Flat roof timber sizes

[jtc]

I am about to build a flat roof and my building regs drawings which passed stated 147x50 SW at 400 centres. The span at one part of the roof is 4m. I have just seen some tables for timber sizes and it states that the max you can do with 147x50 at 400 is 3.08m. So it appears my building regs drawings are incorrect, can anyone confirm this. Thanks

 

[masona]

I have just seen some tables for timber sizes and it states that the max you can do with 147x50 at 400 is 3.08m. So it appears my building regs drawings are incorrect, can anyone confirm this. Thanks

You may have been looking at the floor joists table which got to take the weight of furnitures and people etc and the flat roof won't have any weight on it apart from the boards and roofing felt

 

[noseall]

a 4m span would normally require 175 or even 200mm deep joists.
400mm centres are fairly close together, so maybe 175 x 50mm?

give you bco a ring at the council to confirm.

 

[mr_chaela]

Just looked at the TRADA tables

Table 36 Permissible clear spans for joists for flat roofs with access for maintenance and repair only:-
With a dead load (kN/m2) excluding self-weight of joist of not more than 0.50,

A 170 x 47 C16 joist has a max clear span of 4.00 metres @ 400 c/c.
A 170 x 47 C24 joist has a max clear span of 4.35 metres @ 400 c/c.

But as noseall says, check with the bco.

ps Make sure you use structural timber that's properly stamped with the strength grade.

 

[jtc]

Thanks for the info. I checked with the BC and he recommended 175x50. I was going to say, so why did you pass the drawings with 150 timbers but decided against it

 

[masona]

There's maybe some internal bearing walls