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Low-pitched roofs

The Old House Web
Editor's note: This story is adapted from theU.S. Department of Housing and Urban Development's Residential RehabilitationInspection Guide, 2000. Clickhere for other stories in this series.

A roof that is nearly level or slightly pitched is called a low-slope roof. No roofshould be dead level flat; it must have at least a slight slope to drain. Problems inlow-slope roofs are common and more difficult to diagnose than pitched roof problemsbecause the path of water leakage through flat roofs is often quite hard to trace. Lookfor signs of ponded water due to either improper drainage or sagging of the roof deck.

The built-up roof and flashings in this photograph are in poor condition. Patching may work temporarily, but the roof and flashings should be replaced.

If the cause is a sagging deck, it should be structurally corrected before it worsens.Low-slope roofs are expensive to repair, so extra care should be taken in theirexamination. Inspect the flashing and joints around all roof penetrations, includingdrains, soil stacks, chimneys, skylights, hatchways, antenna mountings, and otherroof-mounted elements. Note if metal flashings need painting or reanchoring and ifasphaltic or rubber flashings are brittle or cracked. Check parapet wall caps and flashingfor signs of damage due to wall movement.

Examine all portions of the roof covering. Look for signs of previous repairs that mayindicate trouble spots. There are four categories of low-slope roof covering materials andthey should be inspected as follows:

Built-up roofing

Built-up roofs are composed of several layers of roofing felt lapped and cementedtogether with bituminous material and protected by a thin layer of gravel or crushedstone.

Built-up roofs vary greatly in life span, but those used in residential buildingsusually last about 20 years, depending on their quality, exposure, number of plies, andthe adequacy of their drainage. Because built-up roofs are composed of several layers,they can contain moisture in the form of water or water vapor between layers.

Moisture not only accelerates deterioration, it can also leak into a building. Look forcracking, blistering, alligatoring, and wrinkling, all of which may indicate the need forroof replacement or repair. Consult an experienced roofer for a further evaluation if youare in doubt.

Test: An infrared or nuclearscanner can be used to detect areas of moisture in built-up roofs. Once located, theseareas can be more thoroughly checked with a moisture meter or a nuclear meter. Such testsmust be performed by a trained roofing inspector and are normally used to determine areasthat need replacement on very large roofs.

Single-ply membrane roofing

A single-ply membrane roof consists of plastic, modified bitumen, and synthetic rubbersheeting that is laid over the roof deck, usually in a single ply and often with a topcoating to protect it from ultra-violet light degradation.

Single-ply roofs are installed in three basic ways: fully adhered, mechanicallyattached, and loose laid with ballast. If properly installed and properly maintained, asingle-ply roof should last 20 years. Roof penetrations and seams are the most vulnerableparts of single- ply membrane roofing and should be carefully checked. The material isalso susceptible to ultraviolet light deterioration.

A protective coating can be used to protect it, but the coating should be reappliedperiodically. Check carefully for surface degradation on an unprotected roof and fading ofthe coating on a protected roof. Check also for signs of water ponding and poor drainage.

Roll roofing

Roll roofing consists of an asphalt-saturated, granule-covered roofing felt that islaid over the roof deck. It can only provide single- or two-ply coverage. Inspect rollroofing for cracking, blistering, surface erosion, and torn sections.

Seams are the most vulnerable part of roll roofing, and should be carefully checked forseparation and lifting. Also check for signs of water ponding and poor drainage.

Metal roofing

The underside of the low-slope roof should be examined during the interior inspection.If it is inaccessible, look for signs of water leakage on interior ceilings and walls.


From the exterior, check all skylights for cracked or broken glazing material, adequateflashing, and rusted or decayed frames.

Skylights will be checked again during the interior inspection. Leaking skylights arecommon. Replacement skylights must comply with the building code.

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