Flashing and ventilation
Flashing is strips of thin sheets that work as additional waterproofing and make watertight seal at the intersections and transitions in the exterior surfaces. Flashings are usually applied at valleys, eaves, rakes, chimneys, and other penetrations. Flashing material should be compatible with the roof and last the same time. Copper, zinc, aluminum, galvanized steel, and vinyl are the most common materials for flashing. All the flashing materials should be coated with a primer as additional protection.
Drip edge is a kind of flashing that is installed along the roof edges. Drip edge is fastened with roofing nails using the same material spaced 8 to 10 inches. Roof eave is a vulnerable area for forming ice dams. Flashing is required along the roof eave to avoid water leakage. The drip edge should have 1/4 to 3/8 inch extension over the roof eave. Roof valleys carry high volume of water and have a possibility of water leakage. Valleys are generally in two types of open valley or closed valley. Open valleys are those in which the roof-covering material stops several inches short of the centerline of the valley. The valley is protected from moisture intrusion by a metal lining. For open valley, first the felt underlayment is installed on the valley and fastened with nails in two sides. The flashing comes over the underlayment and extends on each side. The flashing is nailed from the edge to the roof. Roof shingles should overlap the flashing for at least 6 inches. Closed valleys are those where shingles on one or both sides of the roof extend across the valley onto the adjacent roof slope. Because valley linings for closed valleys are protected by the shingles, the linings may be made of thick underlayment or thinner metal than that used for open valleys. There are two kinds of closed valleys: woven valley and closed cut valley.
In hot climates, the roof system requires attic ventilation to expel solar-heated hot air from the attic to lessen the building cooling load. Roof ventilation inhale the air from soffit vents and exhale it from gable vents or ridge vents. Some types of box vents or turbine vents are also used in some buildings. In some cases, electric turbines or fans are required for proper ventilation.
