Decking Calculator

Quick Answer — How Many Decking Boards Do You Need?

For a 4 by 4 metre garden deck (16 m²) in the UK, plan on roughly 32 boards' worth of 145 mm decking by area, plus joists at 400 mm centres and a couple of boxes of screws. Enter your exact dimensions in the calculator above for the precise count — it works in whole 3.6 m boards and adds 10% for wastage, so it rounds up above the by-area estimate. Buying to size, materials run roughly £400 to £700 in softwood, £900 to £1,500 in mid-range composite, or £1,400 to £2,200 in hardwood Balau. Decking is permitted development in England only if it meets all three Class E tests: no more than 30 cm above ground, covering no more than 50% of the garden, and not forward of the principal elevation (Planning Portal, verified 2026-06-19).

Last verified: 2026-06-19 against B&Q, Wickes, Travis Perkins, eDecks and Checkatrade. UK supplier prices change quarterly — re-check before you buy.

Where to Buy Decking Materials

Compare prices at the UK's leading retailers before committing to your boards and subframe materials.

Prices vary, so always compare before buying. As an Amazon Associate we earn from qualifying purchases; other links may also earn us a commission at no extra cost to you.

UK Decking Costs 2026 — by Material Type

Different boards behave differently on price, lifespan and upkeep. We've split each material into per-metre and fully-installed numbers so you can match the right line to what you're buying. All ranges last verified 2026-06-19 against named UK retailers.

Wickes treated boards are on promotion at around £2.50 per metre (normally about £3.75); B&Q lists treated softwood from about £3.33 per metre. Trade-side prices (Travis Perkins) run higher — £8.85 to £14.36 per metre ex-VAT for pressure-treated softwood — because grading and moisture content are higher. Labour for installation runs £260 to £340 per day per the Checkatrade cost guide.

Decking Boards — Softwood, Hardwood, Composite Honestly Compared

Most decking calculators online are run by composite-decking brands. They'll quote you their own product price and downplay the alternatives. We don't sell boards, so here's the honest version.

Treated Softwood

Cheapest by a wide margin. Typically pine or spruce, pressure-treated to Use Class 3 (above-ground outdoor). The TDCA gives a 15-year minimum service life as the standard for a quality install, so factor in annual cleaning and re-oiling. If you skip the maintenance, expect 8-10 years before boards start cupping or greying. Wide retail availability: B&Q, Wickes and Travis Perkins all stock it. Best value if you're prepared to do upkeep.

Hardwood (Balau, Iroko, Cumaru)

The longest-lasting natural option. Balau is the most common in the UK and runs from around £10.80 per metre at online retailers like eDecks for 145mm wide smooth-faced boards. Naturally durable to durability class 2-3, so it doesn't need pressure treatment to stay sound for 25+ years. Still wants an oil every 2-3 years to keep its colour. Heavier than softwood, harder to cut, needs pre-drilling for screws. Premium hardwood retailers price Balau higher than eDecks.

Composite (Wood-Plastic, e.g. Trex, Cladco, NeoTimber)

Highest upfront cost but almost zero maintenance. Most composite boards are 60-95% wood flour mixed with recycled plastic. UK retailers including Cladco, NeoTimber and Trex carry 25 to 30 year residential warranties. The trade-off: composite is not always the greenest choice. End-of-life recycling for wood-plastic composite is limited in the UK, and some early budget brands fade noticeably in 5-7 years. Stick with major UK brands (Trex, Cladco, NeoTimber, Composite Prime) that publish material specifications and offer real warranties.

Our calculator uses a treated softwood baseline of £3.50/m, mid-range composite at £8/m, and eDecks Balau at £10.80/m as the hardwood baseline. Update these in your head if you're buying premium-tier products.

How to Use This Decking Calculator

Step 1: Measure the Deck Length

Measure the longest side of the area where you want the deck. This is usually the dimension running along the back of your house or along a fence. Measure in metres — if you only have feet, divide by 3.281 to convert.

Step 2: Measure the Deck Width

Measure how far the deck will project out from the house, or the shorter side of your deck area. Boards are laid across this direction. A typical UK garden deck is 2.5 to 4 metres wide.

Step 3: Choose Your Board Type

Treated softwood is the most affordable and widely available. Composite costs more upfront but needs almost no maintenance. Hardwood Balau is the longest-lasting natural option. The dropdown shows the per-metre price each tier uses for the cost estimate, all verified against UK retailers on 2026-06-19.

Step 4: Decide on the Joist Subframe

Select "Yes" if you're building a new deck from scratch — the calculator will work out joist numbers at 400mm centres. Select "No" if you're replacing the boards on an existing frame and only need a board count.

Step 5: Calculate

Hit the button to see boards (with 10% wastage), joists, screw boxes and an estimated material cost broken down line by line. Always confirm prices and quantities with your supplier before buying, since UK timber prices move with seasonal demand.

Joist Spacing & Span Rules (UK)

Joist spacing is one of the most over-simplified parts of decking advice. The headline rule is right but it isn't the whole story.

Standard joist spacing: 400mm centres

For straight-lay timber and most composite boards, joists sit 400mm centre-to-centre. That number comes from board flex tests: most boards span 400mm without bouncing. Drop to 300mm for diagonal or herringbone patterns, or for any composite board where the manufacturer's spec calls for it. Wider spacing makes the deck creak.

