Help sizing joist for garden building flat roof

[Workers99]

Hi all,

I'm new here and am looking for advice on the size of timber to use for a flat roof on a garden timber frame building I'm planning to build.

The shed I'm planning:

1. The dimensions are 5.6m (front/back) x 4m (sides) x 2.45m (height) to stay within permitted development.
2. 4in concrete base.
3. The walls will be made from 2x4in timber with studs at 400mm. These will be constructed (outside to inside) cladding, battens, membrane, OSB3 11mm sheeting, stud work (insulation within studs), vapour control layer, plasterboard.
4. The the roof span between the front and back walls for the joists will be 3.8m. I plan to build the roof with the joists spaced at 400mm, cover the roof with 18mm T&G OSB3 and then EPDM. I need help with the size of the timber I should use for the roof joists.

I have been searching the net for information and looked as some span tables - these seem to suggest 2x8in C24 - this seems too big to me, or it it?

Any help and advice appreciated.

Thanks

Shaz

 

[nickjb]

Mine is about the same size and that's what I used. Roof is totally solid. You could probably get away with less if you really wanted, maybe stick with 2x8 but at 600 centres if you feel it is overkill.

 

[d000hg]

When you say flat, are you putting a slope on it?

 

[Workers99]

Mine is about the same size and that's what I used. Roof is totally solid. You could probably get away with less if you really wanted, maybe stick with 2x8 but at 600 centres if you feel it is overkill.

Great, that's reassuring. Thanks.

When you say flat, are you putting a slope on it?

Yes, there is a slope. As I'm using EPDM the drop does not need to be as big as other materials.

 

[d000hg]

2x8 does sound massive but at least this way you can put a patio on top

 

[mwatsonxx]

And a baby grand piano.

 

[r896neo]

You'll thank the span tables when there's 4" of snow sitting on top of it.

 

[d000hg]

Sure if he moves to Finland!

 

[Workers99]

Thank you all . I'll just go with the span table guidance.