What would cause this???

[Bobby Hamilton]

Was there only one layer of shingles, or was the new roof installed over an existing one? If so that could be part of the problem.

[dbucknavich]

Originally Posted by mnorthrup

Looks like installation problem. Heat usually does not cause cracks. It causes blisters and makes the granuals fall off. If it is a manufacture defect you will usually see it repeats on the run. And is in the same area of the strip. These are laminate shingles and have two layers glued together.
1 does it happen on more than one strip( being the 3 foot long shingle if american style and if metric than a little longer) Repeat
2 does it go thru both layers or just the top.
3 Is in in the same area of the strips if it repeats.

Wow. Very nice detective work Mark. Sounds easy now that you said it first.

[Bob Elliott]

Originally Posted by cbrown1

Who is Bob?

I am

[Stephen W. Stanczyk]

Originally Posted by cbrown1

And I am still waiting for the fish to bite!

Have to use a float or a "Bob".

[Carl A. Brown]

Originally Posted by relliott

I am

I know that but you had not shown up yet!

BOBert

[Frank Albert]

I have many pics ot those. One roof, it was caused by lack of cross bracing between the rafters, from front to rear.
All the others? Can't verify cause and effect, but the majority were missing their 4th. nail on one end. I lifted the tabs to look.

Notice the cracks are at 3' intervals. My own positively non technical opinion has to do with fiberglass. It's soft as silk when in it's woven state. Once it's been embedded in anything, it is brittle. I've worked in bodyshops and built boats, so I know this for a fact.
And, you can't get splinters when in it's soft state. Shingles throw splinters like quils.

I think the slight movement allowed by the one un-nailed end 'could' be enough to cause splitting.

I have no scientific data, just observations from experience.