Decking Joist Spacing Calculator

Summary

Decking substructure design has two separate spacing questions that get conflated on site, and mixing them up is the single most common decking calculation error. The first question is joist spacing — how far apart the joists sit, which is governed entirely by how far the decking boards themselves can span unsupported before they flex, splinter, or fail under foot traffic. The second question is joist span — how far each joist can run between its own points of support (bearers, beams, or a ledger and a bearer), which is governed by the joist's timber size, grade, and the load it carries, using the same structural principles as any timber floor joist.

Get the first one wrong and the boards bounce, creak, or crack over time even though the frame underneath is perfectly sound. Get the second one wrong and the joists themselves sag or fail, which is a structural problem, not a cosmetic one. Both numbers come from published tables, but they're different tables answering different questions, and a deck that looks identical from above can have completely different substructure requirements depending on the board thickness chosen.

This calculator works through both steps with a worked numeric example, using the standard reference figures for treated softwood decking timber (C16/C24 to BS EN 338, treated to BS 8417 Use Class 3/4) as commonly quoted in UK trade guidance. For board quantity calculations (not spacing), see decking board calculator; for the full construction and Building Regulations picture, see decking and decking permits; for general indoor floor joist spans, see joist span calculator.

Key Facts

• Joist spacing is driven by board thickness, not by load calculations you do yourself — manufacturers and trade bodies publish maximum unsupported spans for each board thickness; treat these as the starting input, not something to calculate from scratch
• 27-28mm timber decking boards — the most common UK softwood board thickness — need joists at a maximum of 400mm centres for boards laid perpendicular to the joists (i.e., running square across them)
• 32mm+ timber decking boards can span further between joists — up to 600mm centres — because the extra thickness resists deflection
• 19-21mm thin boards (lightweight or budget product) need closer support, typically around 300mm centres
• Diagonal (45°) board laying reduces the effective unsupported span versus perpendicular laying, so joist spacing should be tightened — as a working rule, close spacing by one tier (e.g., 400mm perpendicular becomes 350mm at 45°) unless the specific product's data sheet states otherwise
• Composite decking boards are stiffer or more flexible than timber depending on the core material and manufacturing process — there is no single industry-standard span figure. Always check the specific manufacturer's technical data sheet; commonly seen figures are 300-400mm centres, sometimes closer again at board ends or on angled runs
• Standard decking joist size — 47×100mm C16 treated softwood is the common starting point for light domestic decking; larger sections (47×150mm, 47×200mm) are used for longer joist spans between bearers
• Joist span (between bearers) follows the same structural principles as indoor floor joists (BS EN 1995-1-1 / Eurocode 5, timber graded to BS EN 338), but decking typically carries a lighter dead load than an indoor floor (no screed or plasterboard ceiling below), so joists can often span slightly further than an equivalent indoor span table suggests — published decking-specific joist tables should still be used in preference to indoor tables where available
• Bearer (beam) spacing between posts is the next tier down — bearers pick up the load from multiple joists and span between support posts, typically at 1.8-2.4m centres for standard sizes, wider for larger sections
• Timber treatment — all structural decking timber (joists, bearers, posts) must be treated to at least Use Class 3 (UC3, above ground, weather-exposed) per BS 8417; posts in or near ground contact need UC4
• Timber grade — C16 minimum for standard domestic decking; C24 gives a longer span for the same section size, or allows a smaller section for the same span, useful where headroom under the deck or joist depth is constrained
• Notching and drilling — decking joists follow the same notch/hole rules as floor joists: notches only in the top of the joist within roughly 0.07-0.25 of the span from each support, holes only on the centreline within roughly 0.25-0.4 of the span from each support — relevant where services (lighting cable, drainage) need to pass through the frame
• Deck height above 300mm typically triggers a planning permission consideration, and above 600mm triggers Building Regulations Part K guarding requirements — this doesn't change the joist spacing calculation itself but is a separate check that should happen alongside it (see decking permits)

Quick Reference Table 1: Joist Spacing vs Decking Board Thickness

Quick Reference Table 2: Indicative Joist Span (C16/C24, Decking Load)

These figures follow the same order of magnitude as published UK decking joist guidance for standard domestic loading. Always confirm against the current TDCA (Timber Decking and Cladding Association) or manufacturer span table before finalising a real specification — decking-specific published tables are less standardised than indoor floor tables, and the C24 figures above are an indicative uplift over C16, not a directly published table.

