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Deck joist span calculator
Enter the clear span between the supports and see the smallest joist section that carries it at the standard centres for your country. These are the same figures the deck designer checks every frame against.
Smallest section at 400 mm centres
47 × 175 mm
Taken up to 3.50 m in our figures.
The figures, United Kingdom
At 400 mm centres, following BS 8579 / Approved Document K: joists at 400 mm centres, rise up to 190 mm.
- 47 × 100 mm
- up to 1.90 m
- 47 × 125 mm
- up to 2.50 m
- 47 × 150 mm
- up to 3.00 m
- 47 × 175 mm
- up to 3.50 m
- 47 × 200 mm
- up to 4.00 m
Common questions
How far can a 2x8 deck joist span?
At 16 in centres, the figure we use is about 13 ft 1 in in the US and 12 ft 10 in in Canada. Past that, go to a 2x10 or add a beam with posts under the middle of the run.
What joist size do I need in the UK?
At 400 mm centres, a 47 x 150 mm joist is taken to about 3.0 m and a 47 x 200 mm joist to about 4.0 m. Past 4.0 m the usual answer is an extra beam, not a bigger joist.
Is the span measured from end to end of the joist?
No. It is the clear distance between the supports the joist sits on: ledger to beam, or beam to beam. Any overhang past the beam is separate and should stay short.
Are these figures a structural design?
No. They are common residential figures for standard centres and ordinary timber, used to check a take-off. Timber grade, species, snow load and your local building rules can change them. Ask a structural engineer or building control when in doubt.
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The designer places joists, beams, posts and footings for your deck size and lists what to buy by length. Your supplier prices are optional.
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