Basic factors that affect the potential life of a roof

Design - Design complexity plays a role in roof failures. In brief, the more penetrations, valleys, changes in direction or instances of shingles meeting different materials, such as occurs on a built up roof, the more likely there will be premature failures or leaks.

Roof’s Pitch - Typically, the steeper a roof is the longer it will last, and the less likely it will leak. In general, water runs off a steep roof quickly, and does not back up during high winds. All things equal, a steep roof will dry faster than its shallow counterpart. All of these factors tend to extend the life of a steep roof, or even the steep slopes on a house with varied slopes.

The preferred slope for shingles is 4/12 or greater. Most shingles, however, can be installed on slopes as shallow as 2/12 with waterproof underlayment. Occasionally, a double layer of asphalt felt has been used (sometimes roof cemented together), or some other single-ply material under shingles on shallow slopes as an attempt to compensate for a shallow slope.

Physical damage - Wind, hail, tree limb and scuff damage is common on asphalt composition shingles.

• The rip of the wind can be a threat to a shingle roof. Proper nailing techniques, the sealant strip and the shingle type all play a role in defending against wind damage.
• Hail damage is often an insurance issue; therefore it should be addressed immediately after damage occurs. Hail damage sometimes appears as little spots lacking most or all of the granules. But, depending on the shingle condition and the severity of the storm, hail can practically destroy a roof.
• Limb damage is something that should have been addressed before it occurred by pruning the branches of trees that pose a threat to the roof. Tree limbs that are too close not only can damage shingles, they also shed debris on the roof – slowing the drying process, which can affect shingle life.
• Scuffs are usually from roof walking on hot days.

Physical damage or the possibility of damage should be reported. Even a small piece of broken shingle can leak!

Sun Exposure - Sometimes there is a marked difference between the deterioration of the shingles on two different sides of the house. It can be because the shingles on one side are older than on the other side; but it can also be because one slope faces south and the other faces north. Typically southern exposures age faster than other exposures due to ultra violet light from the sun. Shingle colors may also influence the extent of this difference, with darker colored shingles aging faster than lighter colors on a southern slope.

Roof Over - Most shingle manufacturers do not prohibit shingle roof over; nor does it alone affect the limited warranty. Some experts say it can reduce shingle life as much as one-third. As home inspectors, it is wise to adjust the remaining life estimate accordingly. Roofers have been known to insist on improving ventilation on re-roofs, such as by adding ridge vents or a fan.