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 MIL-HDBK-1001/5A
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;
f)
High vulnerability to degradation in hot, humid
climates;
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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