SPF Roofing: How Spray Foam Roof Systems Work on Commercial Buildings

Most commercial roofs are a stack of separate parts: a deck, a layer of rigid insulation, a cover board, a membrane, and a lot of seams, fasteners and flashing details holding it all together. Each seam is a place a leak can start.

A spray polyurethane foam (SPF) roof replaces most of that stack with one continuous layer. Here is how it works and where it makes sense.

What an SPF roof is

An SPF roof has two parts:

• The foam. A high-density closed-cell polyurethane — typically around 2.5 to 3 lb per cubic foot, denser than wall foam — sprayed directly onto the roof deck or over a prepared existing roof. It expands, bonds and cures into a rigid, seamless layer.
• The coating. Foam does not hold up to sunlight on its own, so it is sealed with an elastomeric coating: silicone, acrylic, polyurethane or polyurea. The coating handles UV and weather; the foam handles insulation, waterproofing and drainage shape.

ArmorThane’s ArmorFoam Roof 200, for example, is a two-component, closed-cell, Class A fire-rated exterior roofing foam with nominal core densities from 2.5 to 3.0 pcf.

Why building owners choose it

• Seamless. No laps, no fastener rows, fewer places for water to get in. Penetrations and parapets are foamed in continuously.
• Insulation built in. Every inch of foam adds R-value, which cuts cooling loads under a hot roof.
• Fixes drainage. Foam can be sprayed thicker in low areas and tapered toward drains, eliminating ponding.
• Light. Foam adds very little weight compared to adding more conventional layers.
• Renewable. When the coating wears, it is cleaned and recoated. Well-maintained SPF roofs can stay in service for decades.

How an SPF roof goes on

Where SPF fits — and where it does not

SPF shines on low-slope commercial, industrial and agricultural roofs, including over corrugated metal where filling the flutes and sealing the fasteners solves chronic leaks. It is also popular on roofs with lots of penetrations and odd shapes, where membranes get complicated.

It is a poorer fit when the existing roof is saturated and the owner will not pay to remove the wet material, or when the site cannot be protected from overspray on windy days. It also needs a crew that does it regularly: weather, foam temperature and surface condition all affect quality.

Maintenance

An SPF roof wants a little attention: an inspection once or twice a year and after major storms, prompt repair of any mechanical damage (a dropped tool, hail bruising), and a recoat when the coating thins. Small foam repairs are fast and inexpensive. It is the opposite of a “set and forget” roof — and that is why it lasts.

Questions to ask a roofing contractor

• What foam density and thickness, and which coating at what dry film thickness?
• How will you survey the existing roof for moisture, and what happens to wet areas?
• Can the design correct ponding and improve drainage?
• What warranty applies, and what maintenance does it require?
• How will you control overspray on this site?