UK Decking Calculator - How Many Deck Boards & Joists Do I Need?

How Do I Calculate How Much Decking I Need?

Working out how much decking you need is straightforward once you break it into boards and joists. Here is the method professional deck builders use:

1. Measure your deck area – measure the length and width in metres. For example, a typical garden deck might be 4m long and 3m wide = 12 m².
2. Choose your laying direction – boards can run along the length or width. This affects how many rows you need and how many boards per row.
3. Calculate board rows – divide the span (the distance perpendicular to the boards) by the board width plus the gap. For example, 3,000mm ÷ 150mm (145mm board + 5mm gap) = 20 rows.
4. Calculate boards per row – divide the run (the direction the boards lie) by the board length. For example, 4,000mm ÷ 3,600mm = 1.11, so you need 2 boards per row.
5. Multiply and add waste – 20 rows × 2 boards = 40 boards. Add 10% waste: 40 × 1.10 = 44 boards total.
6. Calculate the subframe – divide the run by joist spacing and add 1 for the internal joists, then add 2 rim joists for the outer frame. For example, 4,000mm ÷ 400mm + 1 = 11 internal joists (each 3m long) plus 2 rim joists (each 4m long).
7. Add noggins – cross-braces between joists at roughly 1.5m intervals along the joist length. These stiffen the subframe and prevent joists twisting.

For screws, allow 2 per board at each joist crossing. In our example: 20 rows × 11 joists × 2 = 440 screws.

What Are Standard UK Decking Board Sizes?

Wickes, B&Q and Travis Perkins sell decking boards in standard lengths and widths. Here are the most common options:

Type	Lengths	Width	Thickness
Softwood (treated)	2.4m, 3.0m, 3.6m, 4.2m, 4.8m	120–150mm	28–32mm
Hardwood	2.4m, 3.0m, 3.6m, 4.2m	120–145mm	19–25mm
Composite	2.4m, 3.0m, 3.6m, 4.8m	140–150mm	20–25mm

The most popular UK decking board is pressure-treated softwood at 3.6m × 145mm × 28mm. This offers the best balance of price, availability and span for typical garden decks.

How Far Apart Should Decking Joists Be?

Joist spacing is critical for a safe, long-lasting deck. The correct spacing depends on your board type:

• Softwood timber decking – 400mm centres (maximum). This is the standard spacing for 28–32mm thick treated softwood boards. At 400mm, the boards will feel firm underfoot with no flex.
• Composite decking – 300mm centres (recommended by most manufacturers). Composite boards are less rigid than timber and need closer support to prevent sagging, especially in warm weather when they soften slightly.
• Diagonal laying – if you are laying boards at 45°, reduce joist spacing by 25%. So 400mm becomes 300mm for timber, and 300mm becomes 225mm for composite.

Always check the manufacturer's installation guide for specific joist spacing requirements. Some composite brands allow up to 400mm on residential decks, while others insist on 300mm or less.

What Joists and Subframe Do I Need for Decking?

The subframe is the structural skeleton beneath your deck boards. A proper subframe consists of:

• Internal joists – these run perpendicular to your deck boards at regular spacing (300–400mm centres). They carry the weight of the boards and everything on the deck. Standard sizes are 47 × 100mm for most domestic decks, or 47 × 150mm for longer spans.
• Rim joists – two outer joists that run parallel to your boards along the front and back edges of the deck. These form the frame and give the ends of the internal joists something to fix into.
• Noggins – short cross-braces fixed between the internal joists at roughly 1.5m intervals. These stop the joists twisting and add lateral stiffness to the frame. Essential for any deck over 2m wide.

All joist timber should be pressure-treated to UC4 standard (suitable for permanent ground contact). Standard lengths available at UK merchants are 2.4m, 3.0m, 3.6m, 4.2m and 4.8m. This calculator works out how many standard-length timbers you need to buy for the entire subframe, including the cut pieces for noggins.

Timber or Composite Decking – Which Is Better?

