Commercial Roof Flashings

   Flashings are the most common source of roof problems. Many flashing failures, probably the majority, stem from the designer’s failure to provide adequate flashing details on the architectural drawings.  Depending upon contract and fee structure, the designers may not even have the responsibility for coordinating roof design with installed equipment. As a consequence, the roof becomes an elevated dump and work platform for equipment that the designers cannot comfortably include in the building interior. 

   A modern commercial roof is penetrated by a host of components: electrical conduits, piping, ducts, vents, stacks, antennae and others. All roof-mounted equipment required flashing. For this reason, the designer must be well grounded in the principles of flashing design. Flashing must seal the joints at gravel stops, curbs, chimneys, vents, parapets, walls, expansion joints, skylights, drains, built-in gutters and other places where the membrane is interrupted or terminated.

   Base flashing are essentially a continuation of the roof membrane at the upturned edges of the watertight tray. They are normally made of bituminous, elastomeric, plastic or other non-metallic materials, applied in an operation separate from the application of the membrane itself. Counterflashings (or cap flashings), usually made of sheet metal, shield or seal the exposed joints of base flashings.    

   Base flashing for modern single-ply membranes are generally the same as the membrane material – PVC, TPO, EPDM, etc. Its superior plasticity enables it to fit tightly against backing. For inside and outside corners usually use premolded flexible boots.

   For wall and parapets base flashing in built-up membrane systems the best choice may be dictated by anchorage provision. A precast concrete parapet or similarly nonnailable wall section requires cold-applied flashing cement, because hot-mopped flashing must be nailed.

   Uncured EPDM offers a superior combination of qualities for flashing: easy installation, excellent ozone and ultraviolet resistance and flexibility at extreme low temperature. As recent innovation, polyester “fleece” is bonded to the back of EPDM and plastic membrane sheets that creates better adhesion.     

   Counterflashings (or cap flashings) shields the exposed joints of base flashings and shed water from vertical surfaces onto the roof. Counterflashings must be rigid and durable, so metal generally proves as the best material. The metals used for counterflashings include galvanized steel, copper, aluminum and stainless steel. Where no counterflashing is provided, the top edge of the base flashing usually terminated with a mechanically anchored pressure bar.    

Residential Roof Flashings

   The sources of a wide majority of residential roof leaks are roof flashings. For the most part, the correct installation of flashing material is what determines whether or not your roof will leak. The reason for this is quite simple. The vast majority of roofing products are quite easy to install. The manufacturers have engineered these products to be practically leak proof when they are installed on a simple roof.

   However, there are very few simple roofs. Examples of simple roofs are dog houses and small outdoor sheds. You find at list a plumbing vent stack, chimney, fan vent, pot vent, valley or side wall projecting above the roof.

   The engineering concerning flashings has been known for hundreds and hundreds of years. The system depends almost entirely on gravity. Gravity works with the flashing material and sheds the water onto the regular roofing materials. If you use a flashing material that depends on some other method to shed water you can quite possibly develop a leak.

   For the most part, flashings are the transition materials and media between the primary roofing materials and the things that don't get covered with roofing materials. For example, when a chimney projects through the center of a roof, the roofing products must terminate against the chimney. However, this junction must be engineered to allow for expansion and contraction and be leak-proof. This sounds like a difficult task, but it is not. This same condition is true for any other thing that projects through your roof or where two sloping roofs form a valley, saddle or a cricket.

   Many people expect roofs to perform for a minimum of 15 to 20 years. Some roofs can be expected to last 50 to 80 years. This means that the materials which are used for flashings must have the same, or greater, life span as the roofing material itself. These materials must also be able to be shaped easily and have the capability to be easily soldered, brazed or welded. Copper, tin coated steel, lead and galvanized steel are examples of traditional flashing materials.

   Flashing systems must be able to handle the constant expansion and contraction between roofing products and those things which are not roofing products. This battle of expansion and contraction can take place on a daily or seasonal basis. The sun can create extremely hot surface temperatures during daylight hours, causing roofing materials to expand. At night, the temperature drops and the materials contract. Seasonal changes can cause the framing lumber to shrink and swell depending upon humidity and wood moisture content. This shrinking and swelling causes the roof sheathing and rafters to move. The flashing materials must be able to withstand this movement and continue to be leak-proof. Well-engineered and installed two-part metal flashing systems can handle this movement with no problem. On new construction, the framing lumber can shrink dramatically in the first year after construction.

   Two part flashing systems consist of a base or step flashing which is usually in contact or laced into the primary roofing material, and these are then covered by a counter flashing. The counter flashing is a separate piece of metal or building material which laps over the base or step flashing. It should not be attached (mechanically or soldered) to the base or step flashing. The two materials must be able to move independently of one another. The counter flashing can be made of a wide variety of materials. It can be wood siding, stucco, vinyl or aluminum siding, or other material which forms the outside surface of a wall. In chimneys and along brick walls, the counter flashing is almost always the same metal which is used for the base or step flashing.

   Flashing materials which can rust (tin coated steel or galvanized steel) should always be painted. They often are not painted correctly by roofers. These metals must have special primers applied first and then the proper number of finish coats of paint.

   Flashings are, for the most part, the most technical aspect of roofing. Professionals realize the importance of flashings. They know that flashings are the weak link in the chain.

   Our Main Office is located in Toronto, Ontario.