Doubling up joists help

[MaceUK]

Hi

I am building a 5m x 5m cabin with a flat roof. I need to keep the building below 2.5m (to avoid planning) so would like to utilise more internal height by using smaller joist timbers but doubling them up. There are various videos etc on YT, and specifically oakwood gardens who seem to use 2x5 for a lot of spans but doubles them up. Ive only ever seen him go to 4m though...

Could i double up 2x5 (so 4x5) across a span of 5m?

The recommended i believe is 2x8 or 2x9. But of course using this would reduce head height with a cold roof.

Any help would be greatly appreciated.

 

[Mr Chibs]

At 5m span 2x5 will surely sag over time, doubled up, not sure.
Also 5m+ lengths are slightly more tricky to get hold of, most merchants I use stop at 4.8m.

Could you not put a beam(s), metal/wood across the middle at 90º to the rafters and cut the span in half.

Doubt anyone will notice if the roof height is a bit higher than 2.5m... the measurement is from Ground level, can't you ramp it up a bit, remove later?

 

[John D v2.0]

2x5 wouldn't be possible at all if you want your roof to slope the right way, but have you thought about using SIPs instead, then you combine your insulation into your structure. There are span tables for sips with reinforcement, but even then 5m sounds like a stretch to me.
Can you break up the span somehow?

 

[Dereekoo]

Using c24 wood at that size with a roof dead load not exceeding 0.5kn/m2 and a live load of 0.75km/m2 would require the joists to be placed at approx 85mm centres to keep within a permissible deflection of approx 14mm, so no its a none starter unless you assume its never going to snow and then you can go to 200 spacings

 

[MaceUK]

Thanks all for the replies... Ill look at the 9x2 or 8x2 in that case.