I'm buying a '50s modern house with a "gravel" roof. Is the roof going to be a problem?
Wednesday, October 17, 2018
Not necessarily. Tar-and-gravel roofs, also called built-up roofs, are well suited for waterproofing the flat and low-slope roof styles that were popular during the 1950s and ‘60s. They are not as common today for two reasons: 1) low-slope roofs are out of fashion, and 2) the application of the hot asphalt tar used between the layers of the roof requires special equipment and insurance, so many roofers do not install them.
The roof is literally “built up,” like prepping a pan of lasagna, with alternating layers of roofing felts and hot liquid asphalt (instead of tomato sauce), and the black tar paper felts being pressed into each previous layer of asphalt before it cools and hardens. The roof layers are called “plies,” and 3 to 5 can be applied, but the average roof has 4 plies and is referred to as a 4-ply built-up roof.
The final “cheese” 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.
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.
If you decide to do the maintenance yourself, beware: flat and low-slope roofs are statistically more accident-prone than higher pitched roofs. Because people are not afraid of sliding down the roof after a misstep, like on a steep roof, they tend to get too comfortable and end up walking off the edge of a flat roof. So try to always be conscious of where you are in relation to the edge of the roof when you do maintenance on your flat roof or low-slope roof, and don’t walk backwards when clearing off the leaf debris. Also, most skylights were not designed to support the weight of a person walking or leaning on one. So stay off the skylights.
While you or your roofer are up there, here are some common defects to look for:
•• 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.
•• Exposed Roofing Paper - When the tar is gone and the roofing paper is exposed and deteriorating rapidly, it looks like the picture below.
•• 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.
An older blister like the one shown below 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.
•• Flashing Defects - Where roof planes meet, where roofs and walls intersect, and at penetrations of the roof by things like skylights and plumbing vent pipes, a flashing material is added to seal the joint properly. Roof leaks can occur at the flashings, either because they were installed improperly, become damaged or corroded over time.
When a low-slope or flat roof does leak, the water often does always not drip directly though below the leaking area. Instead, it sometimes migrates a few feet away before coming through to stain the ceiling. This can make pin-pointing a leak on a gravel roof difficult.
A built-up tar and gravel roof has an average lifespan of 20 to 30 years, which can be extended by maintaining the roof as outlined above.
Also, see our blog post Why are most house roofs slanted instead of flat?
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To learn more about roofs and attics, see these other blog posts:
How To Look At A House
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