UK Decking Calculator - How Many 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?

UK builders' merchants sell decking boards in standard lengths and widths. Here are the most common options:

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:

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

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.

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.