Remodeling 101: Tar and Gravel Roofs
Perhaps the most striking visual aspect - in fact, a defining feature - of contemporary homes, is the low-pitched roof, often less than 2:12. The most common original roofing material for these is tar and gravel. A tar and gravel roof provides a nice finished appearance, if well-maintained. The life expectancy of such a roof is 20 years.
A tar and gravel roof is also known as a "built up roof", or BUR. They are well-suited for waterproofing the flat and low-slope roof styles that were popular in the modern architecture of the 1950s and 1960s.
Traditionally, a tar and gravel roof is usually made of 3 to 5 laminated layers, which are made from asphalt base sheets, hot tar, and roofing felt. A top layer of bitumen and extra top mineral coating is also added.
The roof is literally “built up,” with alternating layers of roofing felts and hot liquid asphalt, and the black tar paper felts are pressed into each previous layer of asphalt before it cools and hardens. These roof layers are called “plies.” The average BUR has 4 plies, and is called a 4-ply built-up roof. The final topping is a thick flood coat of asphalt, with gravel raked over and embedded into it while still hot. The layers of roofing felt and asphalt are the actual waterproofing, and the gravel topping protects the black asphalt from the sun’s UV-rays, which would otherwise accelerate the deterioration of the roof surface.
Light-colored gravel is used to cover the dark roofing layers. The gravel also weighs down the roofing materials, in addition to reflecting some of the sun's light, and helping to protect the roofing layers against sun damage. An even layer of gravel must be kept on the roof at all times. Drains and downspouts are also built in to drain any water from the roof.
Compared to other roofing products, built up roofing is quite inexpensive. However, the lifespan of a tar and gravel roof is only about 10 to 20 years, depending on its installation and the climate. Some well maintained tar and gravel roofs may last up to 25 years.
Strong sun can also damage the roofing membrane if it is exposed and not covered by the gravel. Tar and gravel roofing is not normally recommended in areas with lots of snow or rain. It is sometimes prone to leaks, especially if the flashing and underlayments are not correctly installed. Flat roofing is prone to ponding, when water stays on a roof surface for more than 48 hours. A tar and gravel roof must have an adequate drainage system to avoid this problem.
Gravel Roof Maintenance
The primary concern with a built-up roof is maintenance. You should have your roof checked at least once a year, and preferably every six months, to remove leaf debris that collects on the surface and does not wash away like it would on a higher-pitched roof.
The acidity caused by rotting organic debris on the roof will shorten the life of the roof. Also, plan on occasionally having gravel added to any areas where the black asphalt has become exposed to deterioration from sunlight. On low-slope roofs the gravel tends to slowly migrate downward, with the ridge regularly losing its gravel covering and needing a sprinkling of fresh gravel.
Some Common Issues to Look For in Older Gravel Roofs
• Alligatoring - As the roof approaches the end of its lifespan, the asphalt topping becomes brittle and multiple rows of cracks form, loosely similar to the pattern on an alligator’s back, exposing the roofing plies below. Water seeps into the seams and between the plies, causing Blisters.
• Blisters - When moisture seeps between the plies of the roof, then the sun comes out and heats up the surface, the water turns to vapor, and the expansion of the trapped gas causes a raised pocket, known as a Blister, to form. The blister will often loosen the gravel and some of it will slide away from the area, allowing more deterioration, and leakage. A older blister is not just visible; when you step on it, the blister makes a squishy sound from the trapped water inside.
• Ponding - The puddling of water on an area of a roof is called Ponding. A built-up roof is designed to accept water puddles on the surface without leaking, but the rule-of-thumb is that any area of standing water on the roof 48-hours after a rain is considered a problem.
Even what is called a “flat” roof of a house is usually built with a very slight slope to allow water runoff, but a poorly framed roof structure, or one that has had some sagging of the rafters over time, will create areas of ponding. The evidence of ponding is often still visible to a inspector, even after all the water has evaporated, due to the debris rings around the ponding area. A roofer can add fillers to re-slope a problem ponding area when re-roofing the home.
Fixing an Older Tar and Gravel Roof
An older tar and gravel roof can be fixed by patching the roofing membrane. Adding additional roofing layers made out of fiberglass can add further insulation. Also, applying a new acrylic/elastomeric “cool roof” coating can further weatherproof a tar and gravel roof.
Although these refurbishing measures can extend the roof’s lifespan, it should only be done if the roof is still structurally sound. Otherwise, the entire roof will need to be replaced.
Installing or fixing a tar and gravel roof is very different from installing or fixing conventional roofing shingles. It’s important to find a roofer who has worked extensively with tar and gravel roofing and ask for references.
see also: http://www.eurekamodern.com/roofs.htm
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