Spray Polyurethane Foam (SPF) Roofing

Spray polyurethane foam (SPF) roofing is formed on site by mixing two liquid components, an isocyanate (A side, typically polymeric MDI) and a polyol blend (B side), in a heated spray gun. The mixture expands within seconds into closed-cell rigid foam that bonds to the deck or existing roof. Because foam degrades in sunlight, it is always covered with a protective coating or aggregate.

Properties

• Density: typically about 40-64 kg/m3 (2.5-4.0 lb/ft3) for roofing foam.
• Compressive strength: usually above 276 kPa (40 psi).
• Insulation: about R-5.5 to R-6.5 per inch (RSI roughly 0.97-1.14 per 25 mm).
• Weight: coated SPF adds less than about 2.4 kg/m2 per 25 mm (0.5 lb/sq ft per inch); aggregate-covered SPF adds about 49 kg/m2 (10 lb/sq ft).

Installation

Foam is applied in passes of at least 13 mm (1/2 in), usually no more than 38-51 mm (1 1/2-2 in) each, to a minimum total of 25 mm (1 in). Thickness can be varied to build slope toward drains and fill low areas, which makes SPF popular for re-covering old roofs. The base coat should be applied the same day or within 24 hours.

Coatings and surfacing

Coatings must be elastomeric. Common types are acrylic, silicone, polyurethane, polyurea and butyl, typically 0.5-1.0 mm (20-40 mils) dry. Granules can be broadcast into the wet coating for traffic and hail resistance. Loose aggregate surfacing is limited to slopes of about 1/2 in per 12 in or less; coated SPF can be used on any slope.

Codes and performance

The IBC requires a minimum design slope of 1/4 in per 12 in for SPF roofs. Fire classification follows ASTM E108/UL 790, and uncoated foam must have a flame spread index of 75 or less (ASTM E84). Because foam bonds without fasteners, uplift resistance usually depends on the substrate; pull tests on decks have reached about 43 kPa (900 psf). Hail usually causes surface damage that seldom leaks and can be repaired with sealants.

Service life

Maintained SPF roofs have documented lives exceeding 30 years, with recoating usually needed every 10 to 20 years. Inspections in spring and autumn are recommended. The US trade body is the Spray Polyurethane Foam Alliance (SPFA), founded in 1987.