Built-Up Hot-Mop Roofing- Continued
Mopping in the Felts
The roof is laid up from low to high, a section at a time. We
apply the asphalt with a large fabric mop. The first coat goes
on the base sheet and is immediately followed by a layer of
felt, which must go on while the asphalt is hot. We then lay
successive layers of felt onto freshly applied asphalt. The
roofing stiffens and solidifies as the asphalt cools.
The roof shown in this article has four layers — a base
sheet plus three layers of felt (Figure 9). Built-up roofs
typically have three, four, or five layers. A roof with three
layers should last about 15 years. Each additional layer adds
another five years.
Figure 9.The same materials can
be laid up different ways. Most residences have framed wood
roofs, so the first waterproofing layer is a nailed-on base
sheet. Successive layers of felt are bedded in hot asphalt and
covered with a cap sheet, an emulsion coating, or, as is shown
here, a flood coat of asphalt and loose aggregate. Because the
base sheet qualifies as a layer, a base sheet plus three layers
of mopped-in felt constitutes a four-ply roof.
Laps. The number of layers
determines how the felts should lap. The felt we use comes in
36-inch-wide rolls, so if there are to be three layers, we
install a 12-inch strip along the edge of the roof, lap it with
a 24-inch strip, and then lap them both with a full 36-inch
piece (Figure 10). That gives us three layers of felt at the
edge of the roof — four layers if you include the base
sheet below. Each felt is mopped with hot asphalt before the
next layer is applied.
Hot asphalt is mopped onto the base sheet
and felt in preparation for the next layer of material (left).
These roofers are applying the first full-width piece of felt.
There's already a 12-inch and a 24-inch strip below
Once the joint offset is established, the remaining felt layers
are applied full-width, lapping 24 inches onto the sheets below
The roofers apply successive layers of
felt over hot asphalt. On this roof, full sheets lap 2/3 onto
the sheets below, which means there will be at least three
layers of felt plus one layer of base sheet at any given
We take special care at edges and around penetrations like
vents and scuppers: If a roof ever leaks, it's likely to be at
a penetration or termination point (Figure 12).
Figure 12.It's possible to flash penetrations after
the bulk of the roof is done, but this roofer is laying the
roof over the flashings, an approach that creates fewer
To seal the joint between a plumbing vent and the flashing that
surrounds it, for example, we apply a coat of plastic roofing
cement, embed a strip of fiberglass mesh, and then give the
joint another coat of roofing cement. The cement extends down
far enough to cover the joint between the felts and flashing,
and we slope it so it sheds water (Figure 13).
Figure 13.Most leaks happen at flashings and
penetrations. To keep water from entering, crew members seal
terminations with roofing cement (above). They then coat the
cement with aluminum paint to protect it from UV rays
Because UV rays will eventually break down exposed roofing
cement, we paint it with an aluminum roof coating.
Parapets. We waterproof
parapets with a two-ply finish that consists of a base sheet
and a cap sheet with hot asphalt between. The sheets start at
the top outside edge of the wall and lap onto the cant strip.
It would be messy and dangerous to mop hot asphalt onto the
parapet wall, so instead we mop the back of the cap sheet
before lifting it into place (Figure 14).
Figure 14.Loose aggregate will not stick to
vertical surfaces, so the crew covers parapets with cap-sheet
material. The roofers mopped hot asphalt onto the back of the
sheet and are flipping it into position.
There are a number of ways to tie into the exterior wall
finish. In some cases, three-coat stucco wraps over the wall
and terminates at a weep screed on the inside face of the
parapet. You might think the stucco would leak on top of the
parapet, but it's installed over BUR, building paper, and a
layer of self-healing peel-and-stick membrane. If any water
gets through, it should drain to a weep (Figure 15).
For houses with three-coat stucco
finishes, the standard detail is to run the finish over the
parapet and terminate it at a weep screed just above the roof.
A cap sheet, building paper, and a layer of peel-and-stick
membrane prevent the water that seeps through the stucco from
getting at the framing. If the exterior finish is wood or
masonry, the top of the parapet is covered with a large metal
Metal termination strips are available that allow you to
replace the roof without damaging existing stucco. Or you can
remove the stucco from the inside of the parapet, run the BUR
over the top, and cover the top with a finish metal cap that
laps both sides of the wall.
The asphalt in a built-up roof will deteriorate if it's not
protected from UV rays. One way to provide that protection is
to cover it with a stone aggregate. Crushed granite is the
standard covering in my area, but pea gravel is sometimes used
on commercial jobs.
After the felts are installed and we've coated all the
flashings with plastic cement, the roof is given a final
extra-thick mopping of asphalt. After that, we spread the
aggregate over the asphalt. Some of it sinks in, so we keep
adding it until no asphalt shows through (Figure 16).
Figure 16.Here, one roofer lays down a thick flood
coat of hot asphalt while another spreads crushed granite
aggregate on top (left). Since the aggregate protects the roof
from UV rays, no asphalt should be visible when the roof is
If the roof surface is visible from the street, the client may
choose a decorative aggregate, such as 1/2-inch or 1-inch white
dolomite. We've also used various landscaping rocks and a type
of red lava rock that blends nicely with clay tile on steeper
parts of the roof.
Alternative coverings. Loose
aggregate cannot be used on pitches over 3 in 12 because it
will slide. On steeper roofs, we can protect the BUR with a cap
sheet, which contains felt, asphalt, and a thin coating of
aggregate. Another method is to coat the roof with a clay
emulsion coated with aluminum paint. The problem with this
method is that you need 24 to 48 hours of completely dry
weather to apply the clay and paint.
Maintenance and Repair
Built-up roofs last longer if they are maintained. We recommend
inspecting the roof once every five years. Periodic maintenance
involves removing piled-up leaf debris, which can break down to
form acids, and using roof cement to reseal around vent pipes
BUR is pretty simple to repair. First, we cut out the damaged
area. Then we spud the gravel back from the edge of the cut
about 12 inches.
We clean the area where the gravel was removed and prime it
with an asphalt primer. Next, we patch the damaged section with
layered felts and asphalt that lap and feather onto the area
that was primed. Last, we re-cover the section with gravel (or
a cap sheet or coating, depending on what had been there
has more than 30 years of experience in
the roofing industry and has installed nearly every type of
roofing system. He is the co-owner of Advanced Roofing
Services, Inc., in Alameda, Calif.