the junction because board ends on each side still need fixing to the shared joist. Starter clips cover the first board row, which runs the full straight edge of the L, one per joist. End clips cover both finishing edges (the outer row on each leg), so the end clip count is one higher than the joist count. For fascia, measure the exposed edges of the finished L: the full outer perimeter of an L-shaped deck equals the perimeter of the rectangle that would enclose it. If you build the two legs as separate frames, add one joist back.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.