Both have their place. Here is a practical comparison for UK conditions:

Factor	Softwood Timber	Composite
Cost per m²	£15–£30	£40–£80
Lifespan	10–15 years	25–30 years
Maintenance	Annual oil/stain	Occasional wash
Slip resistance	Poor when wet (add grip strips)	Good (textured surface)
Appearance	Natural wood grain	Uniform, wood-effect finish

Our advice: for a budget deck that you are happy to maintain, softwood timber is hard to beat. If you want a low-maintenance deck that will last decades, composite is worth the extra upfront cost.

How Much Does Decking Cost in the UK?

The total cost of building a deck depends on the size, board type, subframe materials and whether you hire a contractor or DIY. Here is a rough guide to UK decking costs in 2026:

Materials Only

Component	Softwood	Composite
Boards per m²	£15–£30	£40–£80
Subframe (joists, bearers, fixings)	£8–£15 per m²
Foundations (paving slabs or blocks)	£2–£5 per m²
Total materials	£25–£50 per m²	£50–£100 per m²

Example: 4m × 3m Deck (12 m²)

• Softwood DIY – materials typically £300–£600 depending on board quality and extras.
• Composite DIY – materials typically £600–£1,200 depending on the brand.
• Professional installation – add £40–£70 per m² for labour (£480–£840 for a 12 m² deck). Most installers charge more for raised decks, steps or complex shapes.

What Affects the Price?

• Board type – composite costs 2–3× more than softwood upfront, but needs far less maintenance over its lifetime.
• Deck height – raised decks need posts, concrete footings and possibly a balustrade, adding significant cost.
• Ground preparation – if the ground is uneven or overgrown, clearing and levelling adds time and cost. Weed membrane and a gravel bed underneath prevent vegetation growing through.
• Extras – fascia boards, decking oil, LED lighting and balustrades all add up.
• Where you buy – prices for the same board type vary between merchants. Wickes, B&Q and Jewson all stock decking boards and joist timber, so it's worth comparing click-and-collect prices before ordering a full pallet.

Use the cost estimate feature in the calculator above to get a personalised total. Tick “Include cost estimate”, enter unit prices from your local supplier, and the calculator will multiply them through your full materials list.

Do I Need Planning Permission for Decking in the UK?

In most cases, garden decking is permitted development and does not require planning permission, provided:

• The deck is no more than 300mm above ground level (measured from the highest point of the ground next to the deck).
• Together with any other extensions, outbuildings and additions, the deck does not cover more than 50% of the garden area.
• The deck is not forward of the principal elevation (front of the house) facing a highway.
• Your property is not a listed building, in a conservation area, or in an Area of Outstanding Natural Beauty (AONB) – if it is, you may need to apply.

If your deck is higher than 300mm, you will need planning permission. Any deck over 600mm above ground level also requires building regulations approval and must have a balustrade at least 1,100mm high with gaps no wider than 100mm (to prevent a child passing through). Contact your local council's planning department if you are unsure. Similar height and boundary rules apply to garden fences.

How Can I Reduce Waste When Laying Decking?

Decking waste is mostly caused by offcuts that are too short to use. Here are practical ways to minimise it:

• Choose board lengths that fit your deck – if your deck is 4m long, 4.2m boards produce tiny offcuts (200mm). Two 2.4m boards with a butt joint may give less waste overall.
• Plan your cuts – if boards need trimming, save the offcuts and use them to start the next row. Staggering joints also looks better and adds structural strength.
• Lay along the shorter dimension – this sometimes allows full-length boards with no cuts, especially if the width matches a standard board length.
• Use a waste allowance of 5–10% – 10% is standard for most decks. If your deck dimensions are close to multiples of the board length, 5% is usually enough.
• Order accurately – use this calculator to work out exactly what you need, then round up. Over-ordering “a few extra boards” is the most common source of waste.
• Consider the gravel calculator if you are also landscaping the area around your deck – get both material lists right first time.

Frequently Asked Questions

How many decking boards do I need?

