Shallow pitch can present roof problems

Inspector's Eye

April 13, 2003

A reader is buying a house with a shallow-pitch roof.

The roof surface is asphalt shingles, which concerns the reader because it is a roof that looks nearly flat. A home inspector said the roof slope was considered acceptable for asphalt shingles but it was close.

Roofs on nearby houses have the same shallow pitch, but some of them have the sort of roofing you see on flat roofs. One has a tar and gravel surface and some others have roll roofing with white granules.

The reader would like to use one of those flat roof surfaces when it's time to replace the existing roof, believing it would be more reliable. The reader has heard that gravel-coated roofs can end up with a lot of gravel in the rain gutters, so is leaning toward the roll roofing.

The reader wants to know what factors should be considered in making such a choice.

Dear Reader:

A roof surface suitable for a flat roof should work well for you. Although asphalt shingles are more aesthetically pleasing to most people, they can have reliability problems on shallow pitches, especially if workmanship is less than perfect.

Unlike flat roof materials, asphalt shingles do not form a watertight layer. They rely on gravity to keep water from leaking through the joints under the shingles. Shingles installed on shallow pitches are more vulnerable to leakage caused by wind-driven rain, or by water backed up behind leaves or ice.

This is likely to be a problem if roofing nails are overdriven, because the nail heads can tear the shingles and create pathways for leakage. Some incidence of overdriven nails is fairly common, in my experience.

A built-up roof surface with gravel aggregate is an option that can have a nice appearance, is reliable on very shallow pitches and has a life expectancy of 20 or more years. The entire surface of a built-up roof is "flood coated" with hot asphalt, forming a watertight surface. Gravel aggregate then can be applied to protect the asphalt and improve the appearance.

When installed on a pitched roof, gravel migrating into the gutters can be a problem as the roof experiences weather and wear. But that should be minimal if the gravel is properly embedded in hot asphalt. Various types of gravel aggregate are used on built-up roofs, the most attractive being tan pea gravel or white crushed stone.

Modified bitumen roofing also is reliable and long lasting. It is usually torch applied, so that seams are melted together, forming a watertight bond.

Most of these roofs are installed with a white granular surface that, to my eye, can make them look like they belong on a chicken coop. White roofing has its advantages, such as its ability to reflect heat, but other granule colors on modified bitumen make for a better appearance on houses with roofs that are visible from the ground.

Some manufacturers produce modified bitumen roofing in the full range of colors available for asphalt shingles, so both types of roofing can be used without a color mismatch.

Dean Uhler is a certified home inspector and president of Baltimore-based Boswell Building Surveys Inc.

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