MIL-HDBK-1001/SA
h) Exterior damage is localized by its closed cell structure;
i) Recoating will "renew" the system; some coatings are guaranteed for 10 years.
7.2.2 Disadvantages. Compared with conventional built-up roof systems, sprayed polyurethane foam has some offsetting disadvantages:
a) Added cost of recoating the fluid-applied membrane periodically;
b) Greater difficulty in obtaining a level surface and uniform insulation thickness;
c) Extremely high dependence on applicator's skill;
d) High dependence on good substrate preparation;
e) Reduced traffic and impact resistance;
climates;
f) High vulnerability to degradation in hot, humid
g) Vulnerability to bird-pecking. Birds will peck through the outer covering, dig out the foam, and sometimes build nests in the cavity.
7.3 Design Recommendations. Consider both design and field-application requirements. First, consider the following design items:
a) Slope of substrate. Do not rely on applicator varying foam thickness to provide slope-to-drain (minimum 1/2 inch per foot (4 percent) for new construction and 1/4 inch per foot (2 percent) for reroofing);
b) Wind uplift resistance conforming to FM
requirements;
c) Substrate requirements. Most systems prohibit foam application directly to a conventional ribbed steel deck. They require a minimum 3/8 inch (10 millimeters) thick exterior-grade plywood (untreated) fastened to the deck. Fastenings should conform with FM requirements for anchoring insulation boards
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