How to Attach Minimum Slope Roof Insulation for I-90 Uplift

Factory Mutual roofing standards are determined by a laboratory operated under the auspices of Factory Mutual Global, an insurance company. FM 1-90 is one standard for wind resistance. This is a complex formula based on local wind speeds, the type of roof, the pitch or slope, roofing materials and other factors. There is no universal 1-90 standard; it refers to an uplift pressure of 90 pounds per square foot, which will vary by region and roofing material. Insulation is part of the 1-90 equation.

Instructions

- 1
  Match the type of fastener to the insulation; extruded polystyrene (XPS) is the most common low-slope roofing insulation, but polyisocyanurate (polyiso) is also frequently used because it is thinner yet offers the same insulating value. Follow the insulation manufacturer's recommendations for the fastener types; XPS is usually attached with galvanized nails or screws while polyiso may also be attached with some mechanical fasteners.
- 2
  Follow the insulation manufacturer's recommendations for the fastening methods. Check the Factory Mutual Approval Guide, which lists recommendations for every type of insulation for all roofing products and assemblies; a minimum slope built-up roof, with gravel to hold it down, will differ from a single-ply membrane roof.
- 3
  Stagger the fasteners in most cases, rather than simply placing them along the outside edges. One pattern suggested by a manufacturer for a 4-by-8-foot roofing panel alternates fasteners at 6-inch intervals between the outside edges and center sections of the insulation..
- 4
  Use 15 fasteners for insulation under each 4-by-8-foot sheet of standing seam metal roof with another manufacturer's insulation; space these fasteners 24 inches apart around the edges of the panel. Pick your pattern: Factory Mutual suggests four fasteners per 4-by-8-foot insulation panel during roof construction, with the final attachment through the roof decking or roofing material; however, Dow Chemical recommends five fasteners per sheet for its products.
- 5
  Utilize specific fasteners for specific roofs. Galvanized steel nails or screws are used for most installations but check the insulation manufacturer for specific fastener types and placement. Some manufacturers use special types of nails and screws, such as nails with ring or screw tips or screws with wide plate heads. Follow the manufacturer's directions for the installation method; XPS, for example, is usually fastened through the foam, but polyiso coverings must not be penetrated and require different fasteners.

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