Here’s two key differences between flat (low slope) roofs and sloped roofs as they age:

•• The overall condition of a sloped roof is easily observed from the ground at a glance. But out-of-sight means out-of-mind, so a flat roof often doesn’t get checked until water starts dripping from the ceiling under it.

•• Because rain rolls off a sloped roof quickly, minor openings in the roof surface usually do not leak enough water to do more than put a small stain on the bottom of the roof sheathing. The same size opening in a flat roof, however, can slowly leak a lot of water, especially if it happens to be under an area of ponding on the roof. 

    So this means it’s important to check a flat roof regularly—every six months would be good, but at least once a year—and do maintenance chores and repair minor leaks as soon as they appear. This is what to look for:

1) Missing areas of gravel or rock granules covering surface - Gravel or embedded rock granules protect the roofing surface from UV-ray damage. When they are missing it speeds up deterioration of the roof surface.

2) Alligatoring or exposed roofing paper - 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.  

    Further deterioration exposes the roofing paper.

3) Bubble-like blisters or pockmarks - When water seeps between the plies of a built-up and gravel roof, and then the sun comes out and heats up the surface, the water turns to vapor. Expansion of the trapped gas causes a raised pocket, known as a blister, to form. The blister will make a squishing sound when you step on it. Also, the blister will often loosen the gravel and some of it will slide away from the area, allowing more deterioration, and leakage. Sometimes they are as large as the one shown below.

    SPF (Spray Polyurethane Foam) roofs develop pockmarks that grow in size to become holes if not repaired.

4) Accumulation of leaf debris on roof - Because this type of roof is often nearly flat, rain does not fully wash away pine needles and leaves, and they tend to build up over time if not removed regularly. Their decomposition creates acidic areas that speed up the aging of the roof. In this photo, much of the roof surface is buried under decomposing leaves. Check condition of roof surface carefully after sweeping the leaves off the roof.

5) Ponding - When there is an area of a roof that has a puddle of water that does not drain after a rainfall, it is called “ponding.” Although a flat roof is watertight when correctly installed, ponding tends to deteriorate the roof in the ponding area over time. Even a “flat” roof is designed with a slight slope for drainage, and ponding is always the result of settlement or sagging of the roof framing, or poor installation of the roofing. Small areas of ponding are to be expected, but increasing size of ponding areas that last more than a couple of days is an indication that the roof surface is deteriorating.

    Any one of these five signs may only indicate a need for maintenance or a repair to the roof; but a combination of several—along with any indications of leakage underneath—means its time for a new roof.

    The are multiple different materials used for installing flat roof and are they are often known by their acronyms or abbreviations. See our links below to learn more about EPDM, PVC, TPO, and modified-bit roofs.

    Also, we recommend reading Do flat roofs always leak? and What causes bubble-like blisters in a built-up and gravel roof? and What is the purpose of the gravel on a flat tar roof? and Why are most house roofs slanted instead of flat? 

• • • • • • • • • • • • • • • • • • • • • • • • • •  

Here’s links to some links to some of our other blog posts about “FLAT ROOFS (LOW SLOPE)":  

• Why does it cost so much more to replace a steep roof than a low slope roof? 

• Why is there no attic access hatch in the house?

• What is the average life expectancy of a PVC roof?

• What is the difference between roll roofing and modified bitumen? 

 • Can metal roofing be used on a low slope/pitch roof? 

• What is a TPO roof? 

• What is the stuff you paint an old mobile home metal roof with to extend its life?

• Does a gravel roof need maintenance?

• What is the average lifespan of a built-up and gravel roof?

• What is the average lifespan of a modified bitumen roof? 

• What is the average life expectancy of a TPO (Thermoplastic PolyOlefin) roof? 

• What is an EPDM roof?

• What is the average life expectancy an EPDM roof? 

•  I'm buying a '50s modern house with a "gravel" roof. Is the roof going to be a problem? 

• Is "ponding" (standing puddles of water) normal on flat roof?

 • What is a low slope roof?

• What is the average life expectancy of a built-up roof? 

• What is the average life expectancy of an elastomeric roof coating?

• Can you coat a TPO roof to extend its life? 

     Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.