Tim Lamb wrote:
I suspect rain is blowing under the rather narrow steel ridge capping
fitted to one of my barns.

Second hand box section rolled steel sheeting was laid on top of an
existing but leaky felted roof. I am now finding that water is making
its a way under the steel roofing and through the felt.

There are several possibilities... the sheets are secured with heavy
duty roofing nails with a trapped plastic seal. The laps are screwed at
18" intervals but there is no mastic seal. The ridge cap is narrow so
rainwater could splash and blow under.

I have in mind to fabricate a template/former in 12mm ply to fit tightly
to the roof sheet, pierced with holes to allow conventional canned
expanding foam to be fed into the ridge and form a tidy seal. Freehand
attempts leave an unsightly bulge and reveals large gas pockets when
trimmed.

How do I stop the foam sticking to the plywood? And, how long does it
take before full expansion is reached?

Any other constructive thoughts?

regards
Varnish the ply and then spread a film of cheap grease on it including
the holes.
Unhelpfully, you possible need to leave it for a few hours.
Maybe misting with some water might help to accelerate the cure?
Try experimenting.

Bob