The roof deck is most vulnerable to leaks where it meets a vertical wall, at penetration sites such as a soil pipe or chimney, or at changes in slope such as at a valley, saddle, mansard, hip, or ridge. This vulnerability is due to:
- Deferential movements (e.g. the roof deck moves but the chimney does not).
- An accumulation of turbulent water (e.g. in valleys and on the high side of chimneys)
- An accumulation of melting snow or ice (e.g. in valleys and on the high side of chimneys)
- Breaks in overlapped shingles (e.g. at hips and ridges)
Flashing is installed at these locations to bridge adjoining structures and prevent water penetration. Flashing materials include sheet metal; cements, caulks, and sealants; and flexible sheets such as waterproofing shingle underlayment. At hips and ridges the cap shingles, not normally called flashing, serve the same function.
Leaving out ice-dam backups and catastrophic damage from severe storms, old age, or gross manufacturing defects, leaks are most likely to originate at a flashing that has failed or was improperly installed.