Worked Example — 4.8m × 3.6m Timber Deck, 28mm Boards

Scenario: A rectangular garden deck, 4.8m along the house wall (boards run this direction) by 3.6m out from the house, using standard 120mm × 28mm softwood decking boards laid perpendicular to the joists. Deck height is kept under 300mm above ground to avoid triggering planning/Part K guarding.

Step 1 — set joist spacing from board thickness. The boards are 28mm, laid perpendicular. From Table 1, this requires a maximum joist spacing of 400mm centres.

Step 2 — lay out the joists. Boards run the 4.8m direction, so joists run perpendicular to the boards — across the 3.6m width — spaced along the 4.8m length at 400mm centres. Joist count: 4.8m ÷ 0.4m = 12 spaces, plus 1 = 13 joists.

Step 3 — check whether a single joist span covers the full 3.6m width. From Table 2, a 47×200mm C16 joist at 400mm centres spans approximately 3.6m — but this is right at the published table limit and flagged for verification on wide bays. Rather than push a single joist section to its published limit, introduce an intermediate bearer at the midpoint of the 3.6m run, splitting each joist into two 1.8m spans.

Step 4 — resize the joists for the shorter span. With a mid-span bearer, each joist now only needs to span 1.8m. From Table 2, a 47×100mm C16 joist at 400mm centres spans approximately 1.8m — exactly at the limit — so specify 47×125mm C16 (span ~2.4m) for a reasonable safety margin and stiffer feel underfoot.

Step 5 — size the bearers. Three bearer lines run parallel to the boards (along the 4.8m length): one at each 3.6m edge (picking up the joist ends) and one down the centre (picking up the mid-span point of every joist). Each bearer line is 4.8m long and needs its own posts. Using a 47×200mm C16 bearer, which spans approximately 2.4m between posts, three posts per bearer line (at 0m, 2.4m and 4.8m) give two bays of exactly 2.4m each — within the bearer's span capability.

Step 6 — size and count the posts. 9 posts total (3 bearer lines × 3 posts). At a low deck height (under 300mm), 100×100mm timber posts on 300×300×300mm concrete pad footings are standard. Check the load on each post: tributary area per post ≈ post spacing (2.4m) × half the joist span either side of the bearer (1.8m) = 4.32m². At a typical decking design load of around 1.7 kN/m² (1.5 kN/m² imposed + ~0.2 kN/m² dead — lighter than an indoor floor because there's no screed or ceiling load), each post carries roughly 4.32 × 1.7 ≈ 7.3 kN (approximately 730kg). A standard 300×300×300mm pad footing is adequate on firm, well-drained ground; enlarge the pad or use deeper footings/screw piles on soft or made ground.

Summary: 13 no. 47×125mm C16 joists at 400mm centres (each spanning 1.8m in two bays), 3 no. 47×200mm C16 bearers at 2.4m post centres, 9 no. 100×100mm posts on 300×300×300mm pad footings. This structure supports 120×28mm softwood decking boards run perpendicular to the joists at the correct 400mm spacing for that board thickness.

Detailed Guidance

Why joist spacing and joist span are calculated separately

It's easy to assume a single "joist size vs span" table answers everything, but decking substructure design genuinely has two independent limits stacked on top of each other. The board thickness sets the maximum distance between joists (how closely packed the support has to be to stop the board itself from deflecting or cracking). Once that spacing is fixed, the joist size and grade set how far the joist can run before it needs its own support from a bearer. Changing the board spec (say, switching from 28mm to 32mm boards) only changes the first number — it doesn't let a joist span further, because the joist's own span capability depends on its size and grade, not on what's screwed to the top of it.

Composite decking — why there's no single spacing figure

Timber decking boards behave predictably because solid softwood has well-established stiffness properties. Composite boards are manufactured from a mix of recycled plastic, wood fibre, and binders, and the stiffness varies significantly between manufacturers and even between product lines from the same manufacturer. Some composite boards need closer joist spacing than an equivalent-thickness timber board because the composite core is more flexible; others match or exceed timber performance. There is no shortcut here — always pull the specific manufacturer's technical data sheet before finalising joist spacing on a composite deck, and never assume a competitor's product spacing carries across.

