Seattle Home Inspector's Blog

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Is the sun really destroying your roof?

It is a widely held belief that roof ventilation is necessary to prevent overheating of the roof shingles which would result in shortening their life expectancy.

This is simply not true.

This whole conversation is a bit technical and difficult to come to terms with, especially when we cannot eliminate the old information running in the background that keeps us wanting to come to the conclusion that venting is necessary.  Venting “can” be necessary, but it is not “necessarily” necessary—and not for the reasons we think.

In cold climates we vent attic spaces to keep the roof temperature cold--and this way prevent ice dams and flush away minor amounts of moisture that finds its way into the attic space.  Good air sealing of the building envelope is essential to minimize the flow of warm air and moisture from the interior into the attic space.

In hot climates we vent attic spaces to keep the roof cooler to reduce the house cooling loads.  Good air sealing of the building envelope is essential to minimize the flow of warm air and moisture from the exterior into the living space

None of these have anything to do with the life expectancy of shingles as is evidenced by the fact that shingle manufacturers do not have different warranties based on where their shingles are installed.  The Building Science Corporation has lots of good information on all of this for those interested in the hard data.

The maximum temperature that the shingles of an unvented roof can achieve is still well below the maximum for what the shingles are rated.  Frequently shingles with blisters that are the result of factory defects is used to support the notion that the roof is overheating.  These shingles will fail anywhere--regardless of venting or not venting.  Here are two pictures of roof shingles blisters.  The first picture show the black dots where the surface has popped off the blisters.

Shingle blisters from factory defect

This second picture shows the blisters that are raised little bumps and the surface has not yet popped off.

Blisters that have not yet popped

Since many inspectors also think that lack of venting is the cause of shingle failure, we frequently hear the call for additional venting as a solution.  More venting can actually exacerbate other conditions (increase the flow of conditioned air into the attic space) and would not lower roof temperatures significantly to alleviate this kind of problem.

More and more we are seeing roofs with no ventilation at all, and the shingles are behaving themselves just fine.

It is interesting to note that the maximum temperature difference in the worst location for solar heating in the country (Vegas) the temperature difference between vented and unvented is about 17 degrees Fahrenheit. The high end of that difference is for black shingles with white shingles being less. Over the life of the shingles the white shingles will last longer whether vented or unvented (less than 5% different). 

In that sense whoever chooses the shingle color is responsible for how long the roof lasts.  Sometimes it makes sense to give that person what they want.

Charles Buell, Real Estate Inspections in Seattle

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Seattle Home Inspector

 

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WA State, Home Inspector Advisory Licensing Board

Comment balloon 42 commentsCharles Buell • May 07 2015 09:57AM
Is the sun really destroying your roof?
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