The two areas where a roof is most likely to start leaking are at roof penetrations and the outside edges. Both have exposed ends of the roofing material and contain flashings as a transition to the adjacent building component. The area called the “field”—the big center area of the roof—is least likely to develop a leak unless damaged by something like a falling tree branch. 

   Here’s a few reasons why roofs leak at the edge:

1) Backed up gutters -The photos below show a roof edge that has suffered from long-term backup of the gutters. The sagging first row of shingles is a clear sign of a problem below. When pulled back at the edge, it was ugly: roof sheathing rotted all the way through below.

2) Lowest row of shingles loses tab adhesion early - The bottom edge of the first row of shingles provides an exposed place for the wind to catch and lift, and these shingles tend to loosen first. This means that the bottom row is also more likely to have damaged, broken shingles. Also, pressure washing the facia and gutters of a home will pop the tab
adhesion of  the first row of shingles. Think of a pressure washer head as a “hand-held hurricane.” 

3) Corroded or missing drip edge flashing - The metal drip edge flashing provides a sharp and angled edge for the water to drip off. Without it, some rainwater will tend to run backwards under the roofing.

4) Tree branches rubbing - Leaves rustling in a gentle breeze in your backyard is pleasant sound, unless some of them are on branches that are rubbing against the surface of the roof shingles. The brushing action removes the granules that protect the roof surface. This occurs most often near the edge of a roof.

5) Antennas - Cable companies require their customers to sign a waiver before they will bolt a satellite dish on the roof—and it’s usually near the edge—for a reason: the bolt penetrations leak.

6) Ice dams - While not a problem in our area, ice dams near the edge of a roof are a common source of leaks in colder climates. The melting ice at the bottom of the mass backs up under the shingles. 

    Also, see our blog posts How do I minimize roof penetrations?

 and How can I tell if the house needs a new roof?