likely leaks through microscopic cracks in the bi-
THE CAUSE
tumen moppings and along the felts themselves
Blisters develop from voids built into the roof
since felts can be microporous. This leads to the
either between the plies or between the bottom of
conclusion that blisters inhale and exhale daily.
the membrane and an impermeable substrate
It is this cyclic breathing that explains why
(Korhonen and Bayer 1986, Korhonen 1989).
blisters continually grow larger. However,
Voids can result from skips in the bitumen mop-
growth is possible only when the volume of air
ping, entrapped debris, bitumen bubbling, or
sucked into the blister at night exceeds the vol-
curled felt edges. Tight quality control on the job
ume of air forced out of the blister the following
can minimize many of these problems, but
day; the internal positive pressure that is created
is great enough to break the peripheral bond at
regardless of how carefully work is done, a per-
the edge of the blister.
fect, void-free membrane is difficult to fabricate
Contrary to popular belief, blisters do not
even under the best of conditions. Thus, some
grow day-by-day during the hottest part of the
blistering is likely in any bituminous built-up
year. That is because blister growth is very much
roof system.
dependent upon relatively cool night air as well
Blisters grow from the expansion of the gas (air
as on high daytime temperatures. During the
or water vapor) within a void under the intense
cooling phase, the raised membrane must stiffen
heat of the sun. The mere expansion of this en-
so as not to subside when a vacuum forms. If the
trapped gas does not account for large blisters,
membrane stays warm and pliable and is unable
however. What makes small voids grow into the
to maintain its shape, it deforms, reducing the
characteristic bloated humps several inches high
vacuum and thus the volume of air drawn into
to a few feet across is a breathing action driven by
the blister. With less inhaled air, the blister is less
thermal cycling. Pressure changes within mem-
Figure 1. The top graph shows the air and blister surface temperature
and the bottom graph shows the pressure difference between the inside
of a blister and the atmosphere. Note how small the pressures are.
likely to expand the following day, no matter
brane voids that cause blister growth are docu-
how hot the day may get (Korhonen 1989). The
mented elsewhere (Korhonen 1989), but they are
optimum growing conditions occur when the
summarized here to explain why blisters grow in
days are hot and the nights are cool, which corre-
size.
sponds to spring and fall.
Blisters cycle from positive to negative pres-
sures on a daily basis. As shown in Figure 1, a
blister typically goes from over-pressure during
WHAT TO DO
the heat of the day to under-pressure during the
Very few options are available for dealing
cool of the night. What's more, the pressures that
with existing blisters. The Roofing Industry Edu-
do develop are considerably below what might
cational Institute's "Roof Maintenance" manual
be calculated from theoretical considerations,
(RIEI 1980) provides guidance for repairing blis-
suggesting that a blister is not air-tight. Indeed,
ters. The number one repair procedure is to do
instead of fully containing pressures, air most
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