Deck Boards Calculator

Deck Building Guide

Gap Spacing

Decking boards need a small gap between them to allow for drainage, expansion, and a tidy appearance. A 5 mm gap between boards is the standard for pressure-treated and hardwood timber decking — wide enough for water and small debris to drain through, narrow enough to walk over comfortably (and avoid trapping heels or small dog claws). For green or freshly-sawn timber (sometimes sold as 'wet' decking): use a 3–5 mm gap at installation, because the boards will shrink as they dry over the first few months, increasing the gap to 6–8 mm — install too tight and the boards may bow as they shrink unevenly. For pre-dried (kiln-dried or air-dried) timber and composite decking: a fixed 5–6 mm gap works as the material is dimensionally stable. Composite boards have specific manufacturer recommendations (often 4–6 mm), and getting these right matters because composite expands and contracts more than timber with temperature — installation in cold weather needs slightly wider gaps to allow for summer expansion, and vice versa. A worked example: a 4 m × 3 m deck with 145 mm wide boards laid across the 4 m length, with 5 mm gaps between, needs 3,000 / (145 + 5) = 20 boards. (Cross-check: 20 boards × 145 mm + 19 gaps × 5 mm = 2,900 + 95 = 2,995 mm — almost exactly 3 m, the last gap being against the last edge.) The gap also makes installation easier — you can use a spacer (a 5 mm offcut or purpose-made plastic spacer) between boards to keep gaps consistent without measuring each one. This calculator handles the gap in its quantity calculation; specify a slightly higher figure (6 mm) for green timber that will shrink, or a slightly lower (4 mm) for pre-dried hardwoods. The board-to-board gap is separate from the board-to-wall gap, which should be 10–15 mm to prevent water trapping against the building.

Board Direction

Which way you run decking boards affects both the look and the material wastage of a deck, and the standard choice is usually parallel to the longest dimension of the deck — typically parallel to the house wall on a rear deck. Running boards along the length (parallel to the house): the most common pattern, minimises end-cut waste because boards are cut once at each end to length and any short offcut from one board can sometimes be used as the start of another row, and gives a calming sight-line that draws the eye along the length. A worked example: a 4 m × 3 m deck with boards running parallel to the 4 m side, where boards come in 3.6 m lengths, means each row needs one full board (3.6 m) plus an extra piece of 400 mm cut from another board — 20 boards covering the 3 m width plus an extra 2–3 boards for the short end-pieces, total about 22–23 boards including waste. A diagonal pattern at 45° to the deck edges uses 15–20% more material because every board needs an angled cut at both ends, and the offcuts are usually unusable triangular pieces — but it's visually striking, especially on smaller decks where the diagonal pattern gives an apparent enlargement of the space. Herringbone, parquet, or borders use even more material (20–30% extra) and need much more careful planning and cutting. Whichever direction you choose, always run boards perpendicular to the joists below them, never parallel — boards parallel to joists won't have proper support across their width and will sag. The joist direction is determined first (usually parallel to the house, span as short as possible) and the boards run across them. Always think about water drainage: a slight fall (around 1:80) away from the house prevents pooling, and boards laid with the slightly-curved 'bark side up' (the convex side facing up) shed water better.

Composite vs Timber

The choice between composite and timber decking is one of the biggest decisions in deck building, and the right answer depends on budget, maintenance willingness, and longevity priorities. Pressure-treated softwood timber (the cheapest mainstream option): installation cost roughly £40–60/m² in materials, plus labour. Lasts 15–25 years with annual treatment (re-staining and sealing every 1–2 years), which costs maybe £100–200/year for materials and time. Total 20-year cost of ownership including maintenance: maybe £80–100/m² in materials over the life. Hardwood timber (oak, iroko, ipe): much more expensive upfront (£100–200/m²) but lasts longer (25–35+ years) and looks better, especially as it weathers to silver-grey. Lower maintenance than softwood but still needs occasional treatment. Composite decking (wood-plastic composite, typical UK brand examples include Trex, Fiberon, Millboard): higher upfront cost (£90–200/m² for materials) but needs almost no maintenance — no annual treatment, no staining, no sealing. A typical guarantee is 25–30 years. Over a 20-year ownership period, total cost is often lower than treated timber when factoring in the avoided maintenance time and materials. Aesthetically, modern composite has improved enormously and the best products are convincing even at close range; cheaper composites look more obviously synthetic. Composite gets hotter underfoot in summer than timber (dark composite can be uncomfortable in direct sun). Composite doesn't split, splinter, or warp like timber. The right choice: if you want lowest upfront cost and don't mind annual maintenance: softwood timber. If you want best looks at any cost: hardwood timber. If you want lowest long-term cost and minimum maintenance: composite. Most new domestic deck installations in the UK are now composite, reflecting the lifecycle economics, but the choice is genuinely personal.

Safety and Regulations

Most structural and electrical DIY work in England and Wales must comply with Building Regulations and is notifiable to local Building Control. Work that is not notifiable (like-for-like replacements, cosmetic changes) can be done without notification. Notifiable work done without approval is technically illegal and can cause problems when selling the property. Electrical work in most rooms requires a Part P competent person or Building Control inspection. Gas work must always be performed by a