Asphalt shingles are nailed to the roof sheathing below along the top edge of each strip of shingles, which is covered up by the row of shingles above it. But the exposed bottom edge of the shingles has adhesive strips that secures them in place and prevents the shingles from being lifted up by a gust of wind. Over time, usually starting at about 15-years old, a 3-tab shingle roof starts to lose its adhesive grip along the bottom edge. This progresses as the roof ages, with the edges starting to curl (shown in the photo below), until the bottom edge of the shingle is loose.
As the shingles become more brittle with age, any lifting of the leading edge by wind causes part of the shingle to snap off, exposing the nails below. Exposed nails holes are points that will leak.
Also, when shingles are beginning to lose adhesion, the exposed edge can be lifted up by hand and it makes a soft tearing sound. As they totally lose adhesion, lifting makes no sound at all. Checking tab adhesion is one way that a home inspector evaluates the condition of a shingle roof.
Also, see our blog post Why is premature curl of roof shingles a problem?
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To learn more about roofs and attics, see these other blog posts:
• Why is my roof sheathing sagging between the trusses?
• What is "ponding" on a flat roof?
• Is an attic required to have a light by the building code?
• How can I inspect my roof for hurricane damage?
• How can I tell if a roof has more than one layer of shingles?
• What are the common problems with attic insulation?
• What is the life expectancy of an asbestos cement shingle roof?
• What's the average lifespan of a roof?
• Why is it a mistake to replace a roof and not replace its flashings?
• Why is there no attic access hatch in the house?
• What is the building code requirement for an attic access hatch, scuttle, or door?
• Does a roof with multiple layers of shingles last longer?
• What can I do to prevent roof leaks?
• Are roof trusses better than roof rafters (stick framing)?
• Why is a popped nail in a shingle roof a problem? How do I fix it?
• What are the most common problems with wood roof trusses?
• What causes a lump or dip in the roof?
• If my roof is not leaking, why does it need to be replaced?
• How can I be sure my roofing contractor got a permit?
• How many layers of roofing are allowed on a home?
• What are the dark lines running parallel to shingles on my roof?
• Can metal roofing be used on a low slope/pitch roof?
• How can I make my roof last longer?
• What are the warning signs of a dangerous attic pull-down ladder?
• How can I find out the age of a roof?
• Should I buy a house that needs a new roof?
• Should I buy a house with an old roof?
• Why do roof edges start leaking?
• Why do my dormer windows leak?
• Do home inspectors go on the roof? Do they get in the attic?
• Should I put gutters on the house?
• How much of a roof truss can I cut out to make a storage platform in the attic?
• What's the difference between an "architectural" and a regular shingle roof?
• What does a home inspector look for when examining a roof?
• Do stains on the ceiling mean the roof is leaking?
• How can I tell if the house needs a new roof?
• Why does my homeowner's insurance want a roof inspection?
• What are the hazards to avoid when going into an attic?
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.