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 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
• 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).
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
To accommodate differential movement between the roof
penetrations and panels, use a two-piece flashing, as shown in
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
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
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
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.