During my 30 years in the roofing business, I've installed
many types of roofing and seen a number of products come and
go. But one system that never goes out of style is the
traditional built-up hot-mop roof. I learned how to do built-up
roofing (BUR) during my early years in the trade, and my
company continues to do it to this day.
It's not that we have anything against other types of roofing:
We also install wood shingles, asphalt shingles, tile, and
slate. But if a roof's pitch is shallower than 4 in 12, we're
limited to using BUR or some kind of single-ply membrane, and
my preference is the built-up roof.
BUR vs. Newer Systems
I've installed various kinds of single-ply membranes for
previous employers, and until recently my company also did some
modified bitumen torch-down roofing. While single-plies
typically cost more than BUR, the installed cost for torch-down
is about the same. Torch-down became popular with roofers and
GCs several years ago because it's flexible, takes less labor
to install than BUR, and doesn't require the use of liquid
asphalt (see "Torch-Down Roofing Basics," 8/01). The problem
was that so many house fires were started with the torch,
insurers have made torch-down too expensive for many roofers to
Fortunately, we never stopped doing conventional hot-mop
roofs. The method has been around for more than 150 years and
has a great track record. Depending on the design, a properly
installed hot-mop roof will last 15 to 25 years. It's one of
the least expensive low-slope systems out there, so it has a
low life-cycle cost.
I like BUR because, unlike single-ply membranes, it has a
built-in redundancy, thanks to the multiple layers and
As a GC, it's unlikely you'll ever install BUR yourself, but
if you are advising the client or supervising the job, it helps
to understand how the system works and what it takes to lay up
a durable roof.
Components of BUR
My partner, Barry Wilkes, uses the analogy of a layer cake to
describe BUR to customers. The base sheet and felts are the
cake, and the hot asphalt is the icing that gets spread between
the layers and over the top. The aggregate is like the
sprinkles that get scattered across everything.
The asphalt fuses the felts together, forming a waterproof
membrane. The felts reinforce the asphalt and prevent it from
cracking. The aggregate protects the surface of the membrane
from fire and mechanical damage — and, even more
important, it prevents the asphalt from being degraded by the
sun's ultraviolet rays.
A typical built-up roof has a base sheet, two or three layers
of felt, and three or four layers of asphalt. It's about 1/2
inch thick without the aggregate and about 3/4 inch thick with
the crushed stone. A full tar-and-gravel system weighs about
450 pounds per square. If, instead of gravel, you use a cap
sheet (an asphalt-impregnated felt covered with a thin coat of
aggregate), the roof covering will weigh about 220 pounds per
Substrates. We do most of
our residential work on existing homes and additions. The
process of installing a built-up roof is the same for both,
with one difference: On an existing structure, the old roofing
has to be removed and the substrate may need to be repaired.
BUR can be installed over wood, concrete, steel, or foam
insulation — but in residential, it's nearly always
In newer construction, the roof will be sheathed with plywood
or OSB. Houses built before 1950 may have wood board sheathing
(see Figure 1). The 1/4-inch gaps between boards are not a
problem, but if the surface is excessively lumpy it has to be
skinned with 1/2-inch plywood or a layer of perlite insulation
Figure 1.BUR is applied over wood, concrete,
steel, or foam insulation. The original part of this house has
one-by board sheathing, but the addition has plywood. Either
substrate is fine, as long as it's structurally sound and
BUR can be installed on slopes of up to 4 in 12. It can also
be installed on dead-flat surfaces, though it's better to have
at least some slope, so the roof will drain. A flat roof can be
sloped by covering it with a layer of tapered insulation, but
most customers are unwilling to spend the $150 per square it
takes to buy and install this material. Sloping a built-up roof
does not prevent leaks, but it does reduce the amount of water
that gets through if a leak occurs.
Asphalt should not be applied to damp surfaces, because
trapped moisture can cause blistering.
Cant strip. The fiberglass
felts we use have a memory, meaning they'll break or try to
flatten back out if you bend them into the 90-degree inside
corners at parapets, chimneys, and curbs. If the felts lift,
this defect, called a fish mouth, weakens the membrane and
creates paths for water to get into the building.
To avoid this problem, we install 45-degree cant strips at
inside corners. The strips, made from a composition material
containing perlite, are nailed 12 inches on-center (Figure
Figure 2.Fiberglass felts will not take tight
bends, so the crew installs cant strips at inside
After the cant strips are installed, we cover the sheathing
with a layer of red rosin paper, which functions as a slip
sheet for the base sheet to follow. The base sheet is an
asphalt-coated "fabric" mat. Years ago, these mats contained
asbestos. Manufacturers later switched to organic fibers, which
tend to absorb moisture and rot. The mats are now made with
synthetic materials like fiberglass.
We normally use a Johns Manville base sheet called PermaPly
28. It's a heavier version of the felt used to reinforce the
hot asphalt. The base sheet needs to be heavy because it's
nailed to the deck and you don't want fasteners pulling
through. It also has to span small gaps in the sheathing.
