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Built-Up Roofing: An Historical Perspective

May 15, 2009

Along way from its crude
beginnings in the 1870s,
built-up roofing (BUR) systems
have grown in use and
protect many of the nation’s
commercial buildings. In its
nearly 140-year history, the BUR industry
has continued to evolve and set the bar for
performance criteria in commercial roofing.
Today, the industry faces several new
challenges. Petroleum refiners are able to
extract more from crude oil residuals, an
act that impacts the quality of the asphalt
produced. At the same time, changing economics
have caused refiners to turn from
asphalt production to more lucrative end
products, threatening the availability of
asphalt for roofing.1 Also, the price of
asphalt is expected to rise as the United
States government implements plans to
spend millions of dollars repairing roads
and bridges.2 All of this is taking place while
the commercial roofing industry is facing a
shortage of trained BUR installers.
Two critical changes – raw material
asphalt quality and the shortage of trained
installers – have reduced the latitude of
application conditions necessary to produce
a quality BUR. These factors have
increased the need for a registered roof consultant
to ensure a proper BUR construction.
HISTORY AND DEVELOPMENT
History demonstrates that failure to
account for mistakes of the past can produce
devastating results. In the late 1800s,
BUR materials were used to replace lead
sheets on commercial wooden buildings.3
BURs were constructed in situ, using alternating
layers of jute and tar or lake asphalt.
These constructions, though crude, used
largely unrefined bitumen that was rich in
natural constituents and provided an
improvement over conventional roof constructions
of the time.
Documented use of asphalt for roofing,
in preference to coal tar, began in the
1870s. However, its use became prevalent
as petroleum refining technology improved
in the early twentieth century. Asphalt
gained preference because it was easier to
handle and had a wider functional temperature
range than coal tar.
The fluid catalytic cracking process,
developed in 1942, established the foundation
of modern petroleum refining. Using a
catalyst in the cracking reaction increased
the yield of high-quality products. Several
complex reactions are involved but princi-
18 • I N T E R FA C E S E P T E M B E R 2009
pally, the long-chain hydrocarbons are
cracked into lighter products.
Changes in reinforcements for BUR
impacted performance, but the asphaltic
bitumen that formed the waterproofing
agent retained much of its chemical composition.
4 Since failure mechanisms have been
described as a function of asphalt’s basic
molecular or intermolecular chemistry, the
changes imparted by the refining process
determine the quality of the asphalt produced.
5
AIR BLOWING
Air blowing has been employed through
one technique or another to harden the flux
sufficiently to make it useful as an interply
adhesive for built-up roof construction.
Many use the term “oxidized” when they
think of air-blown asphalt. The process is
actually “dehydrogenation” and “polymerization,”
where hydrogen atoms are split off
from the parent hydrocarbon chains, and
smaller chains attach (polymerize) to form
larger-chain molecules. Most of the oxygen
that reacts with the asphalt in the blowing
process forms water vapor. Only 5% to 15%
of the oxygen introduced into the process
remains bonded in the asphalt. The amount
of oxygen in the finished product increases
with augmented aromaticity of the feedstock.
Thus, highly aromatic crudes produce
an asphalt that is chemically different
from those produced from crudes with low
aromaticity.
Investigators do not agree on the identity
of all compounds in which oxygen is
bound during air blowing, and the reaction
mechanisms that take place in the process
are still being investigated.6
LESSONS LEARNED
In the 1960s and 1970s, paper and
asbestos felts were being offered. Some of
the more interesting events occurred when
conventional “tried-and-true” four-ply BUR
constructions made with asphalt-saturated,
15-pound paper “rag” felts were replaced
with two or three heavier paper felts.
Advertising slogans such as “One Plus One
Equals Four” that promoted two-ply applications
were abandoned when those constructions
experienced widespread failure.
BUR REQUIREMENTS DEFINED
As a result of the extensive failure of
various BUR designs, Bob Mathey and Bill
Cullen of the National Bureau of Standards
(NBS, now NIST) traveled across the country
obtaining samples of installed BUR
membranes of all types of felts available at
the time. They transported the samples to
their laboratory for testing and produced
the first industry-accepted comprehensive
work that described the minimum properties
needed for a BUR system to perform
effectively.
Their results were reported in 1974 in
NBS’s Building Science Series 55, a document
that defined the minimum performance
attributes of a completed BUR membrane.
The study gave the industry a means
to determine the minimum performance of
any BUR membrane, regardless of construction.
