Metal Flashing on
Low-Slope Roofs

Answers:

1. The fundamental components of a flashing system are the base flashing, which continues the roofing membrane that is applied separately; and the various sheet metal elements, including cap flashing or counterflashing, copings, edge flashings, expansion joints, and scuppers. All these parts serve specific purposes and deserve equal design consideration.

2. The major types of sheet metal flashing materials are aluminum, copper, lead, stainless steel, Monel (a nickel and copper alloy), steel, and zinc alloy. Galvanized steel is used most often because it is easy to work with, bends easily, and is less expensive than other materials. Copper is likely to last the longest, but it is expensive and its water run-off stains many building materials, including concrete and stone. Which metal is selected also depends on cost, tensile strength (or stiffness), application details, coefficient of expansion, and compatibility with the base membrane.

3. To accommodate dimensional changes, the architect should specify materials that expand at similar rates or build some flexibility into the design. Two ways to do this are to fasten the laps, usually with solder, so that the system moves as a whole; or to design each joint as an individual expansion control system. If the latter method is used, the metal must adequately lap at the joints—at least four inches—to provide room for movement. Cover and backer plates require more installation time but effectively shield lap joints where water leaks often occur. High-performance, elastomeric caulk also helps keep out water. Metal flashings must be large enough to do the job but small enough to prevent unnecessary movement.

4. Most roof problems are due to water leaking in and around the perimeter and at corners or junctions in the roof plane. Well-known trouble spots are perimeters, non-wall-supported deck junctions, and parapet walls. To avoid these problems, architects must think through the details three-dimensionally. Perspective drawings help architects think through the detailing at corners and transitions. Another cause of roof leaks is poor maintenance. Roofs should be inspected after major storms, heavy winds, and during and after construction.

5. Understanding what lies beneath the flashing determines the quality of the job. The roof deck and the flashing systems may be adequate, but the deck may have poor nail-retention properties. As a result, the flashing will pull out under high winds, temperature extremes, or other stresses. Wood blocking should be of adequate thickness and height to receive nails for both the roofing membrane and the flashing. Details, such as cants and crickets, should be securely anchored as well.