1220x2440 OSB / 1200x2400 Celotex... am I missing something?

Andy7 · 24 Nov 2012

Designing my garden office roof. Warm deck, flat roof 10deg pitch 6m by 6m.

Now, looking at the sizes of OSB verses the Celotex I want to put on the top of it, I'm noticing that generally the size of OSB is 1220 x 2440, which is going to set the spacing of my roof joists. But then when I come to source the insulation, it's sold in 1200 x 2400 which won't line up with my joists.

Am I missing something obvious here - I hope I'm going to have a face-palm moment here.

Can someone illuminate?

Another thing I'd like a sanity check on. Rather then having a single flat span of 6m and making up some kind of 6m furrings, I'm toying with the idea of two 3m furrings back to back to make a kind of very low pitched roof. Bad idea or not?

RonnyRaygun · 24 Nov 2012

2440x1220 is an imperial 8' x 4' sheet. You could simply cut it down to 2400x1200 if it helps.

Andy7 · 24 Nov 2012

RonnyRaygun said:
2440x1220 is an imperial 8' x 4' sheet. You could simply cut it down to 2400x1200 if it helps.

I'd be fine with cutting, though...

a - Seems a waste of materials
b - Surely 'proper' builders would never stand for that extra work.

Materials are a size for a reason. Look at the dimensions of a common brick, elegant design.

RonnyRaygun · 24 Nov 2012

You can get 2400x1200 OSB though. Might just mean searching round a bit for it.

I suspect sheet materials are often sold in the imperial size as a hangover from the pre decimalisation era.
As an example, all steel UB and UC sections are still imperial sizes (e.g. 203x133 UB = 8"x5"), whereas PFC sections are a modern version of imperial tapered flange channels, and are sold in metric sizes.

^woody^ · 25 Nov 2012

IIRC, OSB imported from the US or Canadian mills are still the imperial sizes.

Last time I looked Wickes were selling the imperial sizes, but Selco were selling metric stuff

Andy7 · 25 Nov 2012

^woody^ said:
IIRC, OSB imported from the US or Canadian mills are still the imperial sizes.

Last time I looked Wickes were selling the imperial sizes, but Selco were selling metric stuff

Yep, wickes sell 1200x2440 t&g roof deck. It's £3 per sheet more than just plain old OSB. So, I guess I need to ask myself the question, is £3 more worth the hassle of cutting a strip of my OSB... I think it might be!

theoldun · 25 Nov 2012

Never been able to find out why all panel products, with the exception of t&g chipboard, plaster board and sheet insulation is still made in imperial size.
Always set your joists at either 400 or 600 centres and cut decking to suit, otherwise insulation and plasterboard will not work centres.
The t&g OSB from wickes that you mentioned is still 2440 long and will need trimming. The extra 3 notes per sheet is for the t&g edging, and as you will be cutting of the edging on one end it is not worth the extra money.
For a 6000 span without intermediate support you will require C16 75x225 joists at 400 centres. Minimum cross fall 1in 80 over 6m.= 75mm.
Regards oldun

maltaron · 25 Nov 2012

When I built my extension the spec for the partitions was studs at 400mm centres and standard 8 x 4 plasterboard. I queried this with the architect who said that you cut the plasterboard to 1200 wide. I suggested that it would be simpler to put the studs at 16" centres - He had not thought of that.

theoldun · 25 Nov 2012

Building industry changed over to metric 1970, and to best of our memory plaster board changed from 8x4 to 2400x1200 in 72.
Most common used wallboard is 1200x2400 although you can get other sizes.
Regards oldun

joinerjohn · 25 Nov 2012

Have none of you ever twigged why most timber sheet products are manufactured in Imperial sizes? Same with doors? (unless you specifically ask for metric) Most are produced for the American market (still one of the biggest markets for all sorts of construction materials)

Andy7 · 25 Nov 2012

theoldun said:
...For a 6000 span without intermediate support you will require C16 75x225 joists at 400 centres. Minimum cross fall 1in 80 over 6m.= 75mm.
Regards oldun

thanks for the maths! Actually, I DO have support at 3000mm on the right half and 2000mm on the left half. do you think that C16 47x100 will be okay for that?

I'm aiming at a 75mm drop for 3m (though I'm having a b*gg*r of a job trying to find someone who will ship [inexpensive] firrings to Oxford at the moment).

limerockconstruction · 25 Nov 2012

The support at 2 and 3 meters still leaves your longest unsupported span at4m (unless I'm missing something), so you would need larger timbers - probably 170's

Also a local Timber merchant should be able to cut furrings. My local Travis does mine for me while I wait.

theoldun · 26 Nov 2012

Andy7 said:
thanks for the maths! Actually, I DO have support at 3000mm on the right half and 2000mm on the left half. do you think that C16 47x100 will be okay for that?

I'm aiming at a 75mm drop for 3m (though I'm having a b*gg*r of a job trying to find someone who will ship [inexpensive] firrings to Oxford at the moment).

For access and maintenance and repair only, including snow load of 0.75kn/m2 and max dead load of 0.75kn.m2 . You will need 47x150 regularised to 145 at 400 centres to span 3000
If you fall to each side your firings can be 3000x40 to zero. We always add 10mm on to firings so that they do not run down to zero and cause flat spot, therefore firings will be 50to10.
We cut our own firings with circular saw, but any builders or timber merchants with a saw mill will cut them for you.
Regards oldun

Andy7 · 27 Nov 2012

thanks oldun - I do believe that if it doesn't all fall on my head then I owe you a beer!

mointainwalker · 28 Nov 2012

@joinerjohn

I can't believe that sheet materials are exported from the UK to the US given their low value and the presence of plenty of trees in the States.

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1220x2440 OSB / 1200x2400 Celotex... am I missing something?

joinerjohn

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