The popularity and application of
NBS 55 criteria resulted in the development
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S E P T E M B E R 2009 I N T E R FA C E • 1 9
of ASTM D 2178 Type IV and Type VI fiberglass
felts used today in the construction of
BUR roof membranes.7
CHANGES IN ASPHALT
Among the more significant changes in
asphalt that impact roofing is that today’s
refineries can extract more fractions from
crude oil. Many refiners possess the technology
to rearrange the structures of one
fraction from crude oil to produce a different
fraction. Cracking takes large hydrocarbons
and makes them into smaller ones,
which can be recombined to make the
desired product. Catalysts speed up the
process.8
Additionally, Congress has mandated
production of low-sulfur fuels, which are
accomplished in the refining process by a
coker. Vacuum bottoms (the leftovers of
petroleum distillation) are used as feedstock
to produce either asphalt or coke. If low-sulfur
fuels are in demand, the vacuum bottoms
will be dedicated to fuel production
and not asphalt.
All of this means that today’s refinery
technology can eliminate asphalt production
from slate and remain profitable. It also
means that the quality of asphalt varies
according to the crude slate and the products
the refinery is making. Ultimately,
asphalt quality will be decided by the refining
companies.
Once the quality of the roofing materials
has been established, BUR quality is a
function of the accuracy of the applicator to
construct the roof membrane. Variations in
asphalt quality decrease the latitude of
application conditions, particularly the
application temperature of the asphalt that
forms the waterproofing layer.
Based on these observations, it is
apparent that the best BUR systems will be
constructed under the guidance of a registered
roof consultant who will ensure that
good roofing practices are observed. BUR
systems have come a long way, weathered
many storms, and remain an important tool
for roof consultants.
FOOTNOTES
1 Julia Poppen, “Drivers Face Bumpy
Ride—Blame It on Asphalt,” The
Rocky Mountain News, July 15,
2008.
2 Mike Pesce, Erin Phalen, Joel Rose,
Kai Ryssdal, and Ben Teplitz,
“Asphalt Shortage Makes Recovery
Tough,” Marketplace, American
Public Media, January 6, 2008.
3 Paul Morgan and Alan Mulder, The
Shell Bitumen Industrial Handbook,
Shell Bitumen, Riversdell House,
Chertsey, Surrey, England, 1995.
4 “Petroleum Refining and Catalytic
Crack ing,” The Encyclopedia Bri tan –
nica Online, June 7, 2009.
5 Raymond E. Robertson et al., As –
phalt Behavior as a Function of Its
Chemical Constituents, 1991.
6 John J. McKetta and William A.
Cunningham, Encyclopedia of
Chem ical Processing and Design,
CRC Press, 1977, p. 496.
7 William C. Cullen and Robert G.
Mathey, Preliminary Performance
Criteria for Bituminous Membrane
Roofing, November 1974, U.S.
Depart ment of Commerce, Center
for Building Technology, Institute
for Applied Technology, National
Bureau of Standards, Washington,
DC.
8 Craig Freudenrich, PhD, How Oil
Refining Works, January 4, 2001,
www.HowStuffWorks.com.
20 • I N T E R FA C E S E P T E M B E R 2009
Carter C. Slusher is a modified bitumen and polyiso systems
engineer with Firestone Building Products. He joined the
company in 1992 as its modified bitumen technical service
manager. Prior to joining Firestone, he was the corporate laboratory
manager for TAMKO Asphalt Products, transferring
SBS technology from Germany to the U.S. Prior to that, he
was a senior research chemist with Johns Manville
Corporation, establishing its SBS line and starting up its mod
bit production facility. He earned a BS in chemistry from the
University of Missouri. Slusher holds two patents for mod bit products. He has served
as an officer of ASTM Committee D 08 on roofing and waterproofing and received its
Award of Merit and was named a Fellow of the Society in 2000. Slusher is a past chairman
of the SPRI Modified Bitumen Committee; during his tenure, he developed a new
roofing standard for modified bitumen products.
Carter C. Slusher
Having to wait for the rain to stop before play can resume at Center Court, Wimbledon, England,
is now a thing of the past, thanks to a new retractable roof inaugurated May 17. If it starts to rain,
play is suspended until the roof is closed and the court surface and bowl have reached optimum conditions
for players and spectators. The 30,000-ton concertina-style retractable roof can be deployed in
complete silence in wind speeds up to 43 mph. It closes in a maximum of ten minutes, and the air
management system stabilizes within half an hour. If wind speed increases above 43 mph, the roof will
lock to ensure maximum strength and stability. The air management system controls humidity of the
court and prevents condensation on the inside of the roof or sweating of the grass.
The roof is constructed of 5,2000 m2 of tensile and durable Tenara fabric, concertinaed across the
span of the ceiling. Both 20% and 40% translucent types were used to avoid shadows or bright spots.
Held up by ten 77-meter roof trusses, each weighing 70 tons, the fabric deadens the sound of falling
rain. The main contractor was Galliford Try. The lifespan of the roof is expected to be around 50 years.
One hundred percent of the material used is recyclable.
By the way: if a player whacks a ball hard enough to hit the 16-meter-high roof and it comes back
down on the court, the player loses the point. Just in case you had to know.
— Roofingmag.com
WIMBLEDON’S
CENTER COURT
NO LONGER
SUBJECT TO
RAIN DELAYS