TPO, PVC and EPDM Mechanically Attached Systems - FM Global Perimeter and Corner Patterns

The wind resistance of mechanically attached TPO, PVC and EPDM systems is achieved by the fasteners in the membrane laps. Preliminary fastening of roof insulation under the membrane only requires 4 fasteners per 4' x 8' board according to FM (Johns Manville recommends 6 fasteners per board).
 
Because wind forces have a greater effect on the perimeter and corners of buildings, Factory Mutual requires that fastening patterns be increased in these areas. This is a two step process. First, the dimensions of the perimeter and corner must be determined and then the increased fastener rate in these areas must be determined. Please see the section on "Insulation Fastening Patterns" for instructions on how to determine the dimensions of these areas.
 
Once these dimensions are determined, Factory Mutual states:
 
"In the roof corners and perimeter, the distance between rows of roof cover fastener or batten bars should be the following maximum percentages of the FMRC-Approved spacing. These reduced spacings are used in all Approval classifications:
 
Roof Perimeter: Distance between rows is < or = 60% of the FMRC-Approved field spacing.
 
Roof Corners: Distance between rows is < or = 40% of the FMRC-Approved roof field spacing. An alternative for Class 1-90 and below is to install perimeter fastener rows (60% of roof field as above) in both directions in the corners.
 
For Membranes fastened along the side laps, increased fastening density is obtained by using narrower sheets or intermediate rows of fasteners installed through the sheet with a cover strip applied over the fasteners. Fastening increase is not obtained by increasing the number of fasteners along each row."
 
Example: A fastener in lap system with FMRC-Approved fastener spacing for Class 1-90 of rows 70" o.c. and screws 6" o.c. in these rows would use 42" maximum on center row spacing with 6" on center screw spacing in the perimeter.
 
The corner areas would utilize either 28" on center row spacing with fasteners 6" on center in these rows, or 42" on center row spacing with the rows going in both directions in the determined corner area.