What Can We Tell from This Photograph? (Case Study 2)

Posted by in Simple Forensic Methods | September 22, 2014

This is an exercise to develop the abilities of observation.

What can one know from examining this photograph?

The tabs with the horizontal and diagonal creases (upper left of the photo), the double horizontal creases (lower center of the photo), and the partial crease at the center and near the soil stack are indicative of manual lifting of the tabs “gone wrong.”  That is, some one tried to lift the tabs but fractured them.  They tried the corner, the bottom edge lift, and the center lift, but none worked adequately and the results were unusually fractured shingle stabs.  (This was further supported by the sealed condition of the damaged tabs.)

Since the cap shingles are overlapped to the right, we are probably looking at the north slope or the west slope.  Cap shingles are commonly set with the overlap facing away from the predominant wind direction of the west or the south.

The many creases and fractures from hand lifting indicate that the shingle tabs are previously well sealed and probably not very susceptible to lifting from wind.

The double crease in the tab in the lower center of the photo is indicative of double lifting by hand: first to break the bond of the seal and then bending back to cause a crease.  The small width of shingle tab down slope of the lower crease indicates that it was hand lifted.  The wind pressures and speeds required to cause a crease an inch or so above the bottom edge of the tab are tremendous and would have caused much collateral damage.

The corner breaks/chips and the bottom edge chips indicate that the shingles are relatively brittle and easily broken.  The corner chips are due to brittle fracturing during lifting by finger.

The short course at the top indicates that the shingles were installed by a relatively inexperienced installer. An experienced installer would have adjusted the exposure widths while proceeding up the slope to have a more uniform exposure length in the top course.  (Check the nailing pattern – inexperienced installers often misplace, misdrive or omit nails).

Do you have any other observations that we missed?

Please write to us at engineering@prugarinc.com and let us know.

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