I know that racking of composition shingles can leave a distinct vertical pattern curling on the roof surface. See the attached photos of the 2003 roof I inspected today. Notice the diagonal pattern of the curled tabs. None of these end tabs were properly bonded to the shingles below and there were multiple areas of wind damaged shingles including blow-offs and creased shingles.
This looks to me like the roofer installed them using a variation of the typical racking method, but on the diagonal instead of straight vertical.
Any other theories?
Notice the big patched areas on the roof in the background of the first photo
By definition, racking is installing shingles straight up the roof, alternating back and forth with each course. Because this is diagonal, it’s not racking.
What you describe can happen when the wind blows out of a single direction right after the shingles are installed and haven’t yet had a chance to bond. The same corners of each shingle are lifted by wind and the first few inches of the adhesive strips become contaminated so that they never bond fully. I’ve seen this on roofs, too.
As Ken pointed out what racking is, which most installers I 've come across do, this is called stepping which is the correct method however I suspect the installer simply butted the shingles slightly too close.
The only issue is I could see would be if they edges are not properly nailed then it could possibly present a problem with high winds.
Rarely do I find a roof that I couldn’t write something up… yet I am careful not to nit pick or get technically exhaustive.
Thanks for the feedback guys. This was a 2003 roof. When I look closely at the shingles along the diagonal lines, which are clearly visible they bear the same distinct characteristic that racked shingles do (i.e., the tab has been lifted causing it to curl and lose its bond) except not the same staggered vertical pattern.
What I think they did was to run the fan up the roof with each course extending past the course below rather than the standard fan method of extending the courses from the bottom. This would require them to lift the corner of the tab to slide the next shingle in on the row and nail it in.
As I mentioned, none of these curled ends were bonded and there were already several areas that had experienced blow-offs and creased tabs.