Sun

Heat and ultraviolet rays cause roofing materials to deteriorate over time.

Rain

When water gets underneath the shingles, shakes or other roofing materials, it can work its way to the deck and cause the roof structure to rot.  And, the extra moisture encourages mildew and rot elsewhere in the house, including damaged walls, ceilings, insulation and even the electrical system.

Wind

High winds can lift the edges of shingles and force water and debris underneath them.

Moss and Algae

Moss can grow on wood shingles and shakes if they are kept moist by poor sunlight conditions or bad drainage.  Once it grows, moss holds even more moisture to the roof surface, causing rot, and its roots actually work their way into the wood.  Algae also grows in damp, shaded areas on asphalt shingle roofs.  Besides creating an ugly black-green stain, it can retain moisture, causing rot and deterioration.  Trees and bushes should be trimmed away from the house to eliminate damp, shaded areas, and gutters should be kept clean to ensure good drainage.

Tree and Leaves

Tree branches touching the roof will scratch and gouge the roof material as they are blown back and forth by the wind.  Falling branches from overhanging trees can damage-or even puncture- shingles and other roof materials.  Leaves on the roof surface retain moisture and cause rot, and leaves in the gutters block drainage.

Missing or torn Shingles

The key to a roof's effectiveness is complete protection.  When shingles are missing or torn off, the roof structure and the interior of the home are vulnerable to water damage and rot.  And, the problem is likely to spread-nearby shingles are easily ripped or blown away.  Missing or torn shingles should be replaced as soon as possible.

Shingle deterioration:

When shingles are old and worn out, they curl, split and lose their waterproofing effectiveness. Weakened shingles easily are blown off, torn or lifted by wind gusts. The end result is structural rot and interior damage. A deteriorated roof system only gets worse with time so it should be replaced as soon as possible. 

Flashing deterioration:

Many apparent roof leaks really are flashing leaks. Without good, tight flashings around chimneys, vents, skylights and wall/roof junctions, water can enter a home or building and cause damage to walls, ceilings, insulation and electrical systems. Flashings should be checked as part of a bi-annual roof inspection and gutter cleaning.

Condensation

Condensation can result from the build-up of relatively warm moisture-laden air.  Moisture in a poorly ventilated attic promotes decay of the wood decking and rafters, possibly destroying the roof structure. 

Signs of water-damage or leaking (usually in the form of water stains, or sagging ceilings)

This could be due to an active leak in the roof or to condensation caused by poor roof ventilation.