Asphalt shingles are not waterproof. If it was possible for water to stand still on a shingle roof, it would quickly seep under the laps of the shingles. But shingles are "water resistant" because they overlap and are only used on sloped roofs; so water rolls down the roof after it hits the surface, and does not get a chance to move upward into the laps under ordinary circumstances. Each row of shingles laps so far down over the one below it that the entire roof effectively has two layers of shingle material everywhere.

    But the overlapping makes repairing an area of shingles more complicated, because a roofer has to weave any new replacement shingles into the pattern of the existing ones. Shingles have to be removed around the area, and others gently lifted and pulled back at the adhesive tabs, then resecured manually with adhesive. Even a minor repair, like the replacement of flashing around a skylight shown below, involves more shingle replacement than you would expect.

    Damage to the existing shingles that remain can happen, especially if it is an older roof and they have become brittle. Shown below is an example at the intersection at the valley of the roof of an addition to a house where the old and new roofs meet. Although the addition was done only three years after the original construction, the contractor was not careful with the shingles lifted—and probably should have replaced more shingles around the valley flashing area (along bottom of photo) than was done. The result was cracks and other shingle damage that becomes more apparent over time.

    Here’s another example of an even sloppier repair around a dryer vent.

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Here’s links to a collection of our other blog posts about SHINGLE ROOFS:

• Why is granule loss a problem for an asphalt shingle roof? 

• What would cause a new asphalt shingle roof to have loose tab adhesion (sealant strips)?

• How can I tell if an asphalt shingle roof manufacturer's warranty rating is 20 years, 30 years, or more?

• What causes roof shingles to curl up at corners?  

• Why is premature curl of roof shingles a problem?

• Can I put a new asphalt shingle roof over an existing shingle roof on a manufactured/mobile home?  

• How can I tell if a roof has more than one layer of shingles? 

• What is the life expectancy of an asbestos cement shingle roof?  

• Does a roof with multiple layers of shingles last longer? 

• Why is a popped nail in a shingle roof a problem? How do I fix it?

• What are the dark lines running parallel to shingles on my roof?

• What is the average lifespan of an architectural (dimensional) shingle roof? 

• What is the average lifespan of a regular (3-tab) shingle roof?

• What does "lack of tab adhesion" in an asphalt shingle roof mean?

• What is the minimum pitch/slope of an asphalt shingle roof?

• What's the difference between an "architectural" and a regular shingle roof? 

• Why is an asphalt shingle roof difficult to inspect when wet right after it has rained?

• What causes small areas of pitting or pockmarks in an asphalt shingle roof?  

     Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.