Flat roof joists size

[vixerfixer]

Hi folks.the garage I’m building is 7x4m.question is what size joists do I need to span 4m?how many?spacing i between?,and can i get the fall by cutting them diagonally?thanks.

 

[elite123]

Look up span tables for flat roofs. prob looking at min 195 x 47 C16 @ 400mm.

Easiest way to get the fall is probably to add furring strip (a diagnally cut timber) on top of the joist to give you the required fall

 

[noseall]

Hi folks.the garage I’m building is 7x4m.question is what size joists do I need to span 4m?how many?spacing i between?,and can i get the fall by cutting them diagonally?thanks.

195mm x 47mm @ 400mm c/c. Use furrings (diminishing strips) for falls, as said.

 

[cdbe]

Easiest way to get the fall is to

Fit one end of the joist higher than the other.

 

[vixerfixer]

Fit one end of the joist higher than the other.

Yeah was gonna say,couldn’t I just use slightly larger joists and just trim those

 

[cdbe]

Yeah was gonna say,couldn’t I just use slightly larger joists and just trim those

No, that's the hard way, and the wasteful way, unless you have plenty of time, a seriously good saw and are good at maths - in which case you could get two different height joists, trim the deeper one and use the offcut as a firring for the shallower one! The reason firrings are used is to keep the bottom of the joist level for the ceiling, in a garage that's not usually necessary.

 

[John D v2.0]

Yeah was gonna say,couldn’t I just use slightly larger joists and just trim those

Technically as soon as you cut a graded joist lengthways you have to regrade it.