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Sealing Roof Penetrations

There are a number of ways to handle penetrations, depending on the type of penetration. First, some basic things not to do: • Do not dam or impede the flow of water on the upslope side of roof penetrations. Standing water and metal panels do not agree, and panel corrosion will certainly result. • Do not use lead or copper flashings, which will corrode the panels from galvanic action. • Do not under any circumstance smear black goo anywhere on a metal roof. It is an ineffective and unsightly remedy, and it will require repeated maintenance. Plumbing vents and furnace flues. Round penetrations of almost any size are easily flashed with special rubber boot jacks that have a moldable aluminum compression ring, such as ITW Buildex's Dektite and Aztec Washer's Masterflash. The ring can be formed on site to custom-fit almost any panel rib profile (Figure 3).


Figure 3. To seal round penetrations, use a special rubber boot jack with a moldable aluminum compression ring, like this one from ITW Buildex. The ring can be formed to fit almost any panel rib profile.

These boot jacks are available in various sizes and colors, and some are made to withstand high temperatures. Most boot jacks slip over the top of the stack, but some versions can be wrapped around the pipes. This is especially useful for sealing an electric service mast, for example, when you can't easily disconnect the power. Skylights, chimneys, and roof hatches. Rectangular penetrations can be handled with sheet metal flashings in some cases, and with "preformed" curb flashings in others. To accommodate differential movement between the roof penetrations and panels, use a two-piece flashing, as shown in Figure 4.


Figure 4.

Use a two-piece metal flashing to accommodate the differential movement between masonry and metal roof panels. The base flashing is rigidly fastened to the roof panel, but not the counterflashing. The "L" in the base flashing should extend at least 6 inches above the roof surface. The base flashing is rigidly connected to the roof panel with washered screws. Apply tape sealant between the roof panel and the flashing, and be sure the screw pierces the sealant or is placed on the dry side of the tape, not on the weather side. Use tape sealant between the base flashing and the counterflashing, but do not rigidly connect these with screws. The upslope side of a rectangular penetration is a bit more difficult. The panel itself must end a sufficient distance from the penetration to allow room for a cricket, or diverter, to direct the flow of water around the penetration. This cricket flashing must be inserted beneath the upslope panel and sealed to it with butyl tape. The voids at the cut edges of the roof panel are sealed with rubber closure strips. Most panel suppliers sell closures to mate with their panel profile. The closures are usually available in both male and female (inside and outside) configurations. To make the transition between a cricket and the side flashings watertight, as well as attractive, is a true art that is best learned from an experienced installer. To simplify flashing rectangular penetrations, use preformed curbs. These curbs are one-piece designs of all-welded construction that include a built-in diverter. The curb slides under the roof panel above and overlaps the panel below. It is sealed with one or several beads of butyl tape, and screwed down. The fabricator must know exactly what panel type is being used so that he can match the curb flanges to the rib spacing and the profile of the panel. However, these curbs cannot be color-matched to your roof. If this is a problem, fashion a sheet metal shroud from flat roofing stock to cover the curb. Rake and headwall flashing. A two-piece sheet metal flashing should be used at rakes. To seal along a headwall above a shed roof, add rubber closures to seal the void at the end of each rib. Make sure the bottom leg of the two-piece flashing extends well over the rubber closure to protect it from sunlight.