Choosing Flexible Flashings - Continued
On most job sites, peel-and-stick flashings are installed
without a lot of fuss. Typically, the flashing is cut to
length, the release paper is removed, and the flashing is
pressed in place by hand (Figure 7). But the easy way may not
be the right way. Some manufacturers recommend that substrates
should be primed before installing their peel-and-stick
flashing, and that pressure should be applied with a roller,
not the palm of the hand.
7. Builders use a variety of methods for
flashing windows. This installer has chosen a
belt-and-suspenders approach, installing strips of
FortiFlash, a rubberized-asphalt flashing, on top of
strips of E-Z Seal, a kraft-paper flashing laminated
with polyethylene and fiberglass
Is a primer necessary?
Self-sticking flashings often adhere better to a primed surface
than an unprimed surface. Manufacturers that recommend priming
generally focus on concrete and masonry as the most problematic
surfaces, partly because those surfaces can be dusty or damp.
Other manufacturers specify that OSB and gypsum sheathing need
to be primed, and a few recommend priming metal and plywood.
Most manufacturers agree that in cold weather, a self-sticking
flashing will adhere better to a primed than an unprimed
surface. There is a Catch-22, though: When it's too cold for
peel-and-stick, it may also be too cold to apply primer.
In any case, few residential builders are likely to take the
time to prime plywood or OSB sheathing before using
peel-and-stick, which is one
reason some manufacturers omit the recommendation. If you do
decide to prime, remember to use the primer recommended by the
flashing manufacturer, since the wrong primer may cause
Hand pressure or roller?
Many, but not all, manufacturers recommend that their flexible
flashing should be installed with a steel or hard-rubber
J-roller — the same type of roller used for gluing
plastic laminate countertops. Many manufacturers' reps admit
that this recommendation is widely ignored, but doing so
carries some risk: When it comes to priming and using a roller,
the bottom line is that builders who deviate from a
manufacturer's recommendations can't expect any support from
the manufacturer if something goes wrong.
Use With Care
Despite the versatility of flexible flashings, they have their
limitations and must be used with common sense. Some builders
have reported adhesion problems with peel-and-stick. Others
note that too much peel-and-stick can create a wrong-side vapor
How tacky? Peel-and-stick
doesn't always stick. "I've been to sites where I've seen the
peel-and-stick already half falling off the housewrap," says
Patricia McDaniel, owner of Boardwalk Builders in Rehoboth
Beach, Del. Poor bonding can be due to a variety of factors,
including low temperatures and dirty substrates (Figure 8).
Manufacturers agree that the adhesive bond of peel-and-stick
flashings varies over time. Initially, for the first month or
so, the bond should actually get stronger. But no one really
knows when, if ever, the bond strength may begin to fail.
8. Peel-and-stick flashing adheres poorly to
dirty substrates or when applied in cold
Building scientist Joe Lstiburek urges caution. "A problem
with these membranes is that they can peel away," says
Lstiburek. "Don't rely on the adhesive property for
In an informal JLC test, 21 different peel-and-stick
flashings were bonded to wood for 14 hours. About half of them
failed to make a waterproof seal. Although further curing might
have resulted in a waterproof bond, the test shows the need for
caution when depending on an adhesive alone to seal out
Wrong-side vapor barrier.
Peel-and-stick membranes should be used sparingly on wall
sheathing, since they can create a wrong-side vapor barrier.
"If you put a big hunk of peel-and-stick on the sheathing,
interior moisture can condense behind it, causing rot," says
Lstiburek. "It doesn't happen very often, but it happens
occasionally. You've got to be careful not to get slap-happy
with the stuff."
Several builders in British Columbia report finding sheathing
rot behind peel-and-stick membranes, especially at window
heads. But most investigators say that an important
contributing factor in these cases was the use of damp framing
lumber, and there don't appear to be any reports of such
problems in other parts of North America. In fact,
manufacturers of peel-and-stick membranes confidently recommend
their use at window heads. "Clearly, vapor is an issue," says
Rick Scruggs, technical service specialist at Grace
Construction Products, a manufacturer of rubberized-asphalt
flashing. "We wouldn't like to see you cover the whole wall,
unless there are provisions for the vapor to get out. But if
just a narrow strip of membrane is used around a window, the
vapor can escape from other areas."
Many peel-and-stick manufacturers recommend the use of their
products under siding at areas subject to splashback. However,
because of the wrong-side vapor barrier problem, such an
application is controversial. "The use of self-adhering
membrane at splashback areas concerns me greatly," says Bob
Switzer, chair of the Canadian Home Builders Association of
British Columbia. "You are far better off finding ways to
prevent the splash, like replacing the soil with lava
Yet many builders confidently use peel-and-stick to protect
sheathing from splashback. "We use it at the splashback area
all the time," says McDaniel. "I think you are much more likely
to get water into a structure with bad flashing details than
vapor problems, at least in our climate."
Over or under the housewrap?
One debate that won't be settled soon is whether peel-and-stick
should be applied directly to the sheathing or is best applied
to the housewrap or felt. There are strong advocates for both
Some manufacturers recommend that their peel-and-stick
membrane should be applied directly to the sheathing. "In
general, the membrane should be adhered directly to the wall
sheathing, and not to a layer of felt or housewrap. What's the
value of the membrane if it can't be fully adhered to the
substrate to prevent water from getting behind it?" says
Scruggs. McDaniel agrees. "Installing housewrap and then
slapping windows in and then attaching peel-and-stick to the
housewrap doesn't do anything," she says.
By contrast, builders who are worried about a wrong-side vapor
barrier prefer to see a layer of building paper or housewrap
between the sheathing and the peel-and-stick. "Rarely do I
apply a piece of peel-and-stick directly to sheathing," says
Randy Faustmann, president of Rainforest Envelope Protection
Services, a consulting firm in Langley, B.C. "Usually, it is
installed over the building paper. I think that having the
layer of paper between the peel-and-stick and the sheathing
allows a little bit more drying than it would without
No matter how you assemble your sandwich of flexible flashing,
nailing fins, and building paper, everyone agrees on one point:
Lap all the layers to shed water. "You have to lap your layers,
because at some point the glue's going to give," says McDaniel.
"Physics is going to win over chemistry."
Martin Holladayis an associate editor at The Journal of