Decking Material Calculator — Boards, Joists & Fixings
Calculate the quantity of decking boards, joists, posts, and fixings needed for any deck size. Covers softwood, hardwood, and composite decking.
Decking Guide
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.
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
Softwood vs 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): timber and plastic blend. Low maintenance (clean once or twice a year,
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 (struc
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