Decking Calculator

Decking Build Guide

What do I need to know about Choosing Decking Material?

Softwood (pressure-treated pine): £15–25 per m2 for boards. Affordable, widely available, needs annual oiling or staining to prevent weathering. Hardwood (Ipe, Balau, Bangkirai): £40–80 per m2. Extremely durable (25+ years), beautiful finish, but expensive and requires careful sourcing for sustainable certification. Composite (Trex, Millboard, Eva-Last): £40–100 per m2. Low maintenance, resistant to rot and splinters, but more expensive upfront.

What do I need to know about Structure and Frame?

The frame (joists and posts) typically costs as much as the boards themselves. Allow for deck posts every 1.5–2m, concrete post bases, 47x100mm joists at 400mm centres for most boards, and a rim joist around the perimeter. For a 4x3m deck, expect 8–10 posts, 7–9 joists at 4m, and 3 rim joists. Use pressure-treated timber throughout — untreated timber in ground contact will rot within 3–5 years.

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

In England, a deck is permitted development (no planning permission needed) if it is under 30cm above ground level, within the permitted area (no more than 50% of garden), and not in front of the house. Decking over 30cm high requires planning permission. In conservation areas and listed buildings, additional restrictions apply. Scotland, Wales, and Northern Ireland have slightly different rules.

What's the difference between DIY and Professional Installation?

A professional builder charges £100–150 per day. A 4x3m deck takes 2–3 days for an experienced builder — labour cost £200–450. DIY is achievable with basic carpentry skills and the right tools (circular saw, drill, post hole digger). Allow 2–3 weekends for a first build. DIY saves 40–60% of total cost.

What do I need to know about Joist Spacing and Decking Board Direction?

The substructure beneath decking boards, the frame of joists the boards are screwed down onto, needs its spacing matched to the decking board material and thickness, since boards spanning too far between joists can flex, bounce underfoot, or sag over time. Standard timber decking boards are typically supported on joists at centres around 400-450mm apart for normal use, though this tightens for boards laid at an angle (such as diagonally) rather than straight across the joists, since a diagonal board crosses fewer joists over the same run and effectively has a longer unsupported span between each support point. Composite decking boards generally need closer joist spacing than timber, often specified by the manufacturer at a narrower centre distance, because composite material behaves differently under load and temperature than solid timber, and manufacturer span tables should always be checked rather than assumed to match timber decking rules of thumb. The direction boards run also affects joist layout: boards are usually laid at right angles to the joists wherever possible, since this gives the shortest, strongest span and the most secure fixing, while boards running parallel to joists require noggins or additional support framing to provide fixing points along their length.

What should I know about Calculating Fixings (Screws/Clips)?

Beyond the board material cost this calculator estimates, fixings are a separate but meaningful cost that scales with both board count and joist spacing, since every point where a board crosses a joist needs a secure fixing. Traditional face-screwed decking uses two screws per board at every joist crossing, meaning a board spanning six joists needs twelve screws just for that one board, and stainless steel or specifically coated decking screws are worth the extra cost over standard screws, since ordinary fixings can corrode and stain the decking with rust streaks, particularly on lighter-coloured timber or composite boards. Hidden fixing clip systems, increasingly common with composite decking and some grooved-edge timber boards, clip boards to the joists from the side rather than through the face, giving a cleaner finished appearance with no visible screw heads, but typically cost more per fixing than simple screws and the board itself usually needs a manufacturer-specific grooved edge profile to accept the clips. Whichever system is used, it's worth ordering a reasonable surplus of fixings beyond the exact calculated number, since a damaged screw head or a dropped and lost fixing is a common, minor but irritating cause of a delayed job if supplies run out mid-build.

What do I need to know about Decking Substructure Ventilation and Damp Prevention?

A decking frame that sits too low or too enclosed traps moisture underneath, which accelerates rot in timber joists and can cause damp problems against the house wall. Best practice is to leave at least 150mm of clear, ventilated space between the underside of the deck boards and the ground, and more where the ground is poorly drained or clay-based. Where decking runs close to the house, it should sit at least 150mm below the damp-proof course (DPC) — raising decking above DPC level is a common cause of damp penetration into the property and is something Building Control and NHBC guidance specifically flag. A permeable membrane or weed-suppressant fabric laid over the ground beneath the deck, ideally covered with gravel, helps control weed growth and reduces moisture rising from the soil, but it should never fully seal the area — airflow underneath is what actually prevents rot, not just a vapour barrier. Leaving gaps in fascia or skirting around the deck's edge, rather than boxing it in completely, keeps air moving and dramatically extends the life of the substructure.

Worked Example: Material Quantities for a 4m x 3m Deck

Take a deck measuring 4m x 3m (12m²), with boards running along the 4m length and laid across the 3m width. Using a common timber board width of 120mm plus a 5mm expansion/drainage gap (125mm effective coverage per board): number of boards needed across the width = 3,000mm / 125mm = 24 boards, each 4m long (or made up from shorter lengths joined over a joist). Boards are supported by joists running perpendicular to them — so here, across the 3m width — typically spaced at 400-450mm centres for standard timber decking boards (check the specific board's span table, since thinner or composite boards often need closer spacing, commonly 300-400mm). Over a 4m run that works out to roughly 4,000mm / 400mm + 1 ≈ 11 joists, supported in turn by bearers and posts typically set at around 1.8-2.4m centres depending on the joist size and span capability. Fixings follow the same logic: a standard approach is two screws (or two hidden-fixing clips) at every point a board crosses a joist. With 24 boards and 11 joists, that's roughly 24 x 11 x 2 ≈ 528 screws (or clips) for the deck surface alone, before allowing for the substructure's own joist-to-bearer and bearer-to-post fixings. Hidden-clip fixing systems, increasingly common with composite boards, typically use one clip per board per joist crossing rather than two visible screws, which changes this count but follows the same per-crossing logic.

Comparing Decking Materials: Lifespan and Maintenance Trade-offs

The substructure underneath matters as much as the board choice for long-term performance. Whatever decking material is used on top, the frame (joists and bearers) needs good ventilation underneath to prevent trapped moisture rotting timber joists or corroding fixings — typically achieved by keeping a clear air gap beneath the deck rather than boxing it in solidly, and by ensuring the ground beneath slopes away from the house rather than pooling water under the structure. Composite boards are typically laid on timber or composite joists rather than directly on the ground, and most manufacturers specify closer joist spacing than for solid timber because composite boards flex more between supports — following the specific product's span table is more reliable than applying a generic timber rule of thumb. Post footings also scale with deck size and height: a small, ground-level deck can often use post supports set in a concrete-filled hole or bolted to a paved base, while a larger deck, or one raised more than a step above ground, generally needs deeper footings sized to the specific post spacing and the loads the structure will carry, including people, furniture and, in windier or more exposed gardens, lateral wind loading on any attached balustrade. Planning permission is usually not required for a standard garden deck under permitted development, but two common thresholds are worth checking before building: a deck raised more than around 300mm above the natural ground level often brings it within scope of permitted development limits on height, and decking together with other hard surfacing in the garden counting toward the "more than half the garden" permitted development limit that also applies to extensions and outbuildings. Checking both before finalising a deck's height and footprint avoids finding out partway through a build that an application should have been made first.