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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. 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.

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    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. 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.


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 point.


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.


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 seams.

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.


Most leaks happen at flashings and penetrations. To keep water from entering, crew members seal terminations with roofing cement.


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.


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.

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    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 cap.

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.

Aggregate Surface

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.


Here, one roofer lays down a thick flood coat of hot asphalt while another spreads crushed granite aggregate on top.


Since the aggregate protects the roof from UV rays, no asphalt should be visible when the roof is done.

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 and scuppers.

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 originally).

David Lopez 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.