Asphalt emulsions and cut-backs have their place in the low-slope roofing market, but where does plain asphalt stand?
hot-applied built-up roofing (BUR) has lost considerable market share
to single-ply and modified bituminous systems, it remains attractive to
building owners who are re-covering or upgrading their roof systems. In
some markets, such as the Southwest, the labor force is more familiar
with hot-applied BUR, helping asphalt maintain its regional popularity.
This system has been around since the 1870s, according to this historical perspective on BUR by Carter Slusher. Hot-applied BUR has evolved considerably in its 140-year history, achieving these 10 favorable qualities.
Asphalt is a residual of petroleum distillation, with roughly 75% of
produced asphalt used in paving. Roofing grades of asphalt can be
packaged in cartons or delivered as a hot liquid using bulk tankers. As
the costs of petroleum rise, all related construction products follow
suit, including single ply, modified bituminous roofing, and
conventional BUR. However, this well-understood system remains widely
2) Use of recycled materials. Asphalt paving
is widely accepted with 45-50% recycled material content. Great progress
has also been achieved with recycling of asphalt shingles because these
are not contaminated with adhered underlayments and any fasteners can
be separated magnetically. Recycling commercial BUR has also progressed
considerably, but contamination with adhered insulation, mechanical
fasteners, asbestos fibers, and some surfacings remains more
3) Maintainability. This is a major strength
of BUR. Asphalt is easily soluble in mineral spirits, so cold patches
or tie-ins to the existing membrane are very simple and reliable. This
repairability is possible throughout the BUR’s life because
cross-linking or loss of plasticizer does not occur. Modified bituminous
materials are also compatible with BUR so patches can be made by torch
or cut-backs. Temporary patches, repairs, and even replacement are
possible under a variety of weather conditions.
This BUR quality is also well established thanks to over a century on
the market. Many BUR roof systems have been in place for 30 years or
more. Reinforcements have transitioned from mainly organic (rag or
paper) felts to glass fiber felts, which are less likely to blister,
split, shrink, or rot as a result.
5) Versatility. These
systems can be applied from dead level to vertical and anywhere in
between, making BUR a viable choice for any roof slope. They also offer
an appealing versatility during moisture testing for wet thermal
insulation – the moisture survey is conducted using infrared, nuclear,
capacitance, or vector examinations, and the roofing system will permit
partial replacement as long as leakage and insulation wetting is
6) Low VOC content. Low-fuming asphalts
are now available for situations where odor is objectionable. Pair them
with kettles that use an afterburner to consume volatiles released
during the heating cycle.
7) Unlimited storage. Asphalt
does not change while it’s stored in cartons and is not affected by cold
or heat. When it’s directly exposed to the weather, the thin coat of
applied asphalt can sustain some dehydrogenation (also known as mud
cracking), which is why opaque surfacings such as aggregates or coatings
8) Near-immediate bonding without cure time. Asphalt
is the original thermoplastic. Heated to its application temperature,
it solidifies in seconds and becomes traffic- and weather-resistant in
9) Compatibility. While asphalt-treated felts
and fabrics are the main reinforcements for BUR, hot asphalt is both a
weather-proofer and a construction adhesive. Hot asphalt provides
adhesion to facers on cover boards, thermal insulation boards, primed
structural concrete, and oil-free metals.
reflective coatings such as aluminum roof coatings and acrylics work
well to screen UV but may have adhesion problems where water ponds.
However, they’ll still adhere to weathered asphalt and asphalt
emulsions. Asphalt pairs well with many surfacing options, from glaze
coats, fibrated roof coatings, emulsions, aluminum, and acrylic coatings
to the traditional mineral-surfaced cap sheets, flood coat, gravel, or
10) Code and regulation compliance. BUR systems
meet UL Class A, B, or C depending upon surfacings, layers of
reinforcement, slope, and substrate. As a broad generality,
aggregate-surfaced BURs are typically Class A and mineral-surfaced cap
sheet systems Class B. Comprehensive listings are available in UL
directories, FM Global listings, and reports from other testing
facilities. Hail resistance and wind uplift resistance are also
The Disadvantages of Hot BUR Systems
built-up roofing remains popular, it does come with some disadvantages
that require careful consideration of your project’s requirements. It
requires labor-intensive application, with multiple layers necessary to
provide durability, strength, and puncture resistance. Hot asphalt can
burn skin and melt polystyrene foam insulation, which require extra
attention to safety and roofing product choice.
BUR systems are
constructed in the field, whereas most single-ply systems are produced
in a factory under stringent quality control and merely joined together
in the field. Combined with a tendency to use phased construction, in
which only the base sheet is applied to close in a building and
remaining layers are added later, this can invite malfunction if not
applied properly. Phased construction in particular invites moisture,
which leads to undesirable inter-ply blistering when it’s trapped in the
Richard (Dick) L. Fricklas was technical director emeritus of the Roofing Industry Educational Institute prior to his retirement. He is co-author of The Manual of Low Slope Roofing Systems
and continues to participate in seminars for the University of
Wisconsin and RCI Inc., the Institute of Roofing, Waterproofing, and
Building Envelope Professionals. His honors include the William C.
Cullen Award and Walter C. Voss Award from ASTM, the J. A. Piper Award
from NRCA, the William C. Correll award from RCI, and the James Q.
McCawley Award from the MRCA. Dick holds honorary memberships in both
ASTM and RCI Inc.
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