There are multiple possible causes of a roof leak, and a roof is more likely to leak as it gets older. But roofs don’t leak simply because they are old. There has to be an event, threshold of deterioration after which the material begins to fail, or manufacturing or installation defect to trigger the water intrusion. In the photo above, the roof leaks because of an event: a falling tree limb punctured both a shingle and the sheathing below. You can also see a dip below the hole, where the sheathing has buckled from the impact. 

   Also, a roof is more likely to leak at its flashings (such as drip edge, valley, and wall-to-roof flashing) and penetrations (like plumbing vent pipes, chimneys and skylights) than anywhere else. This means that a home with an elegant roofline that has multiple intersecting planes, a chimney and maybe a couple of dormer windows is more likely to leak than a simple gable roof on a rectangular-shaped house. When we are in doubt about where a leak is coming from, flashings and roof penetrations are where we begin our search.

   Here's our “Top 10” causes of roof leaks, followed by a gallery of photos of examples of these defects that are leaking, or will start leaking soon:

1. Poorly installed or missing flashings
2. Incorrect installation of the roofing material
3. Temporary repairs with mastic or caulk
4. Backed up gutters.
5. Improper deck over roof or low slope roof used as a deck.
6. Bolted-thru satellite dishes, solar systems
7. Damaged plumbing boots
8. Trees scraping roof
9. Impact from falling large tree branch or hail
10. Roofing material deterioration

Temporary repairs to roof damage with roofing mastic typically fail within a year.

Older shingles begin to break away at the bottom edge when a gust of wind lifts them up.

Even a tree branch that is gently brushing against a shingle roof will eventually destroy the shingles in that area.

When flashings start to rust through, pin-hole leaks begin.

The acidity created by leaf debris on a low-slope shortens the life of the roof.

Sloppy nailing of roof shingles causes problems like this.

When the edges of the roof shingles begin to curl up, your roof is pretty much over with.

The flashing around plumbing vents that extend through the roof is called a “boot.” When they deteriorate, as in this photos, minor leakage around the vent pipe begins.

All the nailing of a shingle roof should be buried under an overlapping shingle. This photos shows “face nailing,” which is another cause of roof leaks.

Roof-to-wall flashings are very important where a wall rises above an adjacent roof. Here, the lack of a “kick-out flashing” at the bottom of where the roof and wall meet has allowed water to be funneled behind the wall.

Certain types of flat roofs are designed to allow standing water, called “ponding” in the roofing trade. But a large area of deep ponding will cause premature deterioration of the area under it.

If lifting up the front edge of a roof shingle is effortless, the roof is at the end of its serviceable lifespan. Any significant storm event will rip off the shingles.

When the tar in a “tar and gravel” roof deteriorates, it cracks apart in the pattern called “alligatoring,” and signals that the roof is ready to be replaced.

Poorly installed and cheap skylights always leak, especially ones like this one with no raised curb around it.

Backed-up gutters create leak problems at drip edge of a roof over time.

Diverter strip at edge of roof over walkway has caused standing water that seeps under shingles behind it.  

Satellite dish bolted to roof is leaking, causing stain in soffit below.

Close-up of hail impact damage to shingle

    Also, see our blog posts Do stains on the ceiling mean the roof is leaking? and How can I tell if the house needs a new roof?  

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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?

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

• What are the mistakes to avoid when doing attic improvements?

• What causes roof shingles to curl up at corners?  

• What causes shingles to buckle along a line on the roof?

• What causes leaks at a fake roof dormer? 

• What causes a sagging roof ridge line?• What causes bubble-like blisters in a built-up and gravel roof?  

• Why does it cost so much more to replace a steep roof than a low slope roof? 

• 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?

• Why is premature curl of roof shingles a problem?

• 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? 

• 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? 

• 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.