Field of Roof
The fastener placement links below illustrate where to place insulation fasteners for different fastening pattern rates and different size boards.
Before utilizing these diagrams you must know the "fastener contribution rate" (how many per square foot/how many per board) for the particular insulation/membrane system in the "field of roof" (FOR). For adhered membrane roof systems, find the rate for the "field of roof" (FOR) in the specific FM RoofNav Approval.
Corner and Perimeters
Because wind forces have a greater effect on the perimeter and corners of buildings, Factory Mutual requires increasing the fastening patterns in these areas.
In certain circumstances it is appropriate to increase the fastening patterns in the perimeter and corner areas using a prescriptive method. To determine whether or not to use a prescriptive method please see JM Technical Bulletin T06-03.
In some constructions where wind uplift resistance requirements equal or exceed 90 lbs. per sq. ft., increasing the number of fasteners in the insulation board will not satisfy FM requirements. For detailed information consult FM Loss Prevention Data Sheet 1-29.
Corner and Perimeter Dimension
FM determines the dimension of the corners and perimeters as the smaller of:
- 0.1 times the building lesser plan dimension
- 0.4 times the eave height
- Subject to a minimum width of 4 ft.
In this example a building has a lesser plan dimension of 60 ft. and an eave height of 110 ft. According to the FM formula, the corner and perimeter dimensions are 6 ft.
Prescriptive Increase of Fastener Rate for Corner and Perimeter
Once the corner and perimeter dimensions are established, Factory Mutual directs that:
For all mechanically fastened insulations or thermal barriers, increase the number of fasteners per board over the Factory Mutual Research Corporation (FMRC) approved field-of-roof spacing by:
- 50% in the roof perimeter but not less than 1 fastener per 2 sq. ft.
- 1 fastener per 1 sq. ft. in the roof corners
- Round up to the next whole number of fasteners
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Fastener Placement Patterns
