Joist doubling

[i_am_fubar]

After asking some roofers of the best way to achieve a warm deck roof when the thickness of the roof is restricted. One offered the idea of doubling up shallower joists, rather than using single, deeper, ones.

Provisionally, I was going to use 220x47. Could anyone advise what sizes of doubled up joists would give a comparable load bearing capability and performance? If I could get away with 6x2's (yes, I know, evil mixing of metric and imperial), that would be ideal.

Cheers,
Fubar.

 

[tony1851]

It depends on what deflection you are prepared to put up with. The load/span tables work on what is accepted as reasonable deflection, commonly span x 0.003.
Too much deflection will crack your plasterboard ceiling.
Probably two 6x2s would do if an 8x2 would be OK, but as for the deflection?....it would need to be calcuated.

 

[i_am_fubar]

I'll see if our FEA package at work has structural timbers in it's material database.

Thanks for the info on allowable deflection.

13.5mm sounds like a lot from an aesthetic stand point. But then, I guess that is at peak loading, and not the norm for every day use.

Fubar.

 

[tony1851]

13.5mm sounds like a lot from an aesthetic stand point. But then, I guess that is at peak loading, and not the norm for every day use.

Fubar.

Yes, that's usually under maximum (snow) loading.