Mastering Roof Inspections: Asphalt Composition Shingles, Part 13

Climate Zone

The climate zone in which a home is located determines the type of severe weather to which shingles will be exposed, as well as the length of time they’ll be exposed. Climates may be different combinations of hot, cold, dry and humid, each of which can have a different effect on shingle roofs.

Thermal Cycling

“Thermal cycling” is the term used to describe the process whereby roofs get hot during day and cold during the night. When they get hot, all the roof components expand, including the shingles. When they get cold, they all contract.

As shingles age, they shrink. This is especially true of organic shingles. As they shrink over the warm part of the year and contract with the cold…

...shingles that bridge a joint between underlying shingles may split.

The greater the daily differences between high and low temperatures, the more all the roofing components will expand and contract. Roofs exposed to extreme thermal cycling will be less stable and will deteriorate more quickly than those exposed to less cycling.

The amount of thermal cycling experienced by shingles may be affected by:

• the climate zone;
• the number of shingle layers;
• the adequacy of attic ventilation;
• shingle color; and
• the properties of the granule coating.
  
  

Shingle Color

Shingle color can influence roof temperatures. The temperature difference between light-colored shingles and dark-colored shingles can be up to 40°.

Reducing the high temperatures that asphalt shingles reach each day can decrease thermal cycling and increase their long-term service life. Light-colored shingles installed on homes in hot climates will probably help extend the long-term service life of asphalt shingles.

Home Site Conditions

Gaining experience as a home inspector includes familiarizing yourself with problems common to your local work area. Different home site conditions can create different types of problems.  So many different home site conditions exist across North America that we can’t do much more than mention a few of the conditions common to many.

Overhanging tree branches rubbing on the shingles can cause abrasion or tear damage.

Roofs shaded by trees or mountains may stay cooler or may stay excessively moist, which can lead to microbial growth.

Local wildlife, such as squirrels and raccoons, can damage shingle roofs.

Leaves accumulated on the roof and in the gutters can hold moisture against the shingles, and prevent the roof from draining properly.

Geological or landscape features of the home site may protect the roof from storms, or they may direct storms toward it.

Elevation

Shingles on homes at higher elevations are exposed to more ultraviolet radiation than homes at lower elevations. Since UV radiation damages the asphalt layers of shingles, shingles installed at higher elevations may have a shorter long-term service life.

Quality of Maintenance