Mastering Roof Inspections: Asphalt Composition Shingles, Part 41

Various aspects of building design can affect asphalt shingles. Among the most important are the systems of ventilation devices used to remove hot and moist air from the space beneath the roof deck.

Roof-Structure Ventilation 

Roofing materials absorb sunlight as heat. This heat is then released both upward into the open air and downward into the roof structure.

Well-ventilated roof structures use air movement to exhaust heat from the attic or roof structure to the outside. Poor roof ventilation can shorten the service life of felt underlayment and asphalt shingles due to buildup of excessive heat and moisture.

Cooling Season

During the cooling season, which is the season during which the home is kept cool, ventilation is beneficial in several ways.  It reduces shingle deterioration. Heat deteriorates shingles by accelerating the rate at which volatile compounds in asphalt evaporate. Volatiles help keep shingles durable, flexible and waterproof.

Thermal cycling is the daily fluctuations between high and low temperatures. This cycling causes shingles and sheathing to expand and contract, which can result in shingle splitting and contribute to long-term granule loss.

Ventilating the roof structure helps lower home cooling costs by reducing the amount of heat transmitted to the living space from the roof. It may also make the home more comfortable.

Consider the following scenario:

In a home with no attic ventilation, on a sunny day, let’s say the outside temperature is 90° F (or 32° C):

• The temperature on the roof may be as high as 170° F (or 77° C).
• The temperature at the floor of the attic might be around 140° F (or 60° C).
• The temperature of the living space below would be uncomfortably warm from heat radiating down from the roof and attic.

Cool air passing through the attic spacer (or through rafter bays, if the ceiling is vaulted) and flowing across materials absorbs and carries away heat, reducing the amount of heat radiated to the living space.

Heating Season

Moisture Problems

During the heating season, when the home is kept warm and the doors and windows are kept closed, attic ventilation helps prevent problems related to moisture. Homes produce moisture from a variety of sources, and warm air holds more water vapor than cold air.

In a home with a family of four, watering plants, bathing, washing clothes and dishes, cooking, and even respiration can put 2 to 4 gallons of moisture into the indoor air each day.

As warm air in the home rises, this moisture-laden air can eventually find its way into the attic, where it can be absorbed by the sheathing. Under certain circumstances, such as homes with a strong stack effect, holes cut in ceilings for recessed light fixtures can allow large amounts of moisture to be carried into the attic.

The soil beneath a home can also contribute to moisture problems if the makeup air that replaces the indoor air lost to the attic is pulled from the crawlspace.

In addition to shingle buckling or splitting caused by expansion and/or contraction of the sheathing, excessively high moisture levels can cause other types of problems.

Mold Growth

If moisture levels in attic materials rise above about 20%, mold colonies can become active, increasing the size of the colony.

If moisture levels in attic materials rise above about 27%, fungi start to release spores, which can find their way into the indoor air and become a potential health hazard, especially for those with asthma, allergies, lung disease, and compromised immune systems.

Mold fungi are also responsible for wood decay, which is the same as rot. Decayed roof sheathing loses its ability to support roof loads and to hold fasteners.

Moisture can also corrode structural metal components. Excessively high moisture levels in attics can cause both health and structural problems.

Ice Dams

Heat leaking into the attic can melt snow on the roof. Snow melt running down the roof re-freezes as it reaches the cold roof overhang. As more and more ice accumulates at the roof's edge, it can create an ice dam that prevents snow melt from running off the roof.

Snow melt pooling behind ice dams can find its way through the shingles and underlayment, causing leakage that can damage materials in the home.

In addition to damage from leakage, tools used in an effort to remove ice dams can cause shingle damage.  One way to prevent this type of damage and drain a roof that has ice dams is to fill a nylon stocking with ice melt and lay it across the ice dam.  This will melt a channel through the dam and allow the snow melt to drain from the roof.