How Many Floor Joists Do I Need? Spacing and Strutting (UK)
Floor joists are counted from the length of the floor and the spacing. A floor 5 m long at 400 mm centres needs 14 joists, each 3.80 m long for a span of 3.6 m. The floor joist calculator counts them for your room.
Joists for each length
| Length of floor | 400 mm centres | 450 mm centres | 600 mm centres |
|---|---|---|---|
| 3 m | 9 | 8 | 6 |
| 4 m | 11 | 10 | 8 |
| 5 m | 14 | 13 | 10 |
| 6 m | 16 | 15 | 11 |
| 8 m | 21 | 19 | 15 |
Cut length and strutting for each span
With 100 mm of bearing at each end:
| Span | Cut length | Stock length | Rows of strutting |
|---|---|---|---|
| 2.4 m | 2.60 m | 2.7 m | 0 |
| 3 m | 3.20 m | 3.3 m | 1 |
| 3.6 m | 3.80 m | 3.9 m | 1 |
| 4.5 m | 4.70 m | 4.8 m | 1 |
| 4.8 m | 5.00 m | 5.1 m | 2 |
| 5.4 m | 5.60 m | 5.7 m | 2 |
Strutting
- Up to 2.5 m: no strutting is needed.
- Over 2.5 m up to 4.5 m: one row at the middle of the span.
- Over 4.5 m: two rows, at one third and two thirds of the span.
Before you order
- Get the joist size from a span table, a structural engineer or building control: it depends on the span, the spacing, the timber grade and the load.
- Add trimmers and double joists around stairwells, chimneys and hatches, and under any wall that runs the same way as the joists.
- Check that the joints of the flooring boards land on the middle of a joist.
- Do not notch or drill joists except where the rules allow it.
See also the loft boards calculator and the stud wall calculator. These are estimates: check the building regulations.
Frequently asked questions
How many joists do I need?
The length of the floor divided by the centres, rounded up, plus one: 14 joists for 5 m at 400 mm centres.
What spacing should floor joists be?
Usually 400, 450 or 600 mm from centre to centre, depending on the size of the joist, the span and the thickness of the flooring.
When do I need strutting?
In Approved Document A, joists over 2.5 m up to 4.5 m need one row of strutting in the middle, and joists over 4.5 m need two rows at the third points.
What size joists do I need?
It depends on the span, the spacing, the timber grade and the load. Use a span table, or ask a structural engineer or building control: this page counts joists but does not size them.
How many floor boards do I need?
14 boards of 2400 × 600 mm for a floor 5 m by 3.6 m, with 10% extra.
More guides
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.