This post is a brief, modifed summary of an an excellent article published at Buildings.com.
Roofs on commercial and residential property serve one main purpose … to keep the weather outside! Roof flashing — pieces of sheet metal, aluminum or galvanized steel — act as a last line of defense against water leakage. The metal sheeting is placed over joints, around chimneys, fan and plumbing vents and stacks in the roof to prevent water seeping below the roofing material and into the building. The number one cause of leaks in any roof is faulty roof flashing.
Properly installed flashing works with gravity to help shed water onto the regular roofing materials when it is then channeled harmlessly to the ground. However, climate (especially the hot and varied climate of the desert southwest) can cause all kinds of failures in this otherwise perfect system.
Alternate freezing and baking temperatures can cause dimensional changes in the roof structure as wood and material shrink or expand. This can cause the flashing to pull away from the surface and no longer protect the areas it is supposed to. This is particularly true around vertical dormers, second story walls, parapet walls or any vertical surface on the roof.
In addition, flashing failure can also occur when the original roof specs are unavailable during a roof replacement or repair. A reputable roofing professional should be able to draft plans that will protect your roof during this process.
Finally, and perhaps the most critical element, poor installation is most often the cause for flashing failure. Installing flashing isn’t a DIY project, unless you’ve had plenty of training. Call a reputable roofing professional who is licensed, bonded and insured, and who is experienced roofing in your area and climate.