Decking Calculator

How Many Decking Boards Do I Need?

This free decking calculator works out exactly how many boards, joists and screws you need for your deck. Enter the length and width of your deck area, choose your board type and length, and get a complete materials list instantly.

The calculator divides the deck length by the board width plus gap (5mm between boards for drainage) to get the number of board rows. It then works out how many boards you need based on the board length and deck width, including cuts and wastage. Joists are calculated at your chosen spacing (400mm centres for softwood, 300mm for composite) with an extra joist at each end.

Building on concrete? Our concrete calculator works out pad foundations and bases. Need sub-base material? The gravel calculator handles that.

Standard UK Decking Board Sizes

UK decking boards come in a few standard sizes. Choosing the right width and length minimises waste and speeds up the build.

Softwood vs Composite Decking: Which Should You Choose?

The two most popular decking materials in the UK are pressure-treated softwood and composite boards. Each has clear advantages depending on your budget and how much maintenance you want to do.

Pressure-Treated Softwood

Softwood decking (usually Scandinavian redwood or treated pine) is the most affordable option, typically costing £3–£8 per board. It needs treating with decking oil or stain every 1–2 years to prevent greying and algae growth. With proper maintenance, softwood decking lasts 10–15 years. It can become slippery when wet unless you use anti-slip inserts or grooved boards.

Composite Decking

Composite boards are made from a mix of recycled wood fibres and plastic. They cost more upfront (£8–£25 per board) but need virtually no maintenance — no staining, no oiling, no sanding. Most composite decking comes with a 20–25 year warranty. It won't splinter, rot or warp, making it a good choice for families with young children. Some composite boards expand in heat, so follow manufacturer spacing guidelines.

Which Is Cheaper Over 10 Years?

Softwood is cheaper to buy but factor in decking oil (£30–£50 per year), a pressure washer clean, and potential board replacements. Over 10 years, composite often works out similar in total cost — and you get your weekends back.

Composite Decking Calculator — Worked Example

Composite decking boards vary in size between manufacturers, so the number of boards you need depends on the brand. Here is a worked example for the most common UK composite board sizes.

Common UK Composite Board Dimensions

Example: 4m × 3m Composite Deck

Deck area: 12m². Using standard 2.2m × 145mm hollow composite boards:

• Boards needed: 12 × 3.1 = 38 boards (round up from 37.2)
• Board cost: 38 × £12 (average) = £456
• Joists (at 300mm centres): 14 joists at 3m long = £84–£140
• Starter clips + screws: £40–£60
• Total materials: £580–£656

For comparison, the same 12m² deck in pressure-treated softwood costs £220–£380 in materials. The £200–£300 premium for composite pays for itself within 4–5 years by eliminating annual oiling and maintenance. Use the calculator above to get your exact quantities.

Decking Joist Spacing and Structure

Getting the subframe right is the most important part of building a deck. If the joists aren't properly spaced and supported, the boards will bounce, sag and eventually fail.

• Softwood decking: 400mm joist centres (the distance between the centre of one joist and the centre of the next)
• Composite decking: 300–400mm centres — check the manufacturer's instructions, as some brands require closer spacing
• Joist timber: Use 47mm × 100mm (2" × 4") or 47mm × 150mm treated timber for joists
• Bearer beams: For raised decks, bearers (the beams that support the joists) should be 47mm × 150mm or doubled-up 47mm × 100mm
• Board gap: Leave 5–8mm between boards for drainage and expansion. Use a nail or spacer to keep the gap consistent
• Screws: Use 65mm stainless steel or coated decking screws (not standard zinc screws, which will rust and stain the timber)

Frequently Asked Questions

Types of Decking for UK Gardens

I have built or helped plan decks using every material on this list, and each one suits a different situation. The table below gives you the honest comparison — prices, lifespans, and what you are actually signing up for in terms of maintenance.

Pressure-Treated Softwood

This is where most UK decks start, and for good reason. Pressure-treated Scandinavian redwood or spruce costs £3–£5 per linear metre for a 145mm-wide board, making a 12m² deck achievable for under £1,000 in materials. The treatment process forces preservative deep into the timber, giving it genuine rot resistance for 10–15 years. The catch is annual upkeep: without decking oil or stain every 1–2 years, the boards grey out, grow algae, and become dangerously slippery. Grooved boards help with grip, but you still need to keep on top of the cleaning.

