Buy a lead roll from local supplier, about 9inch wide roll should do.
Retired Jobbing builder. The following was a choice customers had because of the cost of renewal. Proceed as follows.
A bodge job. cut a strip of 3ply felt and mastic over the split.
A really bodgy job is Aquaseal No 5 about 3 coats on the whole valley, but it only lasts about 2yrs if that.
A better and firmer repair would be.
Mark the CUT slates/tiles either side of the valley at two rows above and two rows below the split. Remove the cut slates/tiles. Cut from the roll of lead a strip the same width as the valley lead. Cut the split across the valley and gently lift the top part just enough to slide the patch piece under about 3inches. on the bottom lay a layer of bitumen/mastick [thick stuff] then tap the lead patch down onto the bitumen/mastic that will hold it in position. You have a reasonably neat repair.
Replace the slates/tiles.
I've done many like it and it extended the life of valleys for ages before they finally had to have new zinc pre-formed valleys. Warning you need to number or letter each cut slate/tile so they go back in the same spot they were cut for. If you don't then you'll have a puzzle on your hands believe me. Post how you got on when you've done it.
Bob, October 2006
