Low-Slope Re-Roof Over Lightweight Concrete Deck
A general contractor hires your GS-21 firm to re-roof an 18,000-square-foot single-story warehouse. The existing roof is ballasted EPDM over 2-inch lightweight insulating concrete (LWIC) on a steel deck; infrared scans show saturated areas and standing water 1.5 inches deep at two drains. The designer's specification requires a 60-mil mechanically attached TPO membrane with fasteners at 12 inches on center and a minimum pull-out value of 300 lbf. Your crew runs ten test pulls: the average is 210 lbf and the lowest is 140 lbf. The steel deck flutes are 1.5 inches deep, and the LWIC crumbles around several test locations. The owner wants the schedule held, and the general contractor says, 'Just use longer fasteners.' Your foreman asks whether to start membrane installation Monday.
1. What should you do before any membrane installation, and how should the pull-test results be interpreted?
Read the worked answer
The pull tests do not validate the specified attachment. The manufacturer requires 300 lbf minimum; the average is 210 lbf and the low is 140 lbf, so at least some fasteners will not develop design capacity. Longer fasteners are not a safe field substitute: the failure may be in the crumbled LWIC or in inadequate engagement with the steel deck, and changing fastener length without the designer's approval alters the attachment design.
Before any membrane is installed, stop work and notify the designer and GC in writing. Request an evaluation of the deck and LWIC, additional pull tests in sound areas, and a written attachment detail approved by the membrane manufacturer and the structural designer. Options may include removing saturated LWIC and replacing it with an approved cover board, using an adhered or induction-welded system if the substrate is suitable, or adding a structural overlay. Do not let the schedule dictate an unapproved attachment. Retain test logs, photos, and moisture readings.
2. The GC directs you to proceed with longer fasteners. How should you respond and document?
Read the worked answer
Treat the GC's oral instruction as a request for information, not a directive to proceed. Send a written RFI or notice stating that the specified attachment cannot be achieved, that longer fasteners have not been approved by the designer or manufacturer, and that proceeding would violate the specification and possibly the manufacturer's warranty.
Ask for a written design change and a signed change order covering added cost, schedule, and any delay caused by the substrate evaluation. If the GC issues a written direction to proceed, confirm that the designer has approved the method and that the manufacturer will warrant it; otherwise proceed under protest only after consulting your attorney. Photograph and retain the pull-test locations, the crumbled LWIC, and all correspondence. A verbal 'just use longer fasteners' will not protect your firm if the roof fails or the manufacturer denies the claim.