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Free joiner / carpenter calculator
Decking calculator: boards, joists, posts and screws
Measure the deck, pick your board size and which way the boards run, and get the boards, joists, bearers, posts, postcrete and screws to order. It uses the TDCA code of practice for board gaps, joist centres and fixings, the TDUK C16 span tables for how far a joist can go between bearers, and the Blue Circle datasheet for postcrete.
What you need
Boards
- Deck area
- 12.0 m²
- Treated softwood deck boards
- 42 × 3.6m boards20 boards across the 3.0m at 151mm cover (145 + 6mm gap) × 2 along the 4.0m, plus 5%TDCA: 5–8mm gap, boards Use Class 3
Frame
- Joists
- 11 × 47 × 150mm C16400mm centres along the 4.0m, each 3.0m long. Use Class 4 timberTDCA: 400mm for 27mm boards, 300mm for diagonal
- Max clear span for this joist
- 3.00 myour joists are 3.0m, so a bearer at each end does itTDUK C16 span tables, 0.5 kN/m² dead load
- Bearers
- 2 × 4.0mone under each row of joist ends, and one under each mid-span row
- Noggins (blocking)
- 20 pieces2 rows at 1.2m, each cut to 353mm from joist offcuts
- Joist hangers
- 0 hangersjoists sit on the bearers and are skew-screwed or strapped; hangers are only for a ledger frame
Posts and fixings
- Support posts
- 10 × 100 × 100mm2 bearers × a post every 1.2m plus one on the end. Use Class 4TDCA: 100 × 100 most common; Jacksons: 3 posts per bearer minimum
- Postcrete
- 20 × 20kg bags2 bags per post in a 300 × 600mm holeBlue Circle Postcrete datasheet
- Post-hole gravel
- 5 × 25kg bags150mm of gravel in the bottom of each hole for drainageTDCA
- Deck screws
- 3 × box of 200440 screws: 2 per board and joist crossing, about 37 per m². Length 2.5 × board thickness (64mm for 28mm boards)TDCA
Balustrade
- Balustrade
- Needed if the drop is over 600mm Approved Document K: 1100mm guarding on external decks
Sundries
- Fascia boards
- 4 × 3.6m14.0m of open edge, in the same board
- Weed membrane
- 14 m²under the frame, pegged, with 10% for laps
- End-grain preserver
- 1 litrebrush every cut end, notch and drilled hole
Round up to whole packs and bags. Figures include the wastage you chose and match TradeCalc in the app.
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Or claim your free profile link nowHow the maths works
- Boards run one way and joists run across them. Boards across the deck are the span divided by the cover width, which is the board width plus the gap: the TDCA specification says 5 to 8mm between timber boards and 6mm for solid composite. Boards along the deck are the run divided by the board length, then wastage is added.
- Joists are spaced along the run at the centres you pick, plus one, and each joist is as long as the span. The TDCA code of practice rates a 27mm C16 board to 400mm centres; diagonal boards need 300mm; 600mm is only for 34mm C24 boards.
- How far a joist can go between bearers comes from the TDUK C16 span tables at 0.5 kN/m² dead load: 47 × 150 does 3.0m at 400mm centres, 47 × 200 does 4.29m. If your span is longer, the calculator adds a row of bearers and posts.
- Joist hangers are only for a ledger frame, two per joist. On a freestanding frame the joists sit on the bearers and are skew-screwed. Ledger fixings are M10 coach screws or bolts every 600mm, staggered.
- A raised deck has a 100 × 100mm Use Class 4 post every 1.2m under each bearer, plus one on the end. Each post takes two 20kg bags of postcrete for a 300 × 600mm hole, from the Blue Circle datasheet, over 150mm of gravel.
- Screws are two per board and joist crossing at the outer quarter points, from the TDCA: at 400mm centres with 151mm cover that is about 33 per square metre. Composite boards use one hidden clip per crossing instead.
- A balustrade is needed where the drop is over 600mm, 1100mm high with no gap a 100mm sphere can pass (Approved Document K). Spindles are 41mm at 140mm centres, which leaves a 99mm gap.
- Fascia is the open edge divided by the board length. Wastage is 5% for straight boards of 3m or more, 10% for 2.4m boards, diagonal or picture-frame layouts.
Worked example
A 4m by 3m deck with 145mm boards at a 6mm gap running along the 4m length needs 20 boards across the 3m width and 2 along, so 42 boards at 3.6m with 5% wastage. Joists at 400mm centres are 11 of 47 × 150mm, each 3m, exactly the TDUK 3.0m span, so 2 bearers and, raised, 10 posts with 20 bags of postcrete. Screws are 2 per crossing: 440, so 3 boxes of 200.
Questions people ask
How many decking boards do I need per square metre?
It depends on the board. A 145mm board with a 6mm gap covers 151mm, so about 6.6 board-metres per square metre: 1.85 boards at 3.6m, or 2.75 at 2.4m, before wastage. A 120mm board with the same gap needs 7.9 board-metres per square metre.
What spacing should decking joists be?
The TDCA code of practice rates the standard 27 to 28mm softwood board to 400mm centres laid straight. Diagonal boards need 300mm. 600mm centres are only for 34mm C24 boards, so do not use them with stock boards. Composite makers mostly ask for 300 to 400mm; check the fitting guide.
How many screws does decking need?
Two per board and joist crossing, at the outer quarter points, from the TDCA. At 400mm centres with 145mm boards that is about 33 screws per square metre, not the 20 most guides quote. Screw length should be 2.5 times the board thickness: 64mm for a 28mm board.
How many bags of postcrete per post?
Two 20kg bags for a 100 × 100mm post in a 300mm wide, 600mm deep hole, from the Blue Circle Postcrete datasheet. Put 150mm of gravel in the bottom of the hole first so the post end can drain.
Do I need a balustrade?
Yes if the drop from the deck is more than 600mm. Approved Document K asks for 1100mm guarding on an external deck or balcony, with no gap that a 100mm sphere can pass through, and it must resist a horizontal load, so bolt the posts through the rim joist rather than screwing them to the surface.
Do I need planning permission for decking?
Usually not, as long as the deck is no more than 300mm above the ground and, with other outbuildings, covers less than half the garden (Planning Portal). Anything higher, or near a boundary, is worth a call to your council first.
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