Repairing Flashing

If you can't find obvious damage to your roof, such as missing shingles or shakes, you can usually trace leaks to flashings or valleys.

Roof Valleys
Leaks in valley flashing usually occur where water backs up in the valley due to obstructions. The water flows over the edges of the flashing and runs under the rooding material. Fix this by clearing the valley of debris, which can act like a dam to back up water.

Next make sure that the roofing material in the valley is cut in a smooth, straight line.
If any of the material such as underlayment or a shingle, sticks out of the valley and under the shingles. To ensure against valley leaks, carefully raise composition shingles where they meet the valley flashing and coat the area with roofing cement. Then run a bead of caulk down the valley flashing right net to the shingles. On wood shingles or shakes, just run a bead of caulk along the edges where they extend onto the valley flashing.

A break in the metal flashing itself can cause a leak. If you find a break, repair it with aluminum patching tape and roofing cement. Cut off a length of tape to amply cover the hole, peel off the backing stick the tape in place and cover it with roofing cement.

Vents
Leaks occur around vent pipes when the caulk or roofing cement has shrunk or cracked. A new bead of cailk is the cure.