Roof Blisters
Cause and Cure
CHARLES KORHONEN AND BRIAN CHAREST
INTRODUCTION
obtained the rights to manufacture the vent and
Since 1974, when the National Roofing Con-
in 1990 they introduced the blister vent to the
tractors Association began surveying its members
roofing market.
to identify recurring roofing problems, blistering
Acceptance and use of the vent by building
has been the most commonly reported problem
owners and roofers has been slow. In a potential-
on bituminous built-up membrane roofs. In 1993,
ly huge U.S. market containing thousands of
bituminous built-up roof systems accounted for
built-up roofs, most of which are expected to
nearly 31% of the .4 billion commercial roof-
have at least some blistering, vent sales have not
ing market. Of this market .6 billion, or 77.5%,
been impressive, amounting to only several
was attributable to roof failures (LaValley 1994).
thousand per year. (Cor-A-Vent reportedly gives
more samples away as part of their advertising
Since blistering is a large problem, accounting for
than they sell.) Part of the problem of poor sales,
24% of all deficiencies reported for bituminous
at least initially, can be attributed to the newness
roofs (Cullen 1993), it is safe to assume that blis-
tering is a large cause of roof failures and associ-
of the vent--few people were aware of it, and
ated reroofing expenses each year.
there can be a natural resistance to try anything
new. A high selling price also presents a barrier
Blisters are spongy, dome-shaped areas caused
to widespread use. In addition, the notion that a
by gases expanding beneath or within roof mem-
reactive maintenance approach (repair broken
brane plies. Their effect is to shorten membrane
life by dramatically increasing its vulnerability to
blisters) as opposed to a proactive one (repair
physical damage and to weathering. The sloped
them before they break) was the appropriate
sides of a blister will cause aggregate surfacing to
solution to blisters further dampened potential
roll down hill, exposing the remaining flood coat
sales.
and felt to increased embrittlement by ultraviolet
In an attempt to transfer this new technology
rays. Dropping tools on blisters or stepping on
to the military community, CRREL, as part of the
them can easily burst them, allowing direct access
Corps of Engineers Facilities Engineering Appli-
for water entry into the roof system.
cation Program (FEAP), conducted two demon-
In 1988, after measuring pressures within some
stration projects, one that compared contractor to
blisters and physically examining many others,
in-house vent installation in 1990 and another
the U.S. Army Cold Regions Research and Engi-
that supplied vents directly to various DoD in-
neering Laboratory (CRREL) developed and pat-
stallations for their evaluation and use in 1992.
ented a miniature pressure relief vent designed to
Valuable information was developed from both
be inserted into the top of blisters to deflate them
demonstrations.
and prevent them from returning. This vent was
This paper traces the development of the blis-
seen by many as the perfect way to extend the life
ter vent by reviewing the cause of roof mem-
of a blistered roof and to minimize any risks asso-
brane blisters, details the findings from the two
ciated with blisters. In 1989, Cor-A-Vent, Inc.
demonstration projects, and recommends how to
(16250 Petro Drive, Mishawaka, Indiana 46544)
overcome some of the barriers to its use.
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