Roof flashings serve several purposes. It adds a finished look to your roof while helping to prevent wind and insect damage. However, flashing’s primary purpose is to ensure that your roof remains watertight. This is one roofing element that is sometimes ignored or poorly installed, but aside from the roofing material itself, flashings are the most important roofing components. Once you understand how flashing works, you’ll see why it is crucial to a long-lasting, watertight roof.
What is Flashing?
On any roof, whether flat or steeply sloped, flashing serves several purposes. Installers use it around the edges, in valleys, along walls, and around skylights, chimneys, vent stacks and other protrusions. You’ll find roof flashings available in a variety of materials:
- Painted Galvanized Steel
- Stainless Steel
- Zinc Alloy
- Roofing Felt
Most types of flashing are non-corrosive and non-staining, but some types will corrode when mixed with the wrong kinds of metal. Therefore, when you select your preferred type of flashing, make sure that it is 100% compatible with the roofing material, fasteners and other roofing components.
How Does Flashing Work?
Flashing protects against moisture and seepage in several ways. When properly installed, flashing factors how far wind-driven rain or cross wash can travel. It should extend far enough underneath both roofing and siding that no water can reach the underlayment.
One might assume that in many instances, caulking or a roof sealant would work just as well to seal seams and edges around roof protrusions. However, flashing is much more effective than sealants because sealants are susceptible to wear and thermal expansion. In other words, as the roofing material expands and contracts in response to varying temperatures, sealants tend to crack or lose their adhesion. Flashing, because it extends underneath roofing and siding, continues to provide a moisture barrier no matter how much the roofing material shifts.
Flat Roof Flashings
On flat roofs – particularly those made from EPDM – edge flashing is a two-piece system that does more than prevent leaks. Once the roofing membrane is in place, metal flashing is installed around the edges of the roof so that the flashing covers both the horizontal and vertical edges. This serves two purposes: It prevents wind-driven rain from making its way underneath the roofing membrane, and it prevents damage to the roof’s edge during high winds.
However, rainwater can still make its way underneath the horizontal edge of the flashing. The solution to this is to install a second layer of flashing that covers the horizontal seam between the metal flashing and the roof membrane. This second layer is normally an adhesive rubber membrane that seals to both the flashing and the roofing material. Not only does this flashing stop water from seeping under the metal flashing, but the rubber’s flexibility also means that it won’t crack or tear due to thermal expansion.
As you are planning your building’s new roof, make sure that you pay close attention to the types of flashing you will use and the ways in which it will be installed. Flashings, whether they are flat roof flashings, angled flashings to protect valleys or flashings designed for use around chimneys and stacks, are essential to building a roof that is long-lasting, watertight and resistant to wind damage.