For 35 years, roofing professionals have used asphalt base
sheets as mechanically attached bases for roof systems. But roofing
contractors, specifiers and consultants are faced with different
types of lightweight insulating concrete decks, and the performance
of base sheets over these decks has varied. When not performing
properly, base sheets can contribute to roof system problems,
including displacement from wind, wrinkling, splitting and
During summer 2002, Richard P. Canon, principal with Canon
Consulting and Engineering Co. Inc., Spartansburg, S.C.; William J.
Woodring, director of field technical operations for GAF Materials
Corp., Wayne, N.J.; and I made an effort to document how base
sheets perform by conducting extensive field sampling, laboratory
testing and analysis. We sought to determine whether physical
changes occur in the base sheets over time when installed over
lightweight insulating concrete decks. Eleven roof systems were
selected ranging in age from 7 years old to older than 10 years.
The testing included determining decks' moisture contents and pH
values; decks' fastener pull-out resistance strengths; physical
condition of the base sheets and their respective roof systems; and
tensile testing of base sheets' desaturated fiberglass mats. The
results of this investigation were presented at the Roof
Consultants Institute's Building Envelope Symposium in November
What did we find? The results, though not well-documented until
this research, were not surprising: There is a significant
difference in performance between a utility base sheet and venting
base sheet when the moisture content of the lightweight insulating
concrete deck is above 15 percent to 20 percent. That's the bottom
line. Following is how we arrived at our conclusion.
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