Water is a threat to buildings, and building components are designed to direct water away from or keep water out of buildings. A well-constructed roof with a successful drainage system is the first defense against water penetration. But, “joints in metal panels and trim create the opportunity for water intrusion,” says John Pierson, PE, engineering services manager, The Garland Co. Inc., Cleveland. “Since they are exposed on two sides, the corners of the roof also see the most abuse from wind, rain and snow.”
As a result, a watertight building needs flashing. Flashing keeps water from getting into places it doesn’t belong. Flashing is installed at or around valleys, corners, hips, eaves, vent stacks, mechanical equipment, solar systems and other locations. When installed correctly, flashing makes joints at these junctions watertight, while allowing the natural expansion and contraction of the building and its devices. Flashing also seals against penetrating unwanted air from the outside or heat loss from within a building. The Uniform Building Code in Section 1707 (b) stresses the importance of flashing for exterior openings.
Flashing comes in many different shapes and sizes and in a number of different materials, such as aluminum, tin-coated and galvanized steel, copper, plastic and rubberized asphalt. There are different types of flashing depending on its required function. “Standing seam, flat seam and corrugated metal panels do require different flashing,” Pierson says. “The differences are in the flashing height and locations of the sealants.” Flashing materials must have the same or greater life span as the building material itself. They must be shaped easily and have the capability to be easily soldered, brazed or welded.
Valley flashing is used at sections where a part of the roof meets another section at an ascending angle. The flashing is installed to prevent water flow from collecting in the “valley” between the two slopes. Step flashing is installed around chimney bases. Vent pipe flashing is installed over pipes or flutes which crawl along the roof. In almost all cases, the top of the flashing attaches to the building under the underlayment, and the bottom overlaps the roofing material, so water is directed away from the underlying structure of the building. Intermediate pieces of flashing overlap in the same manner.
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Two-part flashing systems consist of a base or step flashing that is usually in contact or laced into the primary roofing material, and these are then covered by a counter (or cap) flashing. The counter flashing is a separate piece of metal or building material that 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 so they can expand and contract independently under changing weather conditions. 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 that forms the outside surface of a wall. In chimneys and along brick walls, the counter flashing is almost always the same metal that is used for the base or step flashing.
Successful flashing installation
Successful flashing installation is a very complex and technical construction aspect that will determine whether or not your building will leak. This type of work is best left to experienced professionals, not DIYers. The best way to learn correct flashing techniques is to spend time with an experienced craftsman as he or she installs a complete roof. Inexperienced craftsmen too often place flashing against walls or bricks, hammer it into position and caulk to seal. This gets the job done quickly, but creates a relatively short-term leakproof sealant. The reason for this is economics. With countless roofing companies competing and aiming for the lowest quote, quality details are often overlooked. The faster flashing is installed, the cheaper it costs; but often, the less successful the result.
The art of installing flashings is so technical it cannot even be fully described in this article. Many books do not fully cover the subject and it would take several books to get complete coverage of all flashing applications. However, there is a standard installation practice for installing roof corner edges.
Don’t cut corners
For corner edges, the flashing must be bent and overlapped securely to ensure a watertight corner. Measure from the last piece of flashing installed to the corner of the roof. Add two inches to this measurement for the lap. Mark the new piece of metal flashing with a black marker for the cut line. Make two full cuts in the metal flashing with a pair of metal snips. Start at the mark on the top of the flashing. Cut down at a 30-degree angle from this mark. Begin at the same mark and cut down at a 30-degree angle on the opposite side to form a triangle. From there, cut a 1/8-inch slit at the bottom of the metal flashing face directly below the original mark at the peak of the triangle. Bend the metal flashing around the corner using the slit as the bending point at the roof corner. Hammer roofing nails every 6 inches along the top and bottom of the metal flashing until you reach the end. Lap the next piece of flashing over the corner piece 2 inches and continue.
Simple basic hand tools, such as hand tongs, pliers, needle-nose pliers, metal tongs and snips, along with common sense, are required for corners, says Frank Albert, owner and operator of Albert’s Specialty Roofing, Richmond, Va. “The roof material is turned up against penetrations to form a seamless water proof system, and the counterflashing is attached to the penetrations to keep water from entering behind the roof panels. They are ‘apart’ in that they do not get caulked together. The roof, the roof deck and walls all move independently of each other.”
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While flashing installation is very technical and its methods are very diverse, there are some common simple-to-follow standards. Look for loose nails and any damage to the seals at the edges of the flashing. Roofing cement can dry out and crumble away, exposing joints to water. Fasten loose nails and cover exposed nail heads with roofing cement. Renew flashing seals by chipping out old caulking and mortar along the edges of the flashing. Recaulk the joints between the roof and the flashing. It’s much easier to reseal the flashing than it is to patch a water-stained wall or ceiling. Flashings rarely need to be replaced unless they’ve been damaged, such as by a falling tree limb or other impact. Typically, they need only to be refastened and resealed. Fasten flashings using wide-headed roofing nails, or roofing screws with a washer underneath. Don’t drive existing fasteners back into existing holes, as they’ll just come loose again.
For a simple quality inspection, safely follow a drop of rain water as it moves down the side of the roof. It should be carried from surface to surface all the way down, never encountering an open seam or an upturned lip that blocks its progress. The success of a good metal corner is usually dependent upon good details. “This means collaboration between the designer, manufacturer and installer,” Pierson says. “Collaboration is the best way to ensure a successful project.”
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Applying sealant
A continuous bead of sealant is often needed with flashing to prevent water intrusion. Sealant fills imperfections present with metal-to-metal closures. “The adhesive materials, better described as sealants, are the true leakproofing of the corner flashing,” says John Pierson, PE, engineering services manager, The Garland Co. Inc., Cleveland. “They need to be placed so that they are not disturbed by the thermal movement of the roof, and they cannot be exposed to sunlight. UV exposure will cause sealants to fail prematurely.”
Flashing sealants are available in caulking tubes, and come in silicone, butyl rubber, thiokol and styrene butadiene. Indoor sealants must never be used for outdoor applications. To apply, insert the sealant into a standard caulking gun applicator. Cut the end of the sealant at a 45-degree angle, using a utility knife. Squeeze the gun trigger until it is tight against the tube of flashing seal. Apply the flashing sealant to the flashing area. Apply a bead of flashing seal, similar to applying silicone to a tub. Let dry for several hours without getting the flashing wet. Don’t confuse flashing sealant with roofing compound, which should not be used to stop leaks. It’s not a long-term repair and can cause more damage to the core building material. Roofing compound becomes brittle and inflexible within one year exposure to the elements.
