How Many Decking Boards Do I Need? UK Guide

A UK softwood decking board is typically 120 mm wide. Lay it with a 5 mm gap and it occupies 125 mm of your deck. That 125 mm figure, not the 120 mm one, is what you divide into your deck width.

The sum is straightforward once you know that. Take the width of your deck measured at right angles to the direction the boards will run, then divide it by the coverage width of one board.

Boards needed = deck width across the boards ÷ (board width + gap)

A 3.0 m width with 120 mm boards at a 5 mm gap gives 3.0 ÷ 0.125, which is 24 boards. Use the raw 120 mm board width instead and you'd get 25, so you'd have over-ordered by one board. On a small deck that's a few pounds. On a 40 m² deck it's a stack of timber you didn't need.

Boards also come in fixed lengths, so if your deck is longer than the longest available board you'll be joining over a joist, and every joint costs you a bit more board.

Takeaway: divide by board width plus gap, not board width alone. Every board count in this article uses coverage width.

Softwood decking in Britain is sold in a fairly narrow range of sizes, which makes the arithmetic easier than it is for most materials.

Here's the part almost every decking guide gets backwards.

The standard advice is to leave a 5 to 6 mm gap between boards for drainage and expansion. That advice is correct for kiln-dried timber and for composite. It's wrong for the pressure-treated softwood most UK merchants sell off the rack.

Treated softwood decking is usually delivered wet. The preservative is forced in under pressure in a vacuum vessel, and unless it's been kiln-dried afterwards, which costs more and is rarely what you're buying at trade counter prices, the board arrives with a moisture content well above where it'll settle. Over its first summer it dries out and shrinks across the width. A 120 mm board can lose 3 to 4 mm.

So lay wet boards at 5 mm and by September you've got 8 or 9 mm gaps. That's wide enough to lose a phone through, and it looks like a mistake because it is one.

Lay them at 2 to 3 mm instead and let them shrink into the gap you actually wanted. The boards will find their way to roughly 5 to 6 mm on their own.

The counter-intuitive consequence is that a tighter gap needs slightly more boards. At a 2 mm gap your coverage width is 122 mm, so that same 3.0 m deck needs 3.0 ÷ 0.122, which is 24.6, rounded up to 25 boards rather than 24. One extra board to get the finish right is a good trade.

How do you tell whether your boards are wet? Pick one up. Wet treated softwood is noticeably heavier than it looks, and often feels cool and slightly damp on the end grain. If the delivery note says kiln dried, or you've bought composite, use the 5 to 6 mm gap as normal.

Takeaway: if the timber's wet, lay it tight at 2 to 3 mm and let it shrink. Only kiln-dried and composite boards get the 5 to 6 mm gap on day one.

The frame is where deck failures happen, and it's where most of the timber cost sits on a low deck.

Joist spacing for standard 28 mm softwood boards is 400 mm centres. That's the figure in general UK practice and it aligns with the guidance in BS 8605-1, which covers external timber decking. If you're laying boards diagonally at 45 degrees, the effective span between supports increases, so joist centres come down to 300 mm. Some composite boards also specify 300 mm, and the manufacturer's figure always wins over the general rule.

Joists themselves are usually 47 × 100 mm or 47 × 150 mm treated softwood, depending on span and how far off the ground you are.

To count joists over a given deck length: Joists = (deck length ÷ joist spacing) + 1

The plus one catches people out. A 4.8 m run at 400 mm centres needs 13 joists, not 12, because you need one at each end as well as the ones in between.

Treatment matters more than most people realise. UK preservative treatment is classified by use class under BS EN 335, with the treatment itself specified under BS EN 351. Anything above ground in an external deck needs Use Class 3. Anything in contact with the soil, which means your posts and often the bearers on a ground-level deck, needs Use Class 4. Merchants sell both and the difference is not always obvious on the shelf, so ask. A UC3 post buried in the ground will rot in five to ten years. A UC4 one shouldn't.

Takeaway: 400 mm joist centres for standard boards, 300 mm for diagonal, always add one joist to your count, and use UC4 timber for anything touching soil.

A rear garden deck, 4.8 m along the house and 3.0 m out from it. Boards run parallel to the house, so each board is 4.8 m long and they stack across the 3.0 m depth. Standard 120 × 28 mm treated softwood, laid wet at a 3 mm gap.

Wastage guidance for decking is usually given as a flat 10%, and that's too blunt to be useful.

If your boards run the full length of the deck with no cuts, as in the example above, there's almost no cutting waste. One or two spare boards covers splits and delivery damage, well under 10%.

If your deck is longer than the longest available board, you're joining over joists, and every join wastes the offcut from the board before it. That's where 10% earns its place.

If you're laying diagonally, go to 15%. Every board meets the perimeter at an angle and both ends come off as waste, and the triangular offcuts at the corners are rarely reusable.

Picture frame borders, where a board runs round the perimeter and the field boards die into it, add roughly another 5% because of the mitres.

Takeaway: full-length boards need a couple of spares, joined runs need 10%, diagonal decks need 15%.

Two UK regulations catch people out on decking.

Planning permission is usually not needed if the deck surface sits no more than 300 mm above ground level and the deck, along with any other extensions and outbuildings, covers less than half the garden. Go above 300 mm and you may need permission, particularly if you're overlooking a neighbour.

Guarding is a separate question and comes under Approved Document K of the Building Regulations. Where there's a drop of more than 600 mm from the deck surface, guarding is required, and for a raised deck at a dwelling that guarding generally needs to be 1100 mm high. Balustrade posts also need to be structurally fixed to the frame rather than skew-nailed to a joist end, which is a common shortcut and a genuinely dangerous one.

If your deck is raised at all, check with your local building control before you build rather than after.

Takeaway: under 300 mm high and under half the garden usually means no planning permission, but any drop over 600 mm needs proper guarding.

I asked a landscaper who builds decks around Cheshire what he'd tell someone ordering timber for the first time.

His answer was about the frame, not the boards. He said people spend an hour choosing between board profiles and then buy whatever joist timber is nearest the door, and that the joists are what determines whether the deck feels solid in five years. He builds at 400 mm centres as standard and drops to 350 mm on anything that'll take a hot tub or a lot of furniture, because the extra timber is cheap compared to relaying a bouncy deck.

He also said the single most common thing he's called out to fix is posts set in concrete without a gap for drainage, so water sits around the base of the post and rots it from the bottom up. His fix is to set posts on a concrete pad with a metal post base rather than sinking them into the concrete at all.

Takeaway: spend the thought on the frame. The boards are the bit you see, the joists are the bit that fails.

The board count is one division and the joist count is one division plus one, but decks are rarely a neat rectangle, and the gap decision changes the answer. Our decking area calculator handles the area, board coverage and wastage in one go.

If you're pricing the frame timber as well, the timber calculator will work out how many stock lengths you need to buy and how much you'll waste as offcut, which is usually the bigger cost surprise. For steps up to a raised deck, the deck stair stringer calculator handles rise and run.

None of these replace checking the design against BS 8605-1 and Approved Document K, or speaking to building control on anything raised.

Takeaway: the calculators handle the arithmetic; the published standards and building control handle everything else.