Single Storey Extension Roof Joists

RaymoS2

I'm looking for some advice on my flat roof joists. The span table for my 4.7 metres says 9 x 2 at 400-600mm but due to a limit on the room I have I am wondering can I use 7x2 at 300mm spacing a or do I need to go up to 7x3?

Secondly, I will need furrings for these joists as I need a run off for the flat roof but I cannot seem to find anyone who sells them in any size or even will cut them for me, any ideas?

Gumbo

RaymoS2 wrote: »

I'm looking for some advice on my flat roof joists. The span table for my 4.7 metres says 9 x 2 at 400-600mm but due to a limit on the room I have I am wondering can I use 7x2 at 300mm spacing a or do I need to go up to 7x3?

Secondly, I will need furrings for these joists as I need a run off for the flat roof but I cannot seem to find anyone who sells them in any size or even will cut them for me, any ideas?

What is the max span for 7x2 @ 300c/c as per the tables?
If it doesn't appear then it's a design that has to be done by your engineer.

I recently had to use 7x2 joists to tie in with an existing floor, but engineer insisted on them at 250 c/c.

RaymoS2

kceire wrote: »

What is the max span for 7x2 @ 300c/c as per the tables?
If it doesn't appear then it's a design that has to be done by your engineer.

I recently had to use 7x2 joists to tie in with an existing floor, but engineer insisted on them at 250 c/c.

2 roofers told me it was fine but I can't find on tables 250 or 300 centres for 7 x 2 so I'm just a little concerned.

johnny_adidas

I don't have access to the trada tables but that would be where to look. If not listed then an engineer as mentioned already.

7x3 at 250cts is an awful lot of timber though. Hope you don't have any cables or ducting to run across them (you might be able to use the space between the firring pieces)

What's the span in the opposite direction? Can you run a steel or double joist to break the span and use larger spacing then? (this would cut down on timber cost but again, remember cables & ducts if in roof space)

Why can't the roofers cut the firring pieces for you?

BryanF

Op we can't help you this is a structural query. You need a structural engineer

Gumbo

johnny_adidas wrote: »

I don't have access to the trada tables but that would be where to look. If not listed then an engineer as mentioned already.

7x3 at 250cts is an awful lot of timber though. Hope you don't have any cables or ducting to run across them (you might be able to use the space between the firring pieces)

What's the span in the opposite direction? Can you run a steel or double joist to break the span and use larger spacing then? (this would cut down on timber cost but again, remember cables & ducts if in roof space)

Why can't the roofers cut the firring pieces for you?

Easier to counter batten and create a service void. Insulation is unaffected then too.

johnny_adidas

kceire wrote: »

Easier to counter batten and create a service void. Insulation is unaffected then too.

Not if space is too tight to go for 225s rather than 175s
Insulation might be on top also

BryanF

Just finishing my roof. 200mm PIR on ply over joists. Ply is taped for AT.

RaymoS2

I can't seem to find tables that show max span for 7 x 2 at specifically 250 / 300 centres........might need to find an engineer.