When to Use Spray-In-Place Polyurethane Foam for Roofing

When to Use Spray-In-Place Polyurethane Foam for Roofing

When to use Spray-in-Place Polyurethane Foam for Roofing

By Richard L. Fricklas
(click here for bio)

Columns to date have covered the common commercial membrane roofing systems, including bituminous built-up roofing (BUR), polymer-modified bituminous roofing (MB), and the thermoplastic and elastomeric single-ply systems. Others significant roofing systems include spray polyurethane foam (SPF), the subject of this column, and metal roofing systems, the target of a future column.

Why Spray Foam?

Cellular thermal insulations based upon polyurethane chemistry date back to World War II, where it was used to strengthen the wing sections of aircraft. The virtues of spray-in-place urethane foam (SPF) include light weight, high strength, outstanding adhesion, high thermal efficiency and closed cells that resist the passage of water.

The first significant applications of SPF in roofing date to the 1960's.
Since then, it has become clear that a skilled applicator force is needed, at a much higher level than the traditional installer of hot-applied BUR is.

In recognition of this, The Spray Polyurethane Foam Alliance (SPFA-division of the Plastics Council) has established the most stringent accreditation program in the roofing industry. Not only are contractors tested and evaluated, but accreditation extends to consultant/inspectors and materials manufacturers as well.

Material and application standards for SPF are now available. They include the following ASTM standards:
D 5469 Application of New Spray Applied Polyurethane Foam and Coated Roofing Systems
D 6705 Repair and Recoat of Sprayed Polyurethane Foam Roofing Systems
D 6694 Liquid-Applied Silicone Coating
D 6083 Acrylic Coating Used in Roofing
C 1029 Rigid Polyurethane Foam

Recommendations from SPFA for roofing include the use of higher density/compressive strength materials than used in the past. In general, a minimum of 2.7-pcf foam is recommended, far stronger and more suitable to roofing use than the earlier 2.0-pcf material.

The dry film thickness of the protective coatings is an important consideration in screening UV, in resisting abrasion as well as impact damage. SPF recommends that qualified inspectors verify specified thickness during application through the use of slit and core samples.

Proven Performance

The continuous use of SPF since the 1960's indicates its durability. However, with the attention now being given to energy conservation, to fluctuating energy prices and to the global warming/urban heat island phenomena, SPF is even more attractive than ever. It has the potential to be the most sustainable roofing system of all.

Performance was verified by a field and laboratory assessment of SPF by Rene Dupuis for the National Roofing Contractors Association. He studied 140 SPF roofs ranging from new up to 27 years of service.

An assessment of SPF roofs exposed to Hurricane Andrew by Thomas L. Smith (Professional Roofing Magazine, 1/93) indicated that besides being resistant to wind loading, SPF resists progressive peeling failure (as compared to single ply and bituminous roof systems) and resists water infiltration even when impacted by missiles.

In high winds, a roof will fail at its weakest link. In re-cover, the old roof system(s) should be tested by field pull tests, or mechanically reattached to the roof decking before the new SPF system is applied.

Taking advantage of SPF-Because of the extremely light weight and tenacious adhesion, SPF has been used primarily as a roof re-cover system. It can be beneficially applied to an existing, degravelled BUR. Most building codes limit a structure to two roof systems, but because the new SPF roof may weigh less than the gravel removed from the old system, it is exempted from the two-membrane restriction. This can be especially significant if one of the old roof systems contains asbestos, which can be encapsulated by the new SPF roof.

At the same time that the roof is retrofitted, surface drainage can be improved by building-up low spots and by fabricating crickets, diverting water to drains.

While the SPF is water resistant, it is degraded by UV exposure. Acceptable coatings today include polyurethane elastomers, acrylic coatings and silicone coatings. Light colors greatly improve the Urban Heat Island effect, and Energystar® ratings are available from virtually every supplier.

Sustainability

Because SPF systems do not readily leak even when the coating has eroded away or been pierced, building operators have tended to forget about their SPF roofing until significant degradation has occurred. The optimum formula for sustainability of these systems is to conduct regularly scheduled visual inspections and to recoat before the existing coating is heavily pinholed and cratered.

If the coating is neglected to the point where it is pinholed and UV is reaching the underlying foam, it will be necessary to scarify the system back down to sound material and then to apply new coating. It's far more cost effective to put new coating down before this degradation commences.

During the visual inspections, just the pumping of a little compatible caulk into a puncture may be all that is needed to bring the roof back to a serviceable condition.

Maintaining SPF roofs-Training of building maintenance crews is probably the most significant need of a building manager. It is not acceptable to smear some bituminous mastic into a damaged area, nor to punch a hole in the SPF system for a new electrical line and to fill the hole with some goop. Even field packages of spray-foam offered for this use result in very low density, weak patches that will soon fail. It is far better to arrange for a certified foam installer to be present when the penetrations are to be taken so that a proper tie-in and compatible coating are installed.

Precautions with SPF

SPF can be applied to any clean, oil-free and dry substrate. While application to degravelled roofs is certainly a common substrate, corrugated metal, asbestos-cement and common roof deck materials are also acceptable. On steel roof decks, the flute area could be taped or filled in with preformed insulation, but more likely an underlay of 1/4" gypsum board mechanically fastened would be first installed. This would provide upgraded fire resistance as well as a continuous substrate for the foam.

The spraying operation involves fine particulates that are in the process of expanding and polymerizing on the way to the roof surface. Overspray can be controlled by using wind shields when wind speeds exceed 12 mph. Exercise care by cordoning off parking areas that may be affected during spraying operations.

Detailing SPF Roof Systems

Details for flashings, drains and walkways are now available from the NRCA Roofing and Waterproofing Manual as well as the SPFA.

At existing roof expansion joints, foam may be applied up on the curbs, but the foam should be held back from rubber bellows. If needed, the bellows can be treated with the flexible coating.

Traffic and Impact Resistance-For high traffic areas, higher compression resistance in the foam and greater film thickness of the coating will enhance traffic resistance. In some cases, an additional application of coating into which is embedded grit or roofing granules increases both skid and abrasion resistance.

Chemical Resistance-The three major coating types are resistant to most solvents and oils. If it is known in advance exactly what chemicals, temperatures and concentrations of contaminants are expected, the materials suppliers can verify suitability of their products.

Where to find suppliers and applicators-Visit the SPFA web site for listings or check the NRCA Commercial Roofing Materials Guide.

Resources
ASTM International
National Roofing Contractors Association
SPFA (Spray Polyurethane Foam Alliance)