The base sheets are applied shingle-style up the roof with
approximately 2 inches of overlap at the joints (Figure 3). On
wood substrates, the base sheet is typically fastened with 6d
nails with 1-inch caps spaced 6 to 9 inches on-center at the
edges and 12 inches apart in the field. The base sheet runs to
the top of the cant strip and is cut or notched to fit around
plumbing vents and other penetrations.
Figure 3.The base sheet goes over rosin paper and
has 2-inch laps.
After the base sheet is down, we install flashings. The
project pictured in this article was a reroof, so we had to
replace the flashings around vent pipes and at the scuppers. If
there hadn't been a parapet, we would have installed gravel
stop flashings at the edge. Flashings come in aluminum or
galvanized steel; either type works on a hot-mop roof.
We bed the flashings in plastic roof cement, nail them to the
sheathing, and prime them with a spray-on or brush-on asphalt
primer (Figure 4). With torch-down, it's standard procedure to
lay up the entire roof, then patch in around scuppers and
plumbing vents. It's possible to do this with hot-mop, too, but
installing the flashings before the asphalt and felts are
applied leaves fewer seams (Figure 5).
4. A roofer beds a scupper
flashing in roofing cement (left) before fastening it over the
base sheet (right).
Figure 5.Flashings are primed to make the hot
asphalt stick better. Note the caps under the nails that hold
the base sheet to the roof.
The fumes from molten asphalt are a suspected
carcinogen and, with repeated heavy exposure, can
produce a number of negative health effects. This is
primarily an issue only for roofers and people who
manufacture roofing products. However, in some
jurisdictions there are rules about when, where, and if
you can use hot asphalt. In some cases, you can comply
by using a low-fuming product or a kettle that's
equipped with a filtration device. In other cases, you
may need to switch to a cold-applied bitumen or use a
different roofing system. — D.L.
There are two kinds of asphalt used in residential BUR:
standard and modified. Both are made from bitumen, but the
modified materials contain additives that increase flexibility
and lessen susceptibility to fatigue.
There are also different types of standard asphalt. Some are
designed to produce fewer fumes when melted, but the main
difference between types has to do with softening points.
Standard asphalt is available in types 1, 2, 3, and 4 (Figure
6). The lower the number, the lower the softening point,
measured in degrees Fahrenheit. Asphalts with low softening
points weather particularly well because they remain more
flexible in cold weather. However, high outdoor temperatures
can soften asphalt, and if the pitch is steep enough it may
begin to slide down the roof.
Figure 6.Different types of asphalt have different
softening points. The type 3 material shown here softens at
about 200°F but is applied at over 400°F.
To prevent that from happening, choose a type of asphalt based
on local weather conditions and the slope of the roof. Type 1
works for dead-flat roofs and types 2, 3, and 4 suit
respectively steeper pitches.
Where we work, the weather is moderate; temperatures rarely go
below freezing or above 85°F. But if we were to do a job 20
miles farther inland, the roof would have to withstand summer
temperatures that frequently exceed 100°F. The roof in the
photos was made with type 3 asphalt; farther inland, we would
have used type 4.
Melting. Most contractors
have never been up on the roof while BUR was being installed,
but everyone has seen the kettle on the ground, which is built
onto a trailer and uses propane to melt the asphalt (Figure 7,
facing page). My kettle holds up to 350 gallons and is equipped
with a pump that moves asphalt to the roof through steel pipes
(Figure 8). The asphalt we use comes in 100-pound cylindrical
plugs; we break the plugs into smaller pieces with an axe and
put them in the kettle to melt.
Figure 7.A roofer carefully slips another chunk of
asphalt into the kettle. Splashed asphalt can cause serious
burns, which is why he is wearing gloves, long sleeves, and
8. On small jobs, the hot asphalt
is carried to the roof in buckets. This job was large enough
that it made sense to pump the asphalt up in temporary pipes
before transferring it to buckets or wheeled
It's important to maintain the proper temperature when melting
and applying asphalt. If the kettle is too hot, the asphalt
could catch fire or be degraded by overheating. If that
happens, it won't perform properly.
We frequently use Trumbull's type 3 asphalt, which is designed
to be applied to the roof at 410°F, plus or minus 25°F.
At this temperature, it has the correct viscosity to properly
bond the felts. If the asphalt is too hot, it goes on too thin.
If it's too cool, it will go on too thick and will not fully
bond the felts, producing a membrane prone to cracking. Of
course, the kettle must be hotter than the application
temperature because the asphalt will cool on its way to the
Once the hot asphalt gets to the roof, we transfer it to
buckets or wheeled carts and haul it to the area where we're
working. Hot asphalt can give you a really nasty burn, so
whenever we move it we yell "Hot!" to alert everyone around to
be on guard. It's a bad idea to work below a hot-mop roofing
crew: If a bucket is tipped, hot asphalt could slosh over a
skylight curb or pour off the edge of the roof.