On Thu, 03 Mar 2011 21:04:18 +0000, F wrote:

I'll look for something plasticky then!

Tyvek is onee of the makers of modern "plastic" breathable sarking.

And take the advice on a generous overlap:

Wider than the hole, under the old sarking at the top. A cut in the
old at the top of the sides to let the new out from under the old
over the sides, old side edges higher than the old sarking if
possible, new over the old full width of the patch along the bottom.
I have a feeling that is as clear as mud...

Might be wort playing with bits of paper to work out what needs to be
done to make a reasonably water tight patch.

--
Cheers
Dave.