double joists to increase span?

[JetJackson]

I'm planning a loft conversion and trying to plan how big the room can be. This will depend on the span I can achieve.

Looking at span tables such as these : http://www.salford.gov.uk/timber-floors.pdf , using a 147 x 75 I can achieve a span of 3.56m, with a 400mm pitch.

So, my question is, if I double the joists up, what will the span be increased to? I have looked around on the net quite a bit but cant find any info on the increase in strength / span from doubling / trippling timber?

 

[big-all]

cand find me charts at the min so as a very very rough guess about 1/3 more at 4.8m!!!!

 

[JetJackson]

Also, does the same apply to rafters and perlins?

 

[Shytalkz]

Yes

 

[JetJackson]

Can someone point me in the direction of some material that confirms this? Span tables or a formula or something?

Ta,