Divide the deck span (perpendicular to the boards) by the board width plus gap to get the number of rows. Then divide the deck run (along the boards) by the board length to get boards per row. Multiply the two and add 10% for waste. For a 4m x 3m deck with 145mm boards and 5mm gaps laid along the length: 3,000 ÷ 150 = 20 rows, 4,000 ÷ 3,600 = 2 boards per row, 20 × 2 = 40, plus 10% = 44 boards.

How far apart should decking joists be?

For softwood timber decking, joists should be spaced at 400mm centres (maximum). For composite decking, most manufacturers recommend 300mm centres because composite boards are less rigid and can sag between supports, especially in warm weather. If laying boards diagonally, reduce spacing by 25%.

How many screws do I need for decking?

Allow 2 screws per board at each joist crossing. Multiply the number of board rows by the number of joists, then multiply by 2. For example, 20 rows × 11 joists × 2 = 440 screws. Use 4mm × 50mm or 4.5mm × 60mm stainless steel decking screws to prevent rust staining.

What waste allowance should I use for decking?

A 10% waste allowance is standard for most rectangular decks. If your deck dimensions are close to multiples of the board length (meaning few cuts are needed), 5% may be enough. For complex shapes, angled cuts or diagonal laying, increase to 15%. Waste comes mainly from end cuts that are too short to reuse.

What subframe do I need for decking?

A decking subframe consists of internal joists (perpendicular to boards, spaced 300-400mm apart), rim joists (forming the outer frame), and noggins (cross-braces between joists). All timber should be pressure-treated to UC4 standard. Standard joist sizes are 47 × 100mm for spans up to 1.5m, or 47 × 150mm for longer spans up to 2.4m.

Is composite decking better than timber?

Composite decking costs more upfront (£40-£80/m² vs £15-£30/m² for timber) but lasts 25-30 years with minimal maintenance. Timber needs annual oiling or staining and lasts 10-15 years. Composite is also more slip-resistant when wet. For budget builds, timber is excellent. For low-maintenance longevity, composite is worth the investment.

Do I need planning permission for a deck in the UK?

Most garden decks in the UK are permitted development and do not need planning permission, provided the deck is no more than 300mm above ground level, does not cover more than 50% of the garden, and is not in front of the house facing a highway. Listed buildings, conservation areas and AONBs may have additional restrictions. Decks over 300mm require planning permission, and decks over 600mm also need building regulations approval with a balustrade at least 1,100mm high.

How much does decking cost per m² in the UK?

Softwood timber decking costs roughly £25-£50 per m² for materials including subframe, fixings and foundations. Composite decking costs £50-£100 per m² for materials. Professional installation adds £40-£70 per m² for labour. A typical 4m × 3m softwood deck costs £300-£600 in materials for a DIY build, or £780-£1,440 installed by a contractor.

What gap should I leave between decking boards?

Leave a 5-8mm gap between decking boards for drainage and expansion. 5mm is standard for treated softwood. Composite boards may need wider gaps (up to 8mm) as they expand more in warm weather. Always check the manufacturer's guidance for composite products. The gap allows rainwater to drain through and prevents boards pushing against each other as they swell.

What size joists do I need for decking in the UK?

The most common UK decking joist sizes are 47 × 100mm, 47 × 150mm and 47 × 200mm. Use 47 × 150mm as the standard choice for most domestic decks, since it handles spans up to 1.8m between supports. Use 47 × 100mm only for ground-level decks with supports at 1.2m centres. Use 47 × 200mm for longer spans up to 2.4m or heavier loads. All joist timber should be pressure-treated to UC4 standard.

How many joists do I need for a 4m deck?

For a 4m deck with boards running along the length, you need joists running across the 3m width at 400mm centres. That gives 4,000 ÷ 400 + 1 = 11 internal joists, plus 2 rim joists on the outside edges. So 13 joists in total. Each internal joist is 3m long (the deck width), and each rim joist is 4m long (the deck length).

How long do decking boards last?

Pressure-treated softwood decking typically lasts 10-15 years with annual maintenance (cleaning, oiling or staining). Hardwood decking (such as oak or iroko) lasts 15-25 years. Composite decking lasts 25-30 years with minimal maintenance, usually just an annual wash. Lifespan depends on maintenance, exposure and drainage underneath the deck.