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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Carpentry Questions >> Flat roof, can timber Beam be used instead of RSJ? https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1588355727 Message started by Barry378 on May 1^st, 2020, 6:55pm

Title: Flat roof, can timber Beam be used instead of RSJ?
Post by Barry378 on May 1^st, 2020, 6:55pm

Hi folks, I’m currently building a granny annex at the end of my garden, built from concrete blocks with a 100mm cavity. measuring at 7.2m deep, 5.2m wide and 2.5m high with a flat roof

Iv been toying with the idea of bolting two roof rafters together to make a beam to hang the roof rafters from, instead of using a steal I beam RSJ. As I believe this will be more cost effective in materials and labour.
(Beam width approximately 4.8m)

I just wanted to know if this is a good idea or not? And if so what size timber would be most suitable?

Would appreciate any suggestions many thanks

Title: Re: Flat roof, can timber Beam be used instead of RSJ?
Post by woodsmith on May 2^nd, 2020, 8:44am

If this is a granny annex then it's an habitable room and you are going to need building consent and, if the design needs a structural support, then they are gong to want specs from a scructural engineer. Structural engineers love steel and I doubt they would sanction timber. One option may be to use engineered joists and do away with the need for a beam altogether or possibly add a load bearing internal wall?

Title: Re: Flat roof, can timber Beam be used instead of RSJ?
Post by CWatters on May 15^th, 2020, 7:56pm

The alternative might be just to run joists the short way. If that's a clear span of 4.8m then the old Trada tables I have suggest either..

Access for maintenance only:
C16 47 * 220 on 450mm spacing

Unrestricted Access:
C16 63 * 220 on 450mm spacing

Gives you something to propose to Building Control.

Is this going to be a warm deck or cold deck construction?
https://www.tendringdc.gov.uk/sites/default/files/documents/planning/building%20control/Flat%20roof%20guide.pdf