The flashing system serves a number of purposes on the roof. It seals the roof membrane edges at walls, vents, curbs, drains, gravel stops, scuppers, headers, and penetrations. It allows for movement at expansion joints and other places where the roofing material is interrupted or terminated, while sealing out water. It also covers and protects the edges of the membrane at seams and penetrations, and can be used to conduct water from the roof through scuppers.

The material used for flashing depends chiefly on what the base membrane is made of. While there are many types of low-slope roofing, they can be roughly divided into five categories: single-ply, built-up, modified bitumen, sprayed-in-place urethane foam, and structural metal. Achieving a design that is as close to watertight as possible with these materials alone should be the architect’s first priority. There’s no substitute for a well-designed roof.

The basic elements of flashing design apply to all types of roofs. Base flashing is a continuation of the roofing membrane that is typically applied separately from the field application. The components, made of sheet metal, include cap flashing or counterflashing, which is applied to shield the exposed portions of the base flashing or to extend into the wall to divert any interior water to the exterior of the wall. Metal copings are used to seal and waterproof the top of a parapet or building wall. Edge flashings are used to hold the gravel in place on a ballasted roof or to finish off the edge of other types of roof. Expansion joints are structural separations that accommodate movement between two building elements, or at specific locations, such as where the roof deck changes direc- tion. Scuppers create an exit for water through a parapet wall or an elevated edge. Each of these metal flashing components require equal design consideration.

However, few architects are thoroughly schooled in the purpose, mechanics, and theory of flashing design, according to Jack Robinson, director of technical services for the National Roofing Contractors Asso- ciation (NRCA). “Roofing details rarely come from architects fully thought-out,” he says. “They tend to fall back on the manufacturer’s guidelines, which are generalizations, not job-specific details. Architects must take those generalizations and adjust them to the characteristics of the building.”

Ray Corbin, of the Better Understanding of Roofing Systems Institute (BURSI), agrees. “There is no such thing as a typical roofing detail. There is a tremendous amount of variation depending on where the building is, its structure, and the materials used."

Codes and standards provide some guidance for architects, but there's no substitute for education. Several roofing trade and educational organizations, including the NRCA and BURSI, offer courses and seminars. But according to a survey by the NRCA, not a single architectural school offers courses on roof detailing. “Architects aren't getting any training on this in school, but it should be in the core curriculum," Robinson says. "Something like 50 percent of all building-related lawsuits have to do with the roof. This is the most litigious segment of the industry."

Failing to properly detail and maintain flashings is expen- sive. From 1993 to 1998, professional liability insurer CNA/Schinnerer reports, indemnity payments on roof claims against design professionals averaged $77,000 per claim. While 65 percent of roof claims were closed without any payment to claimants, professional liability specialist Mike Maloney, of Petty Burton Maloney Associates in Rochelle Park, New Jersey, notes that the insurer’s average payout excludes the design professional’s related legal costs and hours spent defending the case, which add up quickly, even in cases without financial settlements.

“Good flashing design is certainly not the most glamourous aspect of architecture,” Robinson says. “Given a choice between creating buildings and detailing roofs, no architect would pick the roof. But understanding flashings is what keeps water out, and there is nothing more essential to a building’s survival.”

Flashing basics
Bituminous flashings are created on the roof by combining felts and adhesives. Single-ply and spray-on flashings, often made from the same material as the membrane, are usually made up in the field. Metal, which is used with all types of roofing, may be formed in a sheet metal shop or in the field. Most thermoplastic single-ply manufacturers have metal flashings coated with the membrane so they can be formed in the field and welded directly to the membrane, creating a very durable flashing.

Sheet metal flashing materials include aluminum, copper, lead, stainless steel, Monel (a nickel and copper alloy), steel, and zinc alloy. Galvanized steel is the most commonly used because it is easy to work with and tends to be less expensive than other materials. Copper is likely to last the longest, but it is expensive, and water running off of it stains many building materials. Which metal is selected also depends on cost, tensile strength (the stiffness of the metal), application details, the material’s coefficient of expansion, and its compatibility with the base membrane.

