Roof fastener tributary load
An arithmetic check using supplied design pressure and allowable capacity.
Idealized maximum spacing (in):
Tributary-area arithmetic
Load per fastener = pressure × tributary width × spacing in feet. Therefore spacing in inches = 12 × allowable capacity / (pressure × width). Example: 162.5 lb / (30 lb/ft² × 10 ft) × 12 = 6.5 inches.
This calculation assumes uniform pressure and equal load distribution. Design pressure, allowable capacity and tributary width must come from the applicable design and tested roof assembly. Assembly approvals, membrane tear, seams, substrate, edge zones and load factors can impose tighter requirements. The result is not an installation specification.
The former wind-pressure approximation and minimum-spacing clamp have been removed. A computed maximum must never be rounded upward to a minimum installation spacing. Consult the roof-system manufacturer and responsible designer.