"Christian McArdle" wrote in message
. net...
The rafters are 4"x2".

Well, without sarking, you might get away with an unventilated between
rafters only. The closed cell foam may even act as a form of sarking (but
with big holes in!). I don't know what the BCO would say, though.

How deep are the joists and how long is the span?

They're 6"x2" at 14" centres and the longest span is 13'.

Quick calculation to metric:

152mm x 51mm joists at 355mm centres at a span of 3.96m.

According to Part A, your joists are good for 3.27m. To reach 3.96m, you
need 195mm joists. However, it is likely that the limiting factor is floor
bounce, not final structural strength. Certainly, your 6" joists are
vastly
better than what is normally found (i.e. 3" joists) which don't even make
the tables. A 4" joist (i.e. 38x97) would only allow 1.83m span.

Thanks - definitely time to ring the local BCO and get some advice.

Mark