Decking Material Calculator

Decking Guide

What do I need to know about Decking Calculations?

Boards: total linear metres = deck width / board centres × deck length × 1.1 (10% waste). Board centres = board width + gap (5-6mm for softwood, 6-8mm for composite). For a 4×3m deck with 150mm boards (155mm centres): rows = 3000/155 = 19.4 → 20 rows. Linear metres = 20 × 4m = 80m. Add 10% waste = 88 linear metres. Boards are usually sold as 3.6m, 4.2m, or 4.8m lengths — choose the longest that minimises cuts and joins. Number of 4.2m lengths: 88/4.2 = 21 boards.

What's the key thing to understand about Subframe Design?

Joists: span at 400mm centres across the shorter dimension. A 4m deck width needs: 4000/400 + 1 = 11 joists. For a 3×4m deck with boards running lengthways: joists run across the 3m width, at 400mm centres along the 4m length. Number of joists = 4000/400 + 1 = 11. Joist size: for spans up to 2m — 47×100mm CLS softwood. Spans 2-3m — 47×150mm. Spans 3m+ — 47×200mm or double up. Posts: hardwood or concrete post anchors at 1.5-2m centres under each joist. Height above ground: low-level decks, under 300mm, generally don't need balustrades, while anything higher typically requires them under Building Regulations.

What's the difference between Softwood and Composite?

Softwood (treated pine/spruce): cheapest option (£15-30/m²). Needs annual or biennial oiling to prevent graying. Splinters when weathered. Will eventually rot despite treatment — typical lifespan 10-20 years. Hardwood (ipe, balau, oak): beautiful appearance. Durable (25-40 years). More expensive (£50-120/m²). Ipe (ironwood): almost impervious to decay — lasts 50+ years. Very hard to work. Composite (Trex, Millboard, Cladco): a timber and plastic blend. Low maintenance, needing only a clean once or twice a year, but with a higher upfront material cost than softwood.

What's the key thing to understand about Planning Permission?

Most residential decking does not require planning permission under Permitted Development if: the deck is less than 30cm above ground level. Total area of decking is less than 50% of the garden area. Not adjacent to a highway. Not on a listed building or in a conservation area. Over 30cm above ground level: planning permission required (potential implications for neighbours, visual impact). Building Regulations: decks over 600mm above ground level may require Building Regulations approval (structural building control sign-off), and decking within 2m of a boundary has separate height restrictions — check with your local authority before building.

What do I need to know about Decking Fixings and Spacing?

Decking boards are typically screwed (not nailed) using stainless steel or coated decking screws, two per joist crossing per board, angled slightly to avoid splitting. Pre-drilling is recommended within 50mm of a board end to prevent splitting, especially in hardwood or near-dry softwood. Hidden fixing clip systems (which fasten from the board's underside/edge rather than through the face) give a cleaner finish without visible screw heads but typically cost 20-40% more than face-fixing and need boards with a routed edge groove to accept the clips. Expansion gaps matter too: composite decking expands and contracts more than timber with temperature, so manufacturers specify minimum end-gaps (commonly 5-10mm) between boards and against walls — ignoring this is a common cause of buckling composite decks in hot summers.

What should I know about Decking Maintenance Costs Over Time?

Softwood decking needs annual or biennial treatment with oil or stain (roughly £15-25 per 5-litre tin, covering 15-20m² per coat), plus occasional board replacement as boards near the ground inevitably rot faster than the rest of the deck. Hardwood needs less frequent oiling (every 2-3 years) but costs more upfront. Composite decking needs only occasional cleaning (soapy water, soft brush) with no staining or oiling required, which is the main argument for its higher initial cost — over a 15-20 year deck lifespan, the total cost of ownership for composite often comes close to or below softwood once labour and materials for repeat treatments are included, even though the upfront material cost is roughly double.

What do I need to know about Thermal Movement and Expansion Gaps in Decking?

Decking boards move with temperature and moisture, and composite boards move considerably more than timber. Leave a 5mm gap between composite boards end-to-end and against walls or upstands, and check the manufacturer's spec for gaps between boards in width, which is often 5-6mm to allow for both drainage and seasonal expansion. On a long run (over 3-4m), some composite brands specify expansion gaps at fixed intervals using breaker boards or trim, since a continuous run can expand by several millimetres per metre on a hot day. Timber decking moves less dramatically but still needs a 3-5mm gap when boards are installed slightly damp, since they will shrink as they dry and gaps will widen. Fixing boards too tightly, or butting them hard against a wall, is one of the most common causes of cupping, bowing, and popped fixings within the first summer. Always check the specific manufacturer's expansion tables rather than assuming timber and composite behave the same way, and remember that darker composite colours absorb more heat and move more than lighter shades.

What do I need to know about Decking Material Lifespan and Joist Spacing?

Different decking materials don't just differ in price and appearance — they also need different support structures underneath, because stiffness and long-term sag resistance vary by material, which directly affects how closely joists need to be spaced.

Composite boards generally need tighter joist spacing than timber of similar thickness because the wood-plastic blend is more prone to long-term sag between supports than solid timber — which is also why composite manufacturers publish their own specific joist-spacing tables rather than one universal figure.

What do I need to know about a Worked Example — Boards and Fixings Needed for a Deck?

Worked example: a deck measuring 4 m × 3 m using 145 mm-wide composite boards laid across the 3 m width (running in 4 m lengths), with a 5 mm gap between boards, and joists spaced at 400 mm centres per the manufacturer's spec. Step 1 — effective coverage per board (board width + gap): 145 + 5 = 150 mm = 0.15 m Step 2 — number of boards to cover 3 m width: 3 ÷ 0.15 = 20 boards Step 3 — number of joists along the 4 m run at 400 mm centres: (4,000 ÷ 400) + 1 = 11 joists Step 4 — screws per board (2 screws at every joist crossing): 11 × 2 = 22 screws Step 5 — total screws for the deck: 20 boards × 22 screws per board = 440 screws It's standard practice to add roughly 5-10% extra boards to this kind of calculation to cover cutting waste, mis-cuts, and the occasional damaged board found on site, which for this 20-board deck means ordering 21-22 boards rather than exactly 20. The screw count is usually rounded up to the next full box size rather than by a percentage margin, since running one screw short at the very end of a deck is a far more annoying site problem than having a dozen spare screws left in the box.

What do I need to know about Estimating Total Decking Material Cost?

Once the board count is known, estimating overall material cost just means multiplying by a typical price per board (or per square metre) for the chosen material — but because board prices vary enormously by species, finish, and supplier, it's more useful to compare materials by typical cost per square metre than by price per individual board.

These relative rankings are far more reliable than any specific price figure, since decking material costs move with timber and polymer markets and vary significantly by region and retailer. For the 12 m² deck in the worked example above, the material cost comparison matters most when weighed against the lifespan table earlier on this page: a composite deck costing two to three times as much as softwood per square metre but lasting twice as long works out closer in true cost-per-year than the upfront price difference alone suggests, which is the calculation worth doing before assuming the cheapest material is automatically the best value.