Yes, water can run uphill on a roof as wind-blown rain or due to capillary action. While this is not a problem for sheet-type sealed roofing, it can sometimes lead to roof leaks on a shingle roofs. They depend primarily on down-lapping of t each layer of shingles over the one below it to resist water penetration. As the adhesive tabs near the edge of the overlap begin to loosen, wind-drive rain can get in.

    An example of capillary action is what happens when you touch the bottom of a sponge to water and it pulls the water up into it. But capillary action can also occur when there is a very small unsealed gap between shingles. The surface tension of water also contributes to this phenomenon. And if the water somehow gets through to the wood roof sheathing below, it can be drawn further by capillary action along the wood fibers.

    All three physical states of water—liquid, gas (humid air), and solid (ice)—can cause water intrusion problems on a roof. The condensation of humid air under shingles, and ice dams that cause water to back up under shingles as they melt, are two examples.To learn more, go to our article How does a home inspector find roof leaks?

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  To learn more about roofs and attics, see these other blog posts:

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

• What causes roof shingles to curl up at corners?  

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

• Why is my roof leaking?

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

• What's the average lifespan of a roof?

• Why is it a mistake to replace a roof and not replace its flashings? 

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

• What can I do to prevent roof leaks?

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

• How can I make my roof last longer?  

• How can I find out the age of a roof? 

• Should I buy a house that needs a new roof?

• Why do roof edges start leaking?

• Do stains on the ceiling mean the roof is leaking?

     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.