When to add an intermediate bearer instead of oversizing the joist

As the worked example shows, pushing a single joist size to the very top of its published span range (like the 47×200mm at 3.6m in Table 2) is technically within the table but leaves no margin and increases material cost for a deeper section. In most cases it's more economical, and gives a stiffer result underfoot, to add an intermediate bearer and use a smaller, cheaper joist section over two shorter spans. This also reduces deflection and bounce, which is one of the most common customer complaints about decking that was otherwise built "to spec."

Diagonal board layouts

Laying boards at 45° is popular for its visual effect but tightens every spacing figure in this article. The board's unsupported span reduces because the board crosses joists at an angle rather than square, so the effective support spacing needs to close up — as a working rule, tighten the perpendicular spacing figure by one tier (see Table 1). Diagonal layouts also increase waste on the board order (see decking board calculator) and generally need a slightly more closely spaced joist layout at the perimeter where boards are cut short.

Cross-checking against the board order

Once the joist spacing and layout are fixed, cross-check the total board requirement using decking board calculator — the two calculations are independent inputs into the same job and both need to agree with the final joist and bearer layout before ordering materials.

Frequently Asked Questions

Can I just use a thicker board instead of adding more joists?

Sometimes — moving from 27-28mm to 32mm+ boards genuinely allows wider joist spacing (400mm to 600mm), which can reduce the number of joists needed. But it doesn't change the joist's own span capability between bearers, and thicker boards cost more per linear metre, so it's a trade-off between fewer joists and a higher board material cost — work both routes up before deciding which is cheaper for a specific job.

Does composite decking always need closer joist spacing than timber?

Not always, but it's common. Because composite core stiffness varies by manufacturer, some products need closer spacing than an equivalent-thickness timber board (commonly 300mm rather than 400mm), while premium composite ranges can match timber spacing. There's no way to guess this reliably — check the specific product's span table every time, including for angled or picture-frame layouts, which often have a separate, tighter spacing requirement in the manufacturer's guidance.

Why does my joist span table show a shorter span for decking than an indoor floor of the same joist size?

It shouldn't, structurally — decking usually carries a lighter dead load than an indoor floor (no screed, no plasterboard ceiling below), so a decking joist should in principle span at least as far as an equivalent indoor floor joist under standard domestic imposed loading. Where a specific published decking table shows a shorter span than an indoor table for the same size and grade, it usually reflects a more conservative safety margin (accounting for weathering, point loads from garden furniture, or wet-timber strength reduction) rather than a genuine difference in the loading itself. Use the more conservative, decking-specific figure where one is published.

How do I know if my deck needs an engineer rather than a table lookup?

If the joist span needed exceeds roughly 3.5-4m without an intermediate bearer, if the deck carries an unusual point load (a hot tub, for example), if the deck is elevated more than 600mm and therefore also needs Part K guarding design, or if the deck ties structurally into the house via a ledger board bolted to the building, get a structural engineer or your Building Control body involved rather than relying on standard span tables alone.

Regulations & Standards

• Building Regulations Approved Document A — structure; relevant for elevated or house-attached decking

• Building Regulations Approved Document K — guarding requirements where a deck edge is more than 600mm above adjacent ground level

• BS EN 1995-1-1 (Eurocode 5) — structural timber design, the basis for joist and bearer span calculations

• BS EN 338:2016 — timber strength classes (C16, C24)

• BS 8417 — preservation of wood; Use Class selection (UC3 above-ground exposed, UC4 ground contact) for decking timber

• Town and Country Planning (General Permitted Development) (England) Order 2015 — permitted development height/coverage limits for decking

• Timber Decking and Cladding Association (TDCA) — industry best-practice span and spacing guidance

• Timber Development UK (TDUK) — C16 and C24 span tables

• Planning Portal — Decking and outbuildings

• GOV.UK — Approved Document K

• joist span calculator — general indoor floor joist span methodology and worked examples

• decking board calculator — decking board quantity calculator, cross-checks against this joist layout

• decking — full decking construction guide, foundation design, timber species and durability

• decking permits — planning permission and Building Regulations height thresholds for decking