Flat roof timber size

[Bigjohnny5]

Hi all
I hope in posting this in the right place I need some advice on sizing timber beams for a flat roof.
I'm building a garden office that will be 8m x 4.5m it will have a flat roof so I'm looking for the correct timber size to span the 4.5m the beams will be at 400 centres.
By what I can find out online it looks like I will need 9" x 2" ? But this will push up the overall height of the building a little to much to stay under the 2.5m maximum so I'm hopeing I can use smaller timbers and maybe double them up so some advice is needed please as I dont want the whole thing sagging next time we get 8" of snow.

 

[noseall]

Hi all
I hope in posting this in the right place I need some advice on sizing timber beams for a flat roof.
I'm building a garden office that will be 8m x 4.5m it will have a flat roof so I'm looking for the correct timber size to span the 4.5m the beams will be at 400 centres.
By what I can find out online it looks like I will need 9" x 2" ? But this will push up the overall height of the building a little to much to stay under the 2.5m maximum so I'm hopeing I can use smaller timbers and maybe double them up so some advice is needed please as I dont want the whole thing sagging next time we get 8" of snow.

Have you considered how much extra height you will gain when you add 125mm of insulation above the joists plus the furrings plus deck?

Your 225mm will be double that in height at the high end. You may need to re-think your wall plate height.

 

[tony1851]

8" x 2" C24@ 375 - 400 centres would do fine.
You will always get some deflection under snow load, but it won't be sufficient to crack a plaster ceiling.
But as per the point raised by Nose above; if height dictates that you need to have a cold roof instead of warm roof, you can't put too much Kingspan between the joists as you would need to leave good clearance above for ventilation.

 

[Bigjohnny5]

Thanks for the responce and helpful advice
Is it possible to bolt smaller timbers together to reduce the overall height of the building and still keep the strength over a 4.5m span ?
Also I have been researching the roof insulation and was going to follow someting I found on utube .... or I'm I wrong in doing so ?..