poking a hole in a blister might work, but the hole
nothing. If a blister is intact and unbroken, the
must be left open until the blister collapses unto
manual suggests leaving it alone, because blisters
the roof, which may take several days or weeks,
that hold pressure do not leak. In addition, blis-
and, therefore, is not a recommended approach.
ters should not be intentionally punctured but
The blister vent serves both to relieve pressures
rather should be monitored over time so they can
and prevent leaks. As shown in Figure 3, the vent
be repaired if individual blisters continue to get
consists of a hollow, threaded shaft covered by a
worse (and they will). Eroded bare spots on
special membrane enclosed in a plastic housing. It
blisters should be coated with a cold-process
allows gases to escape but inhibits water from
recoating/resaturant and sprinkled with gravel if
leaking into the roof.
needed.
Blister vents are easy to install. The installer
When a blister bursts, the usual repair process
merely cleans the area of gravel and dust, adds a
is to remove the entire raised portion of the blister
primer to the surface, pokes a hole into the blister,
and patch the remaining void with alternate lay-
applies sealant to the underside of the vent, and
ers of bitumen and successively larger diameters
of felt. Alternately, an x can be cut into the blister,
threads the vent into the hole. Once installed, the
the corners of the cut peeled back, the resulting
vent causes the blister to deflate and the raised
cavity filled with bitumen and the corners
portion of the blister eventually to collapse onto
pressed back into the bitumen. The blister would
the roof. In this position the roof membrane is less
then be patched as before.
susceptible to damage.
Roof breather vents have at times been promot-
The question has arisen as to whether a deflated
ed as a means of relieving internal roof pressures
and collapsed blister might reseal itself to the roof.
that lead to blisters. Unfortunately, as Figure 2
The answer is no, at least not of its own volition.
Figure 3. Cutaway view of the blister vent.
Figure 2. Cutaway view of roof breather
vent. Note its size and that it vents only the
Note its small size.
insulated space of a roof.
shows, such vents only communicate with the in-
Several blisters were cut open the year after they
sulated space of a roof and, as discussed earlier,
had been repaired with the vent and, though
blisters only occur at or above the top surface of
found to be watertight and flat, they were not well
the insulation. Furthermore, pressures large
bonded to the roof. The original, once-raised por-
enough to lift up the roof membrane do not occur
tion could be easily cut away from the roof. In con-
within the insulated space of a roof (Korhonen
junction with this test, several other collapsed blis-
1989). Consequently, roof breather vents do not
ters were coated with a resaturant to determine
prevent membrane blistering.
if it would penetrate the blister wall and reseal it to
The only way to stop an existing blister from
the roof. A year later the blisters were sliced with
growing larger and becoming a maintenance
a knife but could not be delaminated from the
headache is to depressurize it. The cut-and-patch
underlying roof membrane without causing dam-
approach achieves this goal but ends up being
age. What long-term effect adding such a coating
costly in time and money. An average of one hour
has on roof maintenance is unclear at this point.
is required to repair a single blister, which for
More study should be done with this interesting
material and labor can easily exceed . Simply
finding.
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