When preparing a flashing design, select a gauge appropriate to the application. Metals that are too light are more likely to be damaged or to blow off in heavy winds. A 24-gauge galvanized steel, for example, is fine for cap and base flashings, but 22 gauge is necessary for copings greater than 12 inches.

Within the past 10 years, increased attention has been focused on the effects of wind on roofs. The ability of metal flashings to resist uplift is directly related to the thickness of the metal used and the way it is attached. The vertical face is most prone to damage from wind, though the metal edge is also susceptible to problems because it is affected by wind blowing up the side of the building and across the top. Broad sheets of flashing are usually held in place with a continuous cleat so that wind will not go up and under the membrane, potentially causing condensation and other problems.

Different metals chemically react when they come into contact with each other, which leads to corrosion. Copper and steel, for example, are generally incompatible and must be separated when used on the same roof. Some metals will cause streaks and stains on cedar or redwood siding or trim. Compatibility problems can also arise between minor com- ponents, such as nails and screws. Stainless steel is a good choice for these because it will not react with most metals. The membrane manufacturer can offer advice.

Differing rates of expansion and contraction caused by temperature fluctuations also render some flashing materials incompatible with roofing materials, the deck beneath, and other structural elements. All of the elements of a roof move to some degree as weather conditions change. In cold climates, for example, materials shift significantly as sunlight heats the roof by day and later, when temperatures plummet at night. Also, temperatures on rooftops may vary considerably from those at other levels of a structure. The temperature and season when the flashing is installed are also significant. Tightly fastening metal when temperatures are warm means it won’t have room to contract when temperatures drop.

Metal moves more than nonmetallic building elements, such as wood or concrete. Metal flashings must be large enough to do the job but small enough to prevent unnecessary movement.

Dimensional changes are accommodated by specifying materials that move at similar rates or by otherwise building some flexibility into the design. There are two ways to control movement: by joining (soldering or otherwise permanently fastening) the laps so that the system moves as one piece, or by designing each joint as an expansion-control system. If the latter approach is used, the sections of metal must be adequately lapped at the joints and they must be short enough to limit movement. Cover and backer plates, though they require more installation time, shield lap joints—where water leaks are likely to occur. High-performance, elastomeric caulk also helps keep water out.

To allow the membrane and the metal flashing to move independently and still maintain a seal in all climates takes careful design. Sealants, caulks, and mastics, used in accordance with manufacturer specifications, can add an extra element of protection against water, though none of these is a substitute for proper detailing. Adhesives give added strength and should be specified for vertical grades. Keep in mind, though, that adhesive compatibility is an issue, as is proper application. Some solvent-based adhesives won’t adhere to modified bitumen or metal in cold weather, for example, because the solvent evaporates too slowly to form a bond.

Design tips
Thinking three-dimensionally helps architects understand the roof structure and how details work at perimeter joints, corners, and other locations where two-dimensional details don’t tell the whole story, according to roofing consultant and forensic expert Toby Nadel, AIA, in Dewitt, New York. “In situations where eaves and copings change planes or terminate against a wall, three-dimensional sketches can explore these transitions by evaluating the integrity, the aesthetics, and how watertight the termination is,” he says.

PERSPECTIVE DRAWINGS ARE THE ONLY WAY FOR THE ARCHITECH AND THE ROOFER TO INTERPRET METAL DETAILS.

“Perspective drawings are essential,” the NRCA’s Robinson agrees. “It’s the only way for the architect and the roofer to understand and interpret flashing details.”

Drawings are particularly important at well-known trouble spots—perimeters, non-wall-supported deck junctions, and parapet walls, to name a few—where leaks are most likely to occur. The following are typical applications for metal flashing components and techniques for improving their performance.

Roof perimeter. Differing coefficients of expansion between the metal edge and the roof membrane can cause either or both materials to split. Successful designs provide room for expansion, set the edge flashing in some type of mastic, and elevate the edge above water level so that the roof will not leak if there are splits.

