In article , Tim S
writes
fred coughed up some electrons that declared:


When putting celotex between rafters it is even more important to
exclude water. If it creeps into the unventilated trough formed by the
celotex, between the rafters, it wont be able to escape and we all know
where that leads. This is the case even if the area above the celotex is
ventilated.

Definitely repair the sarking[1].

Since you put it like that, it makes sense.

I'll get some spare and work it into the hole so the overlaps are more or
less right, then tape it up with some duct tape.

Spot on in checking the overlaps are right, always to the outside at the
bottom so that any runoff is onto the top of the sheet below.

Watch out on the quality of the tape, I've seen the adhesive on the
cheaper ones fail after time, particularly in extremes of hot and cold
as you get in a roof.

Last time I did a similar, I got the patch in place, then pulled it back
to apply silicone to its upper face along the top and down the sides
(none at the bottom), then pushed it back in place, secured temporarily
with tape and put a couple of tile battens across the back of it to
support it properly. The silicone was really as an adhesive but would
also stop any sideways spread of water, possibly bypassing the patch.

It was blowing a gale at the time so I got a chance to see just what
forces play around the roofspace when the wind is blowing.
--
fred
BBC3, ITV2/3/4, channels going to the DOGs