When working on the PowerPoint for Vegas I found a new study that included a section on bond failure of asphalt shingles. I had always gone section by section tugging randomly on shingles.
It turns out there are patterns of failure related to thermal cycling that follow the pattern of installation (racking or diagonal) and are consistent no matter what which one is used, no matter whether shingles are 3-tab or laminated, and patterns don’t change with age.
Here’s a link to the PDF. Sealant strip stuff starts on slide 64, but there’s good information about wind and impact in other parts of this.
The idea is to start low on the roof at one corner and tug both corners of each tab in a single course until you find bond failure. Chances are very good you just found a bridging tab; a tab that bridges a joint between two shingles in the course below.
Now that you’ve located a joint, work your way up the roof following the pattern of installation, checking the corners of tabs nearest the joint. You’ll be moving straight up the roof for racked shingles, diagonally for stairstep.
Thermal cycling. If the sealant asphalt is stronger than the shingle asphalt and the shingle mat and asphalt layer are relatively thin and poor quality, you’ll see thermal cracking. If shingles are stronger than sealant strips, you’ll see failed bond… IF you see failure. Better quality shingles are able to stretch enough to accommodate the stresses created by contracting shingles at bridging/joint locations.