Another way to fasten

Two-component polyurethane-based adhesives can be a viable option for roofing professionals

  • Two-component urethane-based adhesives do not rely on ambient temperature or humidity for curing and provide a more predictable cure time.
  • Carts are ideally suited for larger projects where the roof is more open. Some carts have hoses as long as 30 feet, increasing mobility and production, so moving around the roof and providing coverage around penetrations do not pose problems.
  • Some two-component insulation adhesives are allowed to rise and spread from a liquid state to a foam state before the insulation board is placed into the adhesive. The foaming action develops cell structure, which, when compressed, increases the contact area of the adhesive between the board and substrate.
  • Several industries preclude the use of mechanical fasteners that penetrate the roof deck. In addition, some owners and designers have gravitated toward environmentally sensitive "green" adhesive materials available on the market.
  • Two-component insulation adhesives are designed for use on virtually every common roof deck. In addition, many can be used to adhere layers of insulation together.

For more than a century, the commercial roofing industry has used a variety of fastening techniques and methods to secure roof coverings and insulation to decks. Materials historically were affixed to building substrates with hot coal-tar or asphalt-based materials or held in place with pavers or stone ballast.

Driven by FM Global, mechanical fastening began to evolve during the early 1980s as an approved method for attaching insulation and single-ply or low-slope roof coverings because of asphalt-based products' contribution to fire. Although not always the best option for all deck types, mechanical fastening remains the dominant method for securing insulation and roof membranes to steel and wood decks. However, adhesive fastening offers roofing contractors, designers and building owners an alternative when it comes to installing insulation on various deck and building types.

Mechanical fastening limits

Often, mechanical fasteners are not the best option for attaching insulation to concrete, gypsum or cementitious wood fiber roof decks. Most concrete—lightweight and structural—and gypsum decks must be pre-drilled to accept most roofing fasteners. Pre-drilling is time-consuming and labor-intensive, requiring roofing workers to spend a larger percentage of their time installing fasteners.

Although not all fasteners require pre-drilling, the installation process and volume of fasteners required on lightweight insulating concrete decks makes the application questionable at best from a productivity perspective.

It should be noted there are two types of mechanical fasteners for structural concrete and some other difficult-to-drill applications: screws and hammer-in fasteners. Hammer-in fasteners with either fluted shanks or split bulb ends require extra care during the installation process and are difficult to remove without considerable effort, often resulting in some level of measurable damage to roof decks.

Regardless of specific deck types, there are a wide assortment of industries and building types that generally preclude the use of any fasteners that penetrate the roof deck. This is particularly true for hospital, health care and medical facilities; freezer and cold storage buildings; some types of school buildings; food-processing plants; facilities that contain sensitive high-tech manufacturing; or buildings containing potentially corrosive processes. Additionally, facilities that have electrical and/or data conduits or other hazardous mechanicals attached to the deck's underside will not allow mechanical fastening.

Noise, odor, aesthetics, environmental concerns and thermal bridging are some major factors that typically play a role in these decisions.

Noise is a great concern to facility owners and managers particularly when it comes to educational, medical, health care and hospital facilities. Generally speaking, building owners do not want students or patients to be disturbed by drilling or hammering when a roof system is being installed.

One of the oldest methods for securing insulation and roof covers to buildings is to use hot asphalt. The asphalt is pumped to the roof and mopped down as an adhesive for the insulation or roof cover. Some building owners shy away from this attachment method because asphalt fumes can be disruptive to building occupants.

A roof system's primary purpose is to keep water out and a building dry. However, in some cases, roofs can add to building aesthetics. For example, a fully adhered single-ply membrane with an adhered cover board will not have insulation plates telegraphing through the membrane. This is especially desirable when attaching extruded ribs to simulate a metal roof profile.

When it comes to structures such as natatoriums, freezer buildings, food-processing plants and bakeries, owners typically do not allow mechanical fasteners to be drilled through roof decks to avoid introducing contaminants into controlled environments.

During the past decade or so, the construction industry has faced a renewed interest in green building technology and energy-efficient construction products. As a result of this movement—not to mention increasing heating and cooling costs—thermal bridging has become a bigger issue among building owners. Although energy loss through fasteners has only been quantified theoretically, some owners prefer adhesive fastening techniques to mechanical fastening.

Adhesive fastening history

Historically, there have been several insulation adhesives on the market: solvent-based asphalt adhesives, solvent-based nonasphalt adhesives, water-based asphalt adhesives, water-based nonasphalt adhesives, polyurethane-based adhesives and hot asphalt. Although some of these have come and gone, it's important to understand the characteristics and limitations of adhesives commonly available.

