A built-up roof (aka “tar and gravel” roof) is one of the most proven effective roofing systems found on commercial buildings. Built-up roofing is one of the oldest forms of roofing that is still being installed today. These roofs can last multiple decades if properly installed and maintained.
What is Built Up Tar and Gravel Roofing?
Built-Up Roofing (BUR) is an asphaltic roofing system constructed with multiple layers (plies) of felt fabric material in between layers of applied liquid asphalt. Most built-up roofs have three or four plies, however some are built with up to 7 plies. The upper-most layer of asphalt is topped with gravel stones which serve to protect the roof from harmful UV exposure, and from storms and debris falling on the roof.
In some cases, concrete pavers are placed on the built-up roof in areas where foot traffic may occur. This is usually near rooftop HVAC units or ladder access areas.
The built-up roof is likely to be installed over foam board insulation and/or a cover board material. These layers insulate the building and protect the roof deck. If insulation is installed beneath the roof deck, the built-up roofing may be installed directly on the roof deck.
How is Built Up Roofing Installed?
There are two primary installation techniques for installing tar and gravel roofs.
Hot Mop Installation
During hot installation, the roofing crew will pour hot asphalt onto the roof surface, then mop it evenly across the surface. This process is done in between each layer of fabric felt that is installed. Hot mopping creates strong fumes, so we recommend the building be vacant during roof installation.
Cold Applied Installation
A more common method of installing built-up roofs in today’s age is cold-applied asphalt. With this method, the roofing crew will use cold asphalt adhesives applied with a rubber-edged blade. Innovations at roofing manufacturers have resulted in the cold-applied installation method being at least as strong as hot mop installed roofs. And, without toxic fumes, cold applied roofs are usually a safer choice.
How Long do Built Up Tar and Gravel Roofs Last?
A properly installed built-up roof should be able to last up to 30 years. But it should be maintained over time to be able to get the most lifespan out of it as possible. Built up roofs should be inspected annually with maintenance items addressed proactively, as neglect of these roofs can result in a quick roof failure.
Built-up roofing has been installed in some form as far back as the 1800s. It is a time-tested method of roofing. And when correctly installed and maintained, can be a great roof for decades.
How Much do Built-Up Roofs Cost?
Commercial roofing installation costs depend on many factors. Each roofing system is a custom-installation unique to that building. With that being said, we see most built-up commercial roofs costing between $10-$14 per square foot of roofing surface.
For many commercial buildings, built-up roofing may be the most economical roofing choice. Other options include single-ply roofing like TPO or EPDM, or modified bitumen roofing.
Commercial roofing installation projects take up a significant amount of many commercial property maintenance budgets. If it’s getting close to time for your building to need a new roof, we recommend getting an estimate prepared so you can plan and budget for the project.
Pros and Cons of Built-Up Roofing
Built-up tar and gravel roofs are the right choice for some commercial buildings. Here are the pros and cons of built-up roofs.
- Built-up roofing lasts a long time. It’s a proven long-term form of roofing that will protect your building for decades.
- Leaks are rare. Because of the multiple layers, and because asphalt is a great waterproofing agent, leaks are not common on properly installed built-up roofs.
- The top layer of gravel protects your roof from hail, tree debris, and foot traffic.
- Built-up roofs are heavy, so your building’s framing and roof deck must be in good shape to support the roof.
- Hot mopped built-up roofs put out toxic fumes that can make it difficult to install.
- Due to the several layers, built-up roofs take longer to install than single-ply commercial roofs.
Repair for Built-Up Tar and Gravel Roofs
A great benefit of built-up roofing systems is they are simple to repair. Experienced commercial roof repair crews should have no issue correcting a built-up roof.
After a repair area is identified, there are a few options for how to perform the roof repair. No matter which is selected, the first step is to remove the gravel top layer, and the asphalt the gravel is embedded in. The repair should be applied to the top layer of fabric paper.
One repair option is to use new layers of fabric felt with cold-applied asphalt material applied to the roof. Another repair technique that is becoming more popular is using commercial roofing silicone instead of layers of felt and asphalt. Depending on the needs of your commercial roof, either option may work great for you.
If the repair is due to damaged flashing or metal work, the only option may be to remove the damaged flashing and install new.
Or, the repair may be more simple than that. Many commercial roof leaks stem from debris in roof drains that lets water accumulate on the roof. A simple drain cleaning may be the solution to your roofing problem.
Built-Up Roofing Manufacturers
Several national roofing manufacturers produce built-up roofing materials. We have installed built-up roofing from Johns Manville, Iko, and GAF, and are comfortable installing built-up roofing from any major manufacturer. We are certified installers, meaning we have been trained on approved installation techniques and are able to provide longer warranty options to our commercial roofing customers.
Can Roof Coatings Be Installed on Built-Up Gravel Roofs?
Yes. The process for coating a built-up roof is different than coating other types of roofing. The gravel must be removed, and the surface scraped down to the top layer of fabric felt. Then the coating layer(s) can be applied.
Silicone coatings over built-up roofs will require a layer of bleed-blocking primer to be installed first. Then the roofing silicone can be applied.
Other types of coatings, like Castagra Ecodur, can be applied directly to the asphalt under the top layer of gravel.
Commercial roof coatings on built-up roofs are one of the best ways to extend the life of the roof, sometimes up to 20 additional years. The best way to know if your roof is a candidate for a coating is to request a commercial roof inspection and assessment.