A roof that is nearly level or slightly sloped is called a low-slope roof. No roof should be actually level and flat. It must have at least a slight slope to properly drain.
Low-slope roofs can be expensive to repair, so care should be taken in their maintenance. Regular maintenance and periodic inspections for low-slope roofs are necessary. Problems with low-slope roofs are common and more difficult to diagnose than sloped roof problems because the path of water leakage through flat roofs is often quite hard to find and trace back to the source. Home inspectors should check for indications of ponding water (or puddle formation) on the surface due to either improper drainage or sagging of the roof deck structure. If the cause is a sagging deck, it should be structurally corrected.
Flashing
Home inspectors should inspect the flashing and joints around all roof penetrations, including drains, stack pipes, chimneys, skylights, hatchways, antenna mountings, and other roof-mounted elements. The inspection image is of an inspector checking the poorly installed flashing around the chimney stack that penetrates through the flat roof. The black roof tar sealant is not reliable.
Check to see if metal flashing needs to be tightened or fastened, and if asphalt or rubber flashings are brittle or cracked. Parapet wall caps and flashing may develop damage due to wall movement or moisture.
There are a few categories of low-slope roof-covering materials, and they should be checked and inspected as follows:
Built-Up Roofing
The first common type of low-sloped roof covering material is built-up roofing, sometimes called roll roofing. Built-up roofs are composed of several layers of roofing felt material that is lapped, cemented together with bituminous material, and protected by a thin layer of gravel or crushed stone. Built-up roofs vary greatly in life span, but those used in residential buildings usually last about 20 years, depending on their quality, exposure, number of plies, and the adequacy of the drainage.
Because built-up roofs are composed of several layers, they can hold moisture in the form of water or water vapor between layers. Moisture not only accelerates deterioration, but it can also leak into a building.
Regular maintenance and periodic inspections are necessary. Home inspectors should look for cracking, blistering, alligatoring, and wrinkling, all of which may indicate the need for roof replacement or repair. The homeowner should consult an experienced roofer or their local neighborhood home inspector for further evaluation if they have doubts about the roof’s apparent condition.
Single-ply Membrane Roofing
A single-ply membrane roof consists of plastic, modified bitumen, and synthetic rubber sheeting that is laid over the roof deck, usually in a single ply, and often with a top coating to protect it from ultraviolet light degradation. Single-ply roofs are installed in three basic ways: fully adhered; mechanically attached; and loose-laid with ballast. If properly installed and maintained, a single-ply roof should last 20 years.
Roof penetrations and seams are the most vulnerable parts of single-ply membrane roofing and should be carefully inspected. The material is also susceptible to ultraviolet light deterioration. A protective coating can be applied, but it will need to be reapplied periodically. Check carefully for surface degradation on an unprotected roof and fading of the coating on a protected roof. Check also for signs of water puddling and poor drainage.
Roll Roofing
Roll roofing should be inspected before and after the winter season in cold climates. Roll roofing consists of asphalt-saturated, granule-covered roofing felt that is laid over the roof deck. Inspect roll roofing for cracking, blistering, surface erosion, and torn sections. Seams are the most vulnerable part of roll roofing and should be carefully checked for separation and lifting. Also, check for signs of water ponding and poor drainage.