Use Class 4 treatment is now standard for substructure

The TDCA recommends pressure-treating all joists, posts and beams to Use Class 4, even if those timbers don't touch the ground. Use Class 4 is the same treatment level used for fence posts in soil. The rule changed because deck substructures sit in moisture much longer than older guidance assumed. Reference standard: BS 8417:2011+A1:2014.

Strength class: C16 minimum, C24 for longer spans

C16 timber is the minimum allowed by UK building regulations for any deck above 600mm in height. The TDCA recommends C16 below 600mm too. For longer spans, smaller cross-sections, or commercial use, step up to C18 or C24, which costs roughly 15-20% more.

Indicative span figures (BS 5268-7.1, floor joists)

Deck-specific span tables sit inside the paid TDCA/TRADA Professionals' Manual. As an indicative cross-check, the BS 5268-7.1 floor-joist tables for C24 timber at 400mm centres permit a 47×170mm joist to clear-span up to 3.975 metres under standard residential dead load. Deck joists exposed to weather should be specified shorter than these floor figures because outdoor moisture cycling reduces effective stiffness over time. If your deck plan asks joists to span more than 3 metres, get a structural engineer to size them.

Do You Need Planning Permission for Decking?

For most UK gardens, no — decking is permitted development. But the rule is conditional, and the conditions catch people out. The summary below is taken directly from the Planning Portal (the official UK planning information service) and verified on 2026-06-19.

The three rules that all need to apply

For a deck to qualify as permitted development in England, all three of these must be true:

1. The decking is no more than 30cm above ground level.
2. Together with extensions, outbuildings and other raised platforms, the decking covers no more than 50 percent of the garden area.
3. None of the decking is on land forward of a wall forming the principal elevation of the house.

Miss any one of those and you need a planning application. The 30cm height rule is the most common slip: deck builders aiming for a flat indoor-outdoor floor often end up over the line.

Special areas where stricter rules apply

In National Parks, the Broads, Areas of Outstanding Natural Beauty (AONB) and World Heritage Sites: any decking sitting more than 20 metres from the house is capped at 10 square metres total area. On Article 2(3) designated land (mainly conservation areas and AONBs in older orders), no decking is permitted on either side of the house. Within the curtilage of a listed building, no decking is permitted at all.

What's not covered by these rules

Permitted-development rights for decking only apply to houses. Flats and maisonettes don't qualify, full stop — you'll need to check with your freeholder and your local planning authority. Houses created by "permitted development changes of use" and new-build dwellings under separate parts of the order are also outside the standard rules.

The legal source

These rules summarise Schedule 2, Part 1, Class E of the Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended). Full government guidance lives in "Permitted development rights for householders: technical guidance" on GOV.UK. If you're uncertain — especially in conservation areas or near a listed building — ring your local planning authority before you start. A quick phone call costs nothing; an enforcement order does.

Building Regulations & Safety Standards for UK Decking

Planning permission and building regulations are different things. A deck can be permitted development (no planning) and still need building regs sign-off. For most low garden decks the regs don't bite, but they kick in once the deck is high enough to need a barrier or spans a structural element.

The 600mm threshold for raised structures

UK building regulations treat any deck more than 600mm above ground as a "raised structure" and require it to be built from C16-grade timber as a minimum. This is also the height at which the TDCA recommends barriers (handrails). The standard for those barriers is BS 6180:2011 — Code of Practice for Barriers in and About Buildings.

NHBC 60-year service life on new homes

Raised decks and balconies built as part of a new home need a 60-year service life specification under TDCA Codes of Practice. That's a much higher bar than the 15-year minimum the TDCA applies to standard installations. Builders working on new-build sites should download the free TDCA Codes of Practice for the full spec.

Key British Standards for decking

• BS 8417:2011+A1:2014 — Preservation of wood (Code of practice). Sets the Use Class treatment levels.
• BS EN 335:2013 — Durability of wood. Defines Use Classes 1 to 5.
• BS EN 338:2016 — Strength classes for structural timber.
• BS 6180:2011 — Barriers in and about buildings. Code of Practice.
• BS EN 1995-1-1:2004+A2:2014 (Eurocode 5) — Design of timber structures. The current UK structural design standard for timber, superseding the withdrawn BS 5268.

You don't need to read the standards yourself unless you're a professional builder, but if you're hiring a contractor, ask which standards their workmanship complies with. A reputable installer will know.

What we've noticed building this calculator

Most decking calculators online are run by companies that sell composite boards. They quote their own product price, skip the regulation check, and use a single point estimate that hasn't been refreshed in years. We built HomeCalc to do the opposite. Whatever material you pick, the UK planning rules don't change with the board choice — the 30cm rule, the 50 percent rule, the AONB cap, and the listed-building exclusion apply the same way to softwood as they do to premium composite. We'd rather you know that up front than find out from a planning enforcement letter. The cost ranges on this page were last checked against B&Q, Wickes, Travis Perkins, eDecks and Checkatrade on 2026-06-19. If you're reading this more than three months after that, prices may have moved — re-check before you buy.

Frequently Asked Questions