"David Pearson" wrote in message
...

What I wanted to do was this: put the 130mm (or similar
correct value, it is about that size) between the roof beams.

I missed out the most important word here. Should read:

"What I wanted to do was this: put the 130mm *insulation* (or similar
correct value, it is about that size) between the roof beams."

Thanks,
Pearson