How far can 5x2 span?

5 Mar 2022
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United Kingdom
Hi there. First time post - nice to meet you!

I'm planning a self build timber frame garden room with a flat roof that has an internal span of 3.3m. From the span tables I've seen, 6x2 on 400mm centres would be best - based on minimal loads as doing a cold roof and wont be up there often and rarely get snow. But I'm trying to give myself more room to play with (headroom inside and room above the door to put in necessary header, cripples) to keep within 2.5m height.

So I'd like to use 5x2 to shave off a few cm. Would putting 5x2 on 300mm centres be enough? Would doubling up and bolting together two 5x2 - at 400mm centre- give sufficient span for 3.3m?

Not sure if relevant but I'm thinking about fitting out the interior in wood panels like birch ply maybe rather than plasterboard so a bit of movement in the rafters won't equate to cracking in the ceiling.

Any thoughts/solutions on this would be really appreciated.
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My garage roof span is 2.8m, joists are 4x2 @ 450mm centres. The deck is 9mm OSB with EPDM on top. I believe the joists are considerably overspanned on paper but in practice are fine. I (17 stone) can walk on it no problems.

Thanks for that cdbe. Great to see this in practice. My father in law was saying just the same about 4x2s yesterday too so this has really helped cement that idea.

Thanks for taking the time to post.
Span tables give max span of 2.87m for 5 x 2 at 400 ctrs so you aren’t far off.

generally that’s worked out by allowing a 1/360 deflection…..which is considered max for plasterboard without cracking.

I think it would be fine - make sure your fall is around 1 in 40 - to avoid pooling
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I rarely crash the car but still have insurance so forget about what may or may not happen with snow loading. Notwithstanding that and assuming C16 timber and a total dead load on the joists of 25 kg/m2 your deflection would be 8 mm against the recommended of 10mm and the forces within the joist would be 60% of permissible. Doubling up (no need to bolt together) at 400 would be well within required spec.
Currently building a garden room myself. Used a set of instructions purchased from a guy who does youtube videos on builds and it feels bombproof! The span on mine is 3.5m and it's spec'd for doubled up 5x2s at 400mm spaces (not centres) so the insulation fits better. We nailed them along the length and used M10 coach bolts at either end and through the middle. Fitted with any remaining bow in the wood facing upwards. The result is a really solid roof, we had three of us working on it last week.
Thanks to everyone who has replied and offered advice, it is much appreciated. In the end I went for doubled up 4x2 - 5 coach bolts in each - at 400mm centres. When I was putting on the edpm roof membrane it felt sturdy up there and whilst I was up there a mate below saw very little bounce in the rafters. Photo of rafters attached but pleased to say the build had come on since then and is nicely watertight.


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