At the risk of seeming very stupid indeed....
'm building a ply/50mm celotex/ply sandwich floor for my shepherd's hut project. I've got my 1200 x 2440mm ply and I assumed that I could run my 75 x 50mm floor joists at 600mm centres.
However it seems that the ready cut Celotex comes in 450mm widths. This is the stuff made for pitched roofs.
The nice drawing I've just made will need to be re-done. It's annoying me because I thought I knew this, and it means that my insulation width will not tie up neatly with my ply width (1.2m). I really, really don't want to be cutting celotex with a handsaw.

I'd always assumed that roof joists/trusses were 50mm thick, at 600mm centres (2ft in old money). However, that can't be right, if Celotex are selling between-joist insulation at 450mm wide.

Either roof trusses are 6" thick or the centre spacing is less that 600mm. So, what is the standard thickness of a roof joist/truss and what centres are they placed at?

The sheet insulation is likely cut for cavity walls where two blocks = 450c/c

Joist centres are often 400 or 600 centres depending on what the engineer says, occasionally 450 centres. And rarely 300c/c for wide spans or high loading.

Have done all mine in 600 ctrs - less thermal bridging - slower pentane loss from the Celotex too... Consider using T+G OSB/3 instead of ply, then you don't have to match up the edges with the joists (and you also have the option of using the OSB as the air-tight layer too - which worked very well for me, and others here use too).

Thanks for the empathy folks, a rant shared is a rant halved. I don't feel so bad now.

TimSmall; good tip about the T&G OSB, saves lining up the edges with studs. I'll probably try that in my walls to save time. Too late for the floor, the ply is all bought and cut.
I got the 9mm WBP Ext hardwood ply from the importer. It cost £14.34 per sheet, inc VAT, compared with £25-£28 at the B&Q shed.
Ply seems to be an international commodity, it's price fluctuates a lot. Could/should I invest in plywood futures?

Posted By: WulbertAll I need now is to discover that celotex is not actually 50mm thick!

It won't be, will be all over the place. Some caused by shrinkage (I called it retrecion once to sound sophisticated), some by storage, some my damage. Wavy stuff it is.

Posted By: WulbertCould/should I invest in plywood futures?

Only if you want to invest in a downward trend.
Now if you want to speculate, that is a different matter. Buy up 30% of the UKs demand, withhold 20% of that stock and watch the price rise. Spend the income on increasing your stock holding until the monopolies commission comes knocking on your door.
You can have that advice for for about 50 sheets of 6mm marine ply if you make a success of it