How Far Apart Should Floor Joists Be? A UK Guide
Published 31 July 2026 · Browse all tools
Ask three builders how far apart floor joists should be and you'll get "400 centres" from all three without much hesitation. They're usually right — but "usually" is doing a lot of work in that sentence, because the honest answer depends on what's going on top of the floor and how far the joists have to reach. This guide covers the standard UK spacings, why 400 mm became the default, how spacing and span pull against each other, and the joist size you actually end up needing. The free Floor Joist Span Calculator UK does the sums; this explains them.
The short answer: 400 mm centres
The default for UK domestic floors is 400 mm centres, measured centre-to-centre between joists. 450 mm and 600 mm both appear in practice too. "Centres" means the distance from the middle of one joist to the middle of the next — not the clear gap between them.
The spacing is usually chosen to suit the deck above as much as the joists below. Standard 2400 × 600 mm chipboard boards land neatly on joists at 400 mm (four gaps) or 600 mm (two gaps), which is why both spacings show up in real floors. In practice, most UK first-fix carpenters set floor joists at 400 mm centres by habit and only drop to 600 mm when a designer specifically calls for it.
Why 400 mm became the standard
It isn't arbitrary. Sheet materials come in 1200 and 2400 mm modules that divide cleanly by 400, so chipboard and plywood always land on a joist with no awkward offcuts. Closer joists also share the floor load between more members, which stiffens the floor and helps keep deflection down — the quality check that decides whether a floor feels solid or springy.
400 mm also happens to be the spacing at which standard C16 carcassing timber spans most normal UK room widths without going to very deep sections. The standard BS 8103-3 (structural design of low-rise buildings — timber floors) sits behind domestic floor design in the UK, and the span tables referenced by that standard are written around the common UK spacings.
400 mm isn't a rule handed down from nowhere — it's the number where sheet sizes, span and stiffness all line up.
Spacing and span pull against each other
This is the mechanism most guides gloss over. Closer joists each carry a narrower strip of floor, so each one experiences less load and can span further. Wider spacing loads each joist more, cutting the span it can manage.
The numbers from the calculator make this concrete. A 47 × 200 mm C16 joist spans about 4.04 m at 400 mm centres but only about 3.39 m at 600 mm. Same timber, same grade — half a metre of span lost to the wider gap. That gap doesn't save you any joists if it forces you to step up to a deeper, dearer section to cover the room.
The Floor Joist Span Calculator UK lets you compare spacings directly for your grade and depth, so you can see whether the wider spacing is actually saving you anything.
Worked example — a 4.2 m living-room floor in Wakefield
A ground-to-first-floor living room in a Wakefield terrace, joists needing to clear 4.2 m between the front wall and a central load-bearing spine wall. C16 carcassing, standard domestic loading.
- At 400 mm centres, C16: 47 × 200 mm spans 4.04 m — short. 47 × 225 mm spans 4.55 m — covers 4.2 m.
- Switch to C24 at 400 mm: 47 × 200 mm spans 4.36 m — covers 4.2 m one size shallower.
- At 600 mm centres, C16: 47 × 225 mm spans 3.79 m — still short. No standard C16 sawn joist covers 4.2 m at 600 mm; you'd need C24: 47 × 225 mm C24 spans 4.29 m at 600 mm — same section as the 400 mm C16 solution but needs the higher grade.
The wider spacing that was supposed to save joists now requires C24 grade — and the same 47 × 225 mm section — to make the span. At 2026 merchant prices (Jewson / Travis Perkins), 47 × 225 mm C16 runs roughly £12–£16 per metre; C24 costs a touch more and usually needs ordering rather than off-the-shelf, which matters if you're on a tight programme.
Match the spacing to the deck, not just the joists
There's a practical constraint people miss. The floor deck above the joists has its own maximum joist spacing, set by the board manufacturer — and if the joists are wider than that, the board spec overrules the span table, regardless of what the joist can manage structurally.
Standard 22 mm T&G flooring-grade chipboard is designed for joists at up to 450 mm centres. Push to 600 mm and you need boards rated for that span — usually thicker material or a different product. Same applies to plywood and OSB: always check the maximum joist spacing on the board's own spec before you set out the floor.
A joiner will often add a row of solid noggins or herringbone strutting at mid-span on anything over about 2.5 m — it's not always on the drawing, but it's what stops a technically-compliant floor from feeling springy underfoot.
Where spacing stops being a table lookup
Standard spacing rules assume an evenly loaded, uninterrupted floor. They break down in several situations where a structural engineer takes over from a spacing chart:
A stairwell or loft hatch cuts joists short. The trimmer joists that carry the cut-off ends pick up extra load that the standard table doesn't account for — they need individual sizing. A stud wall running along the floor or a water tank overhead concentrates load in a way the 1.5 kN/m² domestic figure doesn't cover. Any loft conversion changes the imposed load entirely and is always a Building Control job, regardless of span.
In those cases, asking "how far apart" is the wrong first question. The right first question is "does this need an engineer?" — and for anything heavy, structural or near an opening, the answer is usually yes.
Get your number
The Floor Joist Span Calculator UK works both ways. Enter the clear span, grade and spacing and it returns the smallest joist that covers it — or enter a joist size and get the maximum span it's allowed. Merchants like Travis Perkins and Jewson stock C16 as standard carcassing; you usually have to ask for C24 and wait for it, which is worth knowing before you design around it.
Set the spacing, then let the span table pick the joist — don't eyeball it.