Hardwood (Oak and Iroko)

If budget allows, hardwood decking is in a different league. European oak boards are dense, naturally rot-resistant, and look stunning — but they cost £55–£85 per m² fitted. Iroko is a popular alternative at £50–£75 per m², offering similar durability with a warmer golden tone. Both will last 25–40 years with minimal intervention. You can oil them annually to maintain the colour, or simply let them weather to a silver-grey patina — either way, the structural integrity holds. The downside? Hardwood is heavy, harder to cut, and needs pre-drilling for every screw to avoid splitting.

Composite Decking

Composite boards from brands like Trex, Cladco, and Eva-Last are the fastest-growing segment of the UK decking market. Made from a blend of recycled wood fibres and plastic (usually HDPE), they arrive in a consistent colour, will not splinter, and need no oiling or staining. Most carry 20–25 year warranties against rot, fade, and structural failure. A 3m × 4m composite deck runs £1,400–£2,400 in materials — roughly double softwood. But when you factor in zero annual maintenance costs and no board replacements over a decade, the total ownership cost often comes out similar. The main things to watch: composite boards expand in heat (leave 10mm gaps at edges), and cheaper brands can feel slightly hollow underfoot compared to real timber.

PVC Decking

Fully synthetic PVC or cellular decking is the most weather-proof option and is commonly used around hot tubs, pools, and waterfront areas. It contains no wood fibres at all, so it is completely immune to rot, mould, and insect damage. Prices start around £60 per m² and go up to £110 for premium ranges. The trade-off is that PVC can feel less natural underfoot and gets noticeably hot in direct summer sun. It is also the hardest to recycle at end of life. For most standard garden decks, composite offers the better balance between durability and aesthetics.

How to Build a Deck — Step by Step

I have built more decks than I can count, and the process is the same whether you are using softwood or composite. What separates a deck that lasts 5 years from one that lasts 20 is all in the preparation and the subframe. Here is the method I follow every time, with the specific UK measurements that matter.

Step 1: Prepare the Ground

Clear the area of turf, plants, and debris. Level the ground as closely as possible — you do not need it perfectly flat, but remove any major humps or hollows. Lay heavy-duty weed membrane across the entire area and extend it 150mm beyond the deck footprint on all sides. On top of the membrane, spread 50mm of compacted gravel or MOT Type 1 sub-base to ensure drainage and prevent the ground becoming a muddy reservoir under your deck. This step alone prevents 90% of the premature rot I see on failed decks.

Step 2: Set Out the Frame Supports

Position concrete deck blocks or adjustable pedestal feet at 1,200mm centres in both directions. For a ground-level deck, 400mm × 400mm × 50mm concrete pads work well. For a raised deck (over 300mm above ground), you will need proper post foundations — use our concrete calculator to work out how much you need for each post hole. Check every support for level using a long spirit level or laser level. Getting this stage right saves hours of frustration later.

Step 3: Build the Joist Frame

Use 47mm × 100mm pressure-treated timber for joists (47mm × 150mm if your span exceeds 2 metres between supports). Space joists at 400mm centres for softwood decking or 300mm centres for composite. Fix joists to the outer frame using joist hangers or 100mm coach screws. Apply joist protection tape along the top edge of every joist before laying boards — this prevents moisture sitting on the contact point and doubles the joist lifespan. Check for level and square as you go: measure diagonals to confirm the frame is square (both diagonals should be identical).

Step 4: Lay the Decking Boards

Start from the outer edge (the most visible side) and work inwards. Use 5mm spacers between softwood boards, or the manufacturer's clips for composite. Pre-drill softwood boards to prevent splitting — drill 2 holes per board at each joist position, roughly 25mm in from each edge. Fix with 65mm stainless steel decking screws (never zinc-plated, which rust within months). If you need to join boards mid-span, always make the join over a joist and stagger the joins across alternate rows so they do not line up.

Step 5: Add Fascia and Edge Trim

Once all boards are laid, cut any overhanging ends flush with a circular saw (clamp a straight edge as a guide for a clean line). Fix fascia boards around the exposed edges of the frame to give a clean, finished look and hide the joists. Fascia can be the same decking material or a matching trim board. Leave a 10mm gap between fascia boards and any wall or fence to allow for expansion — this is especially important with composite materials.