Parapet walls. Allow sufficient height for all the components of the system so that any potential route for water to penetrate the roofing envelope can be blocked. An adequate base membrane also means that there is enough room to properly counterflash; otherwise the metal can wind up resting directly on the roof. The top of the base flashing should be protected with a through-wall or surface-mounted counterflashing, or a coping.

Base flashing: Flashing height should be installed eight to 12 inches above the highest point of a roof plane, including at parapet walls, the bases of mechanical units, crickets, and tapered insulation systems. In severe climates with heavy snowfalls, higher base flashing may be advisable to accommodate more thermal movement. Masonry parapets need to breathe, however, and architects often mistakenly detail flashing that seals the entire side of a masonry parapet wall, Robinson says. Cracks and spalling occur when moisture enters the wall and is trapped.

Fastening: Proper fastening on vertical elements is essential to prevent wind uplift and to keep the metal from simply sliding off as it wears. Nailing the counterflashing at eight-inch-on-center intervals is the norm, but six-inch intervals may be needed in some locations. Backing that up with a compatible adhesive adds strength.

Non-wall-supported deck junctions: When a separate roof deck structure abuts a wall, the two elements will expand and contract differently. This is an especially complex junction because it is an expansion joint and a wall flashing detail. Avoid doing anything that would fasten the wall and the deck, inhibiting movement. To that end, the metal flashing should be lapped—four inches is usually sufficient—to allow move- ment. Base flashing should be high enough to protect the adjacent wall and allow adequate space for the counterflashing.

Expansion joints: Premanufactured expansion joints, which come with sheet metal flanges and bellows (the flexible material that absorbs the movement), are easy to install. Sheet metal joint flashings, often with an accordion fold to absorb movement, may also be made in the field, which may be necessary when there are numerous slope or radius changes or joints that are otherwise complex. These must be carefully detailed, since both sides of an expansion joint cannot be fully secured. Compressible insulation within the joint cavity and a vapor retarder in the deck slow heat and moisture transfer. Cap these with sealant to keep water out.

“For some reason, designers think expansion joints end when they hit the end of the roof or slam into a parapet wall,” says Douglas Pearmain, a senior product design engineer with Johns Manville’s roofing systems division. “But continuing the seal is important.” Expansion joints should run up and over the parapet wall and make a logical transition into the edge of the roof.

In the field
To avoid roof failure, Nadel recommends inspecting several key elements during construction. Flashing materials should be clean and dry prior to application, relatively smooth, and of proper thickness. Ensure that flashing is solidly anchored to decks and walls. Understanding what is beneath the flashing determines the quality of the job. For example, the roof deck and the flashing systems may be of adequate quality, but if the deck has inadequate nail retention properties, the flashing will pull out under high winds or other stresses. Wood blocking should be of adequate thickness and height to receive nails for both the roofing membrane and the flashing. Cants and crickets should be securely anchored.

Construction scheduling also influences flashing details, says Richard Koziol, AIA, roofing consultant with Wiss, Janney, Elstner Asso-ciates Inc. in Northbrook, Illinois. Thinking ahead means all of the base flashings can be of the same height, saving the architect from specifying transitions and odd joint details during construction.

Selecting a qualified roofing contractor and conducting regular field inspections helps control quality. For large, complex roofing projects, owners should, and are often required to, hire a full-time roof inspector equipped with a camera, a notebook, and even a cell phone to report from the site. Architects can hire a clerk of the works, who will contact the architect when problems arise. Some federal or public contracts require continuous on-site supervision.

Proper flashing design should include a routine preventive maintenance program to extend roof longevity and minimize costly emergency repairs. Without proper maintenance, even flashing of the best design, workmanship, and materials will eventually fail. Roofs should be inspected after major storms, heavy winds, and during and after construction for damage caused by foot traffic, equipment, and faulty materials. Architects can assist owners by providing seasonal routine maintenance strategies and roof management programs.