Clogged valleys and gutters
A great many reported roof leaks turn out to be a valley that has been holding wet leaf litter for two seasons. The roofing is fine. The water simply has nowhere to go.
Valleys carry the most water
A valley is where two roof planes meet, and it collects the runoff from both. It is the highest-volume path on the roof and the one least tolerant of obstruction. Debris packs in, holds moisture against the shingle, and forces water sideways under the courses on either side.
Gutters push water into the wrong place
A blocked gutter overflows backwards under the eave as readily as it overflows over the front. That soaks the fascia, the soffit and the ends of the rafters the roof sits on — rot that is expensive precisely because it is structural rather than cosmetic. And what does come over the front lands against the foundation instead of being carried away.
Old canopy, old suburbs
The postwar neighbourhoods across this region — Annandale, Towson, Catonsville, Arlington — were planted when they were built, and that canopy is now seventy years old and overhanging. Roofs under mature trees need their valleys and gutters cleared on a schedule that roofs on open lots do not, and a north slope that never dries grows moss which holds moisture against the shingle.