Tar and gravel roofing has been the industry standard in flat roofs since the 1880s, and is still considered the most common roofing material. Back then, it was simply the most durable material available. Although new innovations were introduced in the 1950s, they were not well-received. It wasn’t until the energy crisis of the 1970s—when the petroleum to create asphalt became so expensive—that the industry began looking for other options. This opened the door to other single-ply membrane materials such as TPO and modified bitumen.
In composition, tar and gravel roofs consist of felt and tar paper adhered and fastened with molten asphalt. After lamination, these layers are covered with gravel to protect against solar radiation. Some of this gravel is embedded in the hot asphalt, while the rest remains loose on the roof surface. Oftentimes, tar and gravel roofs are called “built-up roofs”—or BUR—because the asphalt and tar layers can be stacked.
So long as the gravel layer remains intact, these roofs are considered fairly durable and weather resistant. However, the gravel layer can be scraped off by high winds, heavy rains, or by simply walking on it. This then exposes the asphalt layers to excessive heating. With UV exposure, asphalt can be leeched of its plasticizers, which makes it brittle over time. This often causes alligatoring, an effect of drying and cracking like alligator skin (a clever term, right?). This naturally leads to water leaks and delamination. Also, if even a little bit of water gets under the asphalt it can cause blistering, making your roof look like a plague victim.
Anyone who has played kickball on the blacktop at recess knows that asphalt is notoriously heat-absorbing. Gravel does not have a high reflectivity rating unless it’s white, so it’s rare for a tar and gravel roof to be considered a cool roof.
In most cases, these roofs are cheap on the materials but rather labor-intensive. A leaking tar and gravel roof is not considered repairable; at this point you either have to replace the whole thing or “build it up” with another tar and gravel layer. Either way, the removal of the gravel is necessary and it’s both time-consuming and costly. A neat feature at this point is that modified bitumen can be torched down directly to the tar and asphalt layer. This essentially gives you the best of both materials: the self-healing of the tar layer and the durability of the bitumen.
The life expectancy of a tar and gravel roof is a pretty big range: 10-20 years, depending on weather conditions and maintenance. When building up with more layers of tar and asphalt, the life expectancy declines to 8-10 years for the second and third generations. This is often because the gravel of the previous layers is not fully scraped off and stabs into the tar paper stacked on top of it; these roofs are especially vulnerable to penetration. However, with the right team and a high-quality system, any roof’s vulnerabilities will decrease.
Whatever your flat or low-slope roof calls for, we can handle it for you.