The constant exposure to ultra-violet radiation degrades organic ingredients in roof coverings. The covering heats up and dries out over and over again. Too much sun on wood roofs dehydrates the shingles, causing them to become brittle. Thermal expansion and contraction can destroy adhesion materials in asphalt shingles, for example, and cause cracking in other roofing materials.
Although roof covering systems are designed to protect the roof’s structure from water penetration, rain eventually takes a toll on any roof covering. Some ingredients used in roof coverings are soluble and will dissolve over time. Rain washes away granular or gravel finishes in roofing such as asphalt shingles and built-up roofing. Constant wetting of wood shingles can cause them to rot. Older metal roofs are susceptible to rust.
Strong winds can lift shingles off a building. Wind blows rain against a roof and can drive water under the edges of shingles and tiles. Wind can also blow sand against the roof’s surface, causing erosion of the covering. With wood shingles, for example, sand erosion can remove enough of the top layer so they no longer protect the shingles underneath.
We love the cozy effect of trees overhanging the house, but they can do a great deal of damage to a roof. Branches that scrape back and forth over the roof’s surface can remove the granules from an asphalt shingle roof. Trees can provide too much shade and, as a consequence, can keep a roof from drying out properly after a rain. Wood shingles that are not allowed to dry properly become rotted. Leaves block up drainage systems, causing water damage. Falling branches are an obvious danger to any roof.
Moss reacts with the organic materials in wood and hastens its breakdown. Wood and built-up roofs are especially vulnerable to decaying effects of moss. Its root system penetrates the surfaces and creates paths for water to get into and under the roof surface. On other roofing systems, it rusts nails and impedes water runoff.
SNOW AND ICE
Ice damming can occur when melting snow on the roof refreezes at the roof’s overhang. This causes an ice dam to form. Water from melting snow higher up on the roof becomes blocked by the ice dam and cannot escape to the gutters. This water backs up under shingles and seeps into the interior. Ice dams occur when enough heat escapes from the attic at the upper part of the roof to melt the snow at the same time the lower part of the roof at the eave is below freezing. Better attic insulation, ventilation and proper air sealing would remedy this situation.
No roof covering lasts forever. We just haven’t figured out how to make the ideal roof covering that would never have to be replaced.