What would cause this??

[Scott Falvey]

More cracked shingles here.

House is 1960, roof framing is 2 x 6 16" OC with plywood sheathing. These shingles are a second layer and are about 15 years old. I encountered 6 or 7 vertical rows where the cracking went through 23 courses, 3 cracks were about 6' apart from each other and other areas had random spider cracking.

Would this be due to the bottom course, there were no issues of structural failure from within the attic space. It had good Gable venting but the soffit venting did not appear to be as good, with no lights on in the attic, there was not much daylight visible from the soffit vent areas.

Any thoughts?

[Peter C. Russell]

Scott, to much heat, plus a three tab shingle that is 15 years old is at it's end.

If the house had gable vents that where sized properly they should work and no soffit vents would be needed.

PS as a second roof, if the first layer was in bad shape and the second layer installed over them then it makes sense that the second layer will fail, most often prematurely

[Ed Fako]

Excessive adhesion quality on the sealant strips in addition to a thinner, lighter weight 3-tab fiberglass shingle with a reinforcement matt of minimal quality.

My first guess would be the cheap 20 year Owens Corning 3-tab shingles, but other manufacturers followed suit in an effort to cut costs in the manufacturing process.

When the adhesion from the sealant is too great, it does not allow for lateral expansion and contraction, thereby creating the vertical cracks in mutiple courses of shingle or individual shingle tabs.

Ed