If the ice and water shield is missing then you will have to remove the shingles on both sides along the valley put ice and water shield in place and re shingle the roof.
Repair leaking lead valley roof.
Replace cracked broken or rotted shingles with the new ones you purchased.
Renew flashing seals by chipping out the old mortar and caulking along the edges of the flashing.
This repair process is also the same for composition and tile roofs.
Isolate areas when you run the hose.
Use special masonry caulk to seal the joints between the flashing and the chimney.
A properly flashed roof valley should have an ice and water shield installed with roofing felt being properly secured over the ice and water.
If you find that this is a problem then you will need to repair your roof valley before anything serious happens such as plaster loss or even major water damage.
Seal the seam between the cap and step flashing with urethane roofing cement or silicone caulking compound as shown at right.
A flashing is a transitional roofing material that connects the roof material to some other building material or to another part of the roof.
A lead valley is a particularly difficult item to dispose of as lead is toxic and so a repair may involve simply patching up your lead roofing.
Diy and trade repair this video on lead roof repair shows a long lasting repair for a lead roof.
Most commonly this is the technique that is used to fix a leaking bay roof window or flat lead roof but it can also be used in almost all leaks where a split or hole in the lead is the culprit.
For example soak the downhill side of a chimney first then each side then the top on both sides.
And damaged flashing is behind most roof leaks.
Start low soaking the area just above where the leak appears in the house.
In either situation the leaking roof valley can be fixed by removing affected shingles and structural elements and upgrading your roof s flashing.
If a leak is difficult to find enlist a helper and go up on the roof with a garden hose.