roof joists spacing for OSB

[jonnypron]

Hi,

going nuts here a bit. I'm trying to do the dims for my garden office and am doing 5x2 roof joists with firring strips on top for the pitch. I am doing 400mm centres, but this does not seem to work to get the end of the OSB (1220x2440) to end on half the joist. I've read somewhere you do 353 gaps, but this doesn't seem to work either.

Joists are 45mm wide finished size. The end of the roof / 1st joist; the sheet is flush with. The rest the sheet has to end half on the joist for support. Cannot work out the distance from one joist to the other to be exact.

Also i thought 400mm centres was so the gap between joists was 400mm for insulation fitting (comes in 400mm or proportions of that), but centres means centres so have to trim down to 355mm??? very confused, any advice appreciated.

 

[noseall]

Just go with 600mm or 400mm centres then trim the end of the OSB. Its not just about the OSB but also working for other sheet material like plasterboard which is metric as opposed to the sheet materials which tend to be Yank imperial.

 

[jonnypron]

Just go with 600mm or 400mm centres then trim the end of the OSB. Its not just about the OSB but also working for other sheet material like plasterboard which is metric as opposed to the sheet materials which tend to be Yank imperial.

OK, thanks. Just to confirm 400 centres creates a smaller gap of 355mm so have to trim the insulation too if using celotex or squeeze in if using fibre like rockwool.

 

[noseall]

Celotex is sold in 2400mm x 1200mm sheets. You cut it to suit.

 

[^woody^]

Why trim? Just space the joists evenly to suit the board length. An extra couple of mm spacing won't matter one bit.

 

[noseall]

Why trim? Just space the joists evenly to suit the board length. An extra couple of mm spacing won't matter one bit.

That's an option too. However, I prefer to know that once my studs are covered over (inside and out) that there is some rational measurement that is easily referenced. And it will work for plasterboards especially in regards to staggering the boards too.

 

[^woody^]

That's an option too. However, I prefer to know that once my studs are covered over (inside and out) that there is some rational measurement that is easily referenced. And it will work for plasterboards especially in regards to staggering the boards too.

I've never liked two plasterboards meeting on one joist, so tend to fix a batten on the side of one.

 

[catlad]

Why don't you just buy osb3 and all your headaches will disappear as
you don't have to find a joist?

 

[jonnypron]

Why don't you just buy osb3 and all your headaches will disappear as
you don't have to find a joist?

The internal walls aren't the issue really. Happy to add battens or might use ply as the finished internal wall, but for structural strength the OSB on the roof (before rubber roof) had to meet on a joist and agree with noseall (be good to know where the centres are for future ref):

That's an option too. However, I prefer to know that once my studs are covered over (inside and out) that there is some rational measurement that is easily referenced. And it will work for plasterboards especially in regards to staggering the boards too.

 

[catlad]

hmm.

 

[catlad]

I wasn't very clear when I said use OSB3 I should have said OSB3 T&G which means
you do not need to find the next joist, and also eliminates waste.

 

[jonnypron]

Good point. will consider that for the walls. Already got the roof sheets unfortunately.

 

[geraldthehamster]

Remember, the only reason OSB is 2440 x1220 is that these are the metric equivalents of 8' x 4'. When plasterboard went metric (2400 x 1200), timber board didn't. You have 8'x4' boards, so your joist centres need to be 16". Measure using the other edge of the tape

Cheers
Richard

 

[catlad]

Tapes are very convenient but you can make mistakes
if I am doing anything like that were I need things to fall on a joist then I make a staff,
the length of the material ie. plasterboard sheets and I will mark it at the half way point.
I feel it make it easier to mark out studwork ect.