Commercial Building Inspector B2Building EnclosureHard
A commercial building inspector is observing the installation of a mechanically attached single-ply roof membrane. The roof is subjected to a design wind uplift pressure of 60 psf (2.87 kPa). According to IBC Section 1504.4, how must the membrane be secured to resist this uplift, given that the attachment frequency is critical?
- AThe membrane must be ballasted with a minimum of 10 psf (0.48 kPa) of aggregate.
- BFasteners must be spaced uniformly across the entire roof surface.
- CFasteners must be installed at a higher frequency in the perimeter and corner zones.
- DAdhesive must be applied continuously along all seams and edges.
Show answer & explanationAnswer & explanation
Correct answer: C. Fasteners must be installed at a higher frequency in the perimeter and corner zones.
IBC Section 1504.4, in conjunction with Chapter 16 (Structural Design), requires roof assemblies to be designed to resist wind uplift pressures. Wind uplift pressures are significantly higher at the perimeter and corner zones of a roof compared to the field of the roof. Therefore, mechanically attached membranes must have a higher frequency of fasteners in these zones to adequately resist the increased forces.
Why the other options are wrong
- A. Ballast is an alternative securement method, but the question specifies a 'mechanically attached single-ply membrane' and focuses on fastener frequency.
- B. Uniform spacing is inefficient and insufficient for the higher wind loads experienced at edges and corners.
- D. Adhesive application is for fully adhered systems, not mechanically attached systems, and typically supplements, not replaces, mechanical fastening for uplift.
Roof Wind Uplift Zones
Areas on a roof where wind uplift pressures vary, requiring different attachment frequencies for roof membranes.
- Highest pressures at corners, then perimeter, then field.
- Fastener density must increase in higher pressure zones.
- Design based on ASCE 7 and manufacturer's tested assemblies.
Memory trick: Wind grabs corners first, then edges, then the whole field.