For more than 150 years, asphalt has been used widely in the roofing industry. Hot asphalt offers a single-source adhesive for bonding insulation to monolithic roof decks as well as for bonding multiple layers of certain types of insulation and roll goods.

During the 1960s, cold asphalt-based adhesives were developed primarily for use as an interlayer adhesive in built-up roof (BUR) systems that had organic felts and fiber mats. Within a decade, cold asphalt-based adhesives emerged as an accepted method for adhering polymer-modified bitumen roof systems. Based on asphalt or coal tar, these adhesives typically are applied at ambient temperatures rather than heated.

In terms of performance, some polymer-modified bitmuen-based adhesives provide strong wind-uplift resistances but have long and unpredictable cure times, particularly when used in multilayer applications. As with any adhesive, using the correct amount for the specific installation is critical. If excessive amounts of adhesive are used, roofing materials can slide out of place and take longer to cure. Additionally, trapped solvents may soften roofing materials and cause blisters. In contrast, using insufficient amounts of adhesive may result in an incomplete or weakened bond. Lastly, asphalt-based products may be somewhat messy and may not be aesthetically appealing, especially when used with a white or light-colored single-ply membrane.

To help expedite the curing process, manufacturers incorporated moisture-cure urethane into the chemistry of the subsequent generation of asphalt-based adhesives. Although this helped improve cure times, these adhesives typically rely on ambient temperature and humidity to cure. Depending on ambient conditions, it may take several days for a moisture-cure adhesive to fully cure and provide any bond between a substrate and insulation. When using adhesives with cure times that are influenced by ambient conditions, the waterproofing layer cannot be installed until the adhesive is cured.

A new generation of single-component polyurethane-based adhesives was next on the market. Typically, these are cleaner to use than asphalt-based adhesives though they still rely on ambient temperature and humidity to cure. As such, seasonal conditions and geography can affect cure time. Single-component polyurethane-based adhesives can be used in new and reroofing applications on a wide range of substrates and temperatures. The same moisture-cure issues that limit the effectiveness of urethane-modified asphalts also affect some single-component polyurethane-based adhesives.

Two-component adhesives

As an alternative to single-component polyurethane options, two-component polyurethane-based insulation adhesives were introduced during the mid-1990s. These adhesives do not rely on ambient temperature or humidity for curing and offer a more predictable cure time.

Two-component adhesives typically consist of Part 1 and Part 2 components that are mixed during the application procedure. A controlled reaction between the two chemical components ensures a quick setup and cure time.

As with some other types of insulation adhesives, two-component polyurethane adhesives are virtually odor-free. As a result, these adhesives usually are allowed on projects such as hospitals, medical facilities and schools where strong odors cannot be tolerated. Equally important, these adhesives are compatible with most insulation and deck types and are installed with relatively quiet application methods, allowing them to be used in noise-sensitive areas.

Typically, two-component insulation adhesives are applied by either spray or extrusion. Spray application is the oldest method of dispensing such materials and requires certain equipment as well as job-site and personal protection precautions.

For example, when spraying two-component insulation adhesives, operators typically have to wear a respirator, full personal protective equipment (PPE) suit, gloves and boot covers to prevent exposure to airborne particles. Additionally, contractors must take precautions to ensure overspray does not cover adjacent building components, vehicles or equipment in nearby areas.

These cautions aside, spray application provides full adhesive coverage, meaning the insulation's entire surface is in contact with the adhesive. This application method provides the least expensive option as a result of reduced packaging costs, and many roofing contractors who have invested in sophisticated spray rig equipment prefer it to other application methods.

It should be noted the cost of the equipment required to spray these adhesives is high when compared with other application options and requires extensive worker training and maintenance. Additionally, such equipment requires operators to fully understand the need to keep adhesives "on ratio" because using materials "off ratio" can result in a compromised installation. Depending on the system being installed, such applications meet FM Approval ratings that in certain configurations can exceed 900 pounds per square foot.

Large roof systems with few obstructions and where a large area of insulation can be installed without frequent starts and stops are particularly well-suited for this application type.

However, extrusion is the most common method of applying a two-component insulation adhesive. For these applications, contractors typically use a hand-held applicator similar to a large caulk gun or a larger dispensing cart. Whereas full-coverage units hold 50 gallons or more of each polyurethane adhesive component, carts typically hold 5 gallons of each