The "A" and "B" components of SPF are pumped out of separate 55 gallon
drums, are mixed at the spray gun, and are applied in what are known as
"passes". A pass is a single application of foam and can vary in thickness from
1/2 inch to 1 inch or so. Therefore, SPF roofs that are several inches thick are applied
in multiple passes.
SPF must have a protective
elastomeric coating installed over it because it's very susceptible to UV and mechanical damage. Elastomeric
means the coating must be able to stretch with the foam and return to its original shape.
Coatings are spray applied and can be acrylic, silicone, butyl rubber, and different
urethanes. All have different physical characteristics and varying costs. Coatings are
usually installed in three stages: a base coat, a mid-coat, and a final coat. Granules can
be embedded into the mid and top coat or only the top coat for added protection against UV
and mechanical damage and to increase the system's fire resistance.