Flashing: the importance of following the specifications
The importance of properly installing sheet metal flashing on any type of roof cannot be overstated. While modern building materials may be waterproof, they cannot be expected to remain permanently resistant to moisture as the natural movement of buildings and the shrinkage of certain type of materials over the years may eventually lead to leaks.
Unwanted moisture can enter the building’s interior from any point on the roof. Any area where there is a seam, joint, or connection left unprotected can cause damage to the interior, disintegration of mortar and masonry, and rusting of steel spandrels, lintels, etc.
Three common types of flashing frequently installed for protective purposes are counter flashing (or wall flashing), valley flashing, and gravel stops (or other edge metals).
Counter flashing are used where a roof joins a wall and serves the purpose of turning water from a wall on to the roof. Counter flashing is used in conjunction with composition base flashing and must keep water from entering the building and allow for building movement. It is strongly recommended that the base flashing be applied over a cant strip and be extended up the wall a minimum of 10 inches above the roof line and that a minimum of 4 inches is covered by the counter flashing.
Valley flashings are used to protect the valley formed on shingle, slate, or tile roofs. The open portion of the valley should be a minimum of 5 inches. The edges of the valley should be formed with a hook edge, cleated on 2 ft. centers, lapped 8 inches in the direction of the flow, and with the top of each section fastened with nails of material compatible with the flashing. Copper (minimum of 16 oz.), or stainless steel (minimum of 26 Ga.) is recommended material for valley flashing, but painted galvanized (minimum of 24 Ga.) can be used when the expected life of the roof is less than 15 years.
Gravel stops (and other edge metals) are used on the perimeter, rakes, and eaves of roof and serve the purpose directing water in a certain direction or away from the wall. An edge metal should be a minimum of 4 inches by 4 inches, with a minimum of 4” laps, although 6 inches would be ideal.
On some occasions, spec writers, general contractors, or architects fail to realize the importance of having flashing installed on their roofs, choosing instead to use felt or mastic in its place. This will almost certainly lead to water damage and leaks, which could have been prevented through the proper installation of flashing.
In the installation of flashing, if caulking is not used under the flashing before being attached to the roof as well as applying a bead along the seam after placing the flashing down, leaks still may occur as water may be able to leak under the flashing.