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What is a Cap Sheet?

By B. Turner
Updated May 16, 2024
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A cap sheet is a building material used in built-up roofing, or BUR. BUR applications are used for flat roof surfaces, and consist of several layers of material designed to create a watertight surface. Each layer includes asphalt or tar-coated layers of fiberglass mats. The more layers, or plies used in built-up roofing, the more effectively the surface will be able to resist leaks over time. A surfacing component, such as a cap sheet, is used to cover and protect these lower layers and extend the life of the roof.

Over time, constant sun exposure weakens asphalt and causes it to dry out. As it dries out in the sun, a BUR roof becomes brittle and eventually cracks. Rain seeps into these cracks and eventually penetrates the inside of the building, leading to leaks, mold, and rot. The cap sheet not only protects the asphalt from the sun, but also acts as an extra ply to further increase the integrity of the roof.

A cap sheet can be adhered to the roof surface using a number of methods, depending on the base materials. Some installers rely on heat welding to permanently bond the sheet to the lower roof layers. Others apply lap cement or add a bituminous material such as asphalt between the sheet and roof surface. Depending on the climate and wind levels, some installers may add gravel or aggregate on top of the sheet to help weigh it down and hold it securely in place.

Most cap sheets resemble rolled asphalt roofing. They consist of fiberglass mats coated with mineral granules for strength and durability. A roof cap sheet is fairly heavy because of the mineral surfacing, and is widely referred to as 90-pound roofing in North America. A standard roofing square of this material weighs 90 pounds (40.9 kg), though some lightweight materials are used in certain regions.

Some cap sheets are used in retrofit applications to improve the energy efficiency of a building. These sheets feature a light-colored mineral surfacing that creates a white or off-white roof. This light coloring reflects sunlight to keep building's cool in hot, sunny climates. By reflecting sunlight, these roof sheets reduce the need for air conditioning to lower utility costs and help protect the environment. In this type of application, the cap sheet may be used with a standard BUR roof or with a variety of other roof finishes.

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Discussion Comments
By anon998393 — On Jun 02, 2017

Cap sheets are low slope, thick flat roof, materials. They are made with asphalt blended with plastic or rubber to give it better flexibility and weathering. They call these modified bitumen products.

There are plenty of other types of flat roofing. Modbit roofs are usually a more premium roof and will cost more installed than rubber or similar materials. They also should outlast epdm rubber roofs when installed as a system by a trained installer.

Any roof's lifespan is going to be mostly about the skill of the installer. All things being equal, modbit will usually outlast other products, but even the cheapest roof with phenomenal install will outlast premium products put down by unskilled labor.

Hope this helps

By Izzy78 — On Jul 12, 2011

@TreeMan - Those are some great points. Thinking more about the durability - there are tons of buildings you can walk into and see water damage on the ceiling tiles, so obviously there is some way for water to make it through.

There could also be different roofing types besides built up roofing that are not quite as long lasting. Maybe someone else here and shed some more light on what other types of roofing are often used on business and the like.

Something else to think about is the cost, too. If there are other roofing types, they might cost much less and be used on many more buildings. These might be the kinds of roofs that end up with the water marks. I guess it ends up being a decision between higher up front costs or the cost spread out over the life of the roof.

By TreeMan — On Jul 11, 2011

How long does something like a cap sheet last? The article mentions using heat welding to permanently adhere the cap sheet over the asphalt, so it sounds like a long-term solution.

Surely it can't last forever, though. (I doubt roofing manufacturers would keep making it if it did!) I'm mainly curious what causes the material to break down and how quickly. I am assuming this is a material that is rolled into place on the roof. Maybe it is the material connecting the sheets that breaks down the quickest and requires repairs.

It sounds like the built up roofing is a very durable material, so it could take quite a while before anyone realized there was a problem. By that point, though, the asphalt might be damaged and need replaced.

By jmc88 — On Jul 10, 2011

@titans62 - I think I know what you are talking about. I recently had my roof reshingled, so a lot of the terms are still fresh in my mind. If it is the stuff they used on my roof, it is a rubbery coating. I believe it was called modified bitumen. It came on a roll, and the roofers used nails and a sealant to keep it in place.

As for whether a cap sheet could be used on a normal residential house, I'm not sure about that. I have see the gravel covered material that you are describing, though.

I think it is interesting how self-aware we are coming of conserving energy nowadays. When I was growing up, no one would have considered using white or light colored roofing as a way to conserve energy. Now energy use is one of the deciding factors in a lot of our current purchases.

By titans62 — On Jul 09, 2011

When I had my roof redone a few years ago, the roofers put a black, tarp-like material on the flat sections of my roof. I originally thought this was what it was talking about. After reading more, I'm not sure.

This sounds more like a material that is used in commercial roofing. We had a problem with the air conditioner in the building where I used to work, so I climbed on the roof to take a look at it. The roof was flat and had coating that sounds like what this article describes. It was also covered with gravel. I've seen the same material on other buildings.

I guess my questions are: can a cap sheet be used on flat sections of a residential home, and if not, what is it that is being used on my house?

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