The roof on a building I inspected a while ago had two steps where the elevation of the roof changed. Both were flashed improperly. The metal flashing at this transition should never be a continuous piece whether it is installed on top of the shingles:
Or under the shingles:
Regardless of its position, if the metal flashing is continuous, water will eventually either travel across the shingles and then into the roof structure or under the shingles and across the flashing to the roof structure. Of course any water should then encounter the felt paper under the shingles, but the felt paper is not designed to be water proof—it is full of holes from nailing all the shingles down. Eventually either method of installation will result in damage to the roof structure.
This transition should have step flashings. Proper step flashings will result in as many overlapping pieces as there are rows of shingles. This will guarantee proper drainage and prevent damage to the roof.
Obviously the flashings should also be counter-flashed. In both of the installations pictured above it is likely intended (or was intended) for a fascia to be installed to lap over the flashings. While this will likely prevent water from getting behind the improper continuous metal flashings from above—it will result in the higher roof shingles not overhanging the fascia enough to prevent water from wicking behind the fascia or into the upper roof structure.
Installation of some sort of metal counter-flashing will likely be necessary in conjunction with removal of the continuous flashings and installation of proper step-flashings.
By Charles Buell, Real Estate Inspections in Seattle
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