Which base sheet is best?

A study reveals which base sheets perform best over concrete decks

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 ridging.

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 2002.

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.

Decks and base sheets