Step 6: Finish and Protect

For softwood decking, apply two coats of decking oil before using the deck. Allow 24 hours between coats and 48 hours before placing furniture. This initial treatment is critical — bare softwood can absorb moisture and start greying within weeks during a wet British spring. Composite boards need no finish, but give them a wash with warm soapy water to remove any manufacturing residue and construction dust.

Decking Cost Guide — UK 2026 Prices

This is what you should realistically budget for a deck build in 2026. I have broken down four common sizes with both DIY material costs and professional installation prices. These figures are based on standard ground-level decks using 145mm-wide boards.

What is included in the materials cost: Decking boards (plus 10% wastage), joists (47mm × 100mm), stainless steel screws, joist tape, weed membrane, and 50mm gravel sub-base. It does not include concrete foundations (needed for raised decks), balustrades, steps, or lighting — each of those adds £200–£500 per feature.

Labour rates: Professional deck installers in the UK typically charge £150–£250 per m² for a complete build including ground prep, subframe, and board laying. Rates vary by region — expect to pay 15–25% more in London and the South East compared to the Midlands or North. Always get at least three written quotes and check for public liability insurance before hiring.

The most popular size I see in UK gardens is the 3m × 4m deck (12m²). It is large enough for a table and four chairs with room to move around, fits the majority of standard garden widths, and keeps the project manageable for a weekend DIY build. If you want to extend later, building the subframe 600mm wider than the initial board run gives you the option to add boards without rebuilding the frame.

5 Common Decking Mistakes (and How to Avoid Them)

After years of building and repairing decks, these are the mistakes I see over and over again. Every single one of them is avoidable if you know what to look for before you start.

1. Wrong Joist Spacing

Using 600mm joist centres (the standard for indoor flooring) on an outdoor deck is a recipe for bouncy, sagging boards. Decking boards are thinner than floorboards and get wet, which makes them weaker. Stick to 400mm centres for softwood and 300mm for composite. Yes, it means buying more joists, but each joist costs roughly £5–£8 for a 3.6m length — it is a tiny fraction of the total budget and makes the deck feel solid underfoot.

2. No Ventilation Gap Under the Deck

Sealing the sides of a ground-level deck with solid boards or lattice panels traps moisture underneath, creating a greenhouse for rot and mould. You need at least 50mm clearance between the ground and the underside of the joists, and the sides should allow air to circulate freely. If you want to hide the gap for aesthetics, use a ventilated fascia board or leave 15mm gaps between any skirting panels.

3. Using the Wrong Screws

Standard zinc-plated screws are designed for indoor use. Outdoors, the zinc coating breaks down within 6–12 months, the screw rusts, and you get ugly black stains bleeding through the timber. The fix is simple: use A2 stainless steel or coated decking screws (such as C4-rated green screws). They cost roughly £15–£20 per box of 200 versus £6–£8 for zinc, but they will outlast the deck itself. On composite boards, always use the screws or clips recommended by the manufacturer — using the wrong type can void the warranty.

4. Skipping the Weed Membrane

Without membrane under the deck, weeds and grass grow up between the boards within a single growing season. Once established, they push boards apart, hold moisture against the timber, and are nearly impossible to remove without lifting the deck. Heavy-duty 100gsm weed membrane costs about £1 per m² — there is no excuse to skip it. Secure the membrane with landscape staples before laying gravel on top.

5. Not Accounting for Board Expansion

Timber expands when it gets wet and contracts when it dries. Composite expands when it heats up. If you butt boards tightly against a wall or fence with zero gap, the boards buckle, lift, and eventually crack. Leave a minimum 10mm expansion gap at all fixed edges. For composite, some manufacturers specify up to 15mm at end-to-end joins. I always fit boards on a mild, dry day — not in a heatwave (when they are already expanded) and not in heavy rain (when timber is swollen). That way, the gap stays correct through the seasons.

Best Decking Products UK 2026 — Our Top Picks

From boards to screws to finishing oil, these are the most popular decking products in the UK right now.

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Where to Buy Decking in the UK

Once you know how many boards you need, compare prices across these suppliers. Buying boards and joists together often gets a better deal.

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