If it's a leek then get in touch with a gardener
if it's a leak read on.
All depends: Where is the leak?
Is it a felt roof covrered in grano chippings?
If you can see where the leak is on the roof then
1: Materials> Tin thick trowel on mastic, roll thick roofing felt a cheap pointing trowel all from local builders yard.
Clear any grano or pea shingles stones off to reveal the hole/leak clean around and in the hole if there is oen. Apply the mastic in a 4inch square around the and in the hole/split using the trowel. Cut a patch of felt to fit the mastic area then trowel over the edges of the patch, replace the stones. If it's anthing else then haven't a clue as you've not explained type of roof covering.
Bob, June 2010
