Decking Calculator
Calculate decking boards, joists and sub-frame for any deck. Covers softwood, hardwood and composite with standard board widths, joist spacing and fascia boards. Includes planning height check.
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Choose the board length closest to — or longer than — your deck dimension for fewest cuts.
Used to flag planning permission requirements. Decks over 300mm above ground level require planning permission in most cases.
Installation Guide
Building a Garden Deck
Sub-frame — joists and bearers
The sub-frame carries the entire load of the deck surface. Joists run parallel to the short dimension and should not span more than 1.8m without intermediate support from a bearer. Bearers are heavier timber (typically 75×100mm or 47×150mm C24) that span between posts or sit on pads.
For timber decking: joist spacing of 400mm centres is standard. For composite decking: 300mm centres are typically required by manufacturers to prevent mid-span deflection. All sub-frame timber should be C16 or C24 treated softwood at minimum, rated UC4 if in contact with or near the ground.
Board gaps and direction
Always leave a 5–6mm gap between timber boards to allow for expansion, drainage and ventilation. Boards laid too tightly will buckle and lift in wet weather. The gap also allows water to drain freely rather than ponding on the surface.
Boards laid at 45° (diagonal) look striking but require approximately 15% more material due to the additional cuts at every board end. Diagonal laying also means joists run at 45° to the boards, which affects the sub-frame layout significantly — plan this before ordering.
Timber treatment — BS 8417
All softwood used outdoors must be pressure treated to resist rot and insect attack. The treatment class rating determines what exposure the timber can withstand: UC3 covers above-ground use in exposed outdoor conditions; UC4 is required for ground contact or permanently wet conditions.
Hardwood decking (balau, ipe, oak) contains natural oils that resist rot without chemical treatment, but it should still be finished with a suitable oil to maintain its appearance. Never use hardwood off-cuts from untreated internal grade timber for outdoor construction.
Planning permission — the 300mm rule
Garden decking is generally permitted development provided the finished deck surface is no more than 300mm above the adjacent ground level. The deck (along with any other extensions or outbuildings) must also not cover more than 50% of the total garden area.
If your property is listed or in a Conservation Area or National Park, permitted development rights may be restricted and planning permission will likely be needed regardless of height. Always check with your local planning authority if in doubt — enforcement action can require demolition of non-compliant structures.
FAQ
Common Questions
Divide your deck area by the effective board width (board width plus gap). For a 150mm board with 5mm gap, effective width is 0.155m. For a 20m² deck: 20 ÷ 0.155 = 129 linear metres. Divide by board length (3.6m) for board count: 129 ÷ 3.6 = 36 boards. Add 10% for waste = 40 boards. Our calculator does this automatically.
For timber decking at 400mm joist centres, use 47×100mm (4×2 inch) C16 treated softwood as a minimum. For composite decking at 300mm centres, the same size is adequate. Where joists span more than 1.5m between supports, use 47×150mm. All joists should be UC4 pressure treated if there is any risk of ground contact or standing water.
The traditional guidance is grooves facing down on timber decking to allow air circulation and drainage between the board and joist, reducing rot. However, many modern decking boards are now designed with the grooved side up for additional grip and aesthetic appeal. Check the manufacturer's recommendation for your specific product.
Two screws per joist crossing for timber decking — one on each side of the board to prevent cupping. For 400mm joist centres and a 3.6m board, that's approximately 18 screws per board. Stainless steel or A2/A4 grade screws are recommended for outdoor use — galvanised screws can stain hardwood decking.
No planning permission is needed for most garden decks provided the deck is no more than 300mm above ground level and does not cover more than 50% of the garden. Conservation areas, listed buildings and National Parks have additional restrictions. Always verify with your local planning authority before starting work if you are close to any of these thresholds.