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:

  1. Deferential movements (e.g. the roof deck moves but the chimney does not).
  2. An accumulation of turbulent water (e.g. in valleys and on the high side of chimneys)
  3. An accumulation of melting snow or ice (e.g. in valleys and on the high side of chimneys)
  4. 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.