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The following two pictures show what happens when insulation is packed up
against the backside of a plywood roof deck. Heat is trapped at the deck level, between
the shingles and the insulation. This heat destroys the integrity of the glue holding the
plywood together, causing the plywood to weaken and warp. Ventilation would have little to
no effect in correcting this situation.
The following picture shows "fastener backout" or "nail popping".
This occurs because the deck is in poor shape, the nails are too short, or there is severe
building movement resulting from the foundation settling or seismic conditions.
These next pictures show a roof that's long overdue for replacement. This particular
roof has experienced at least two severe and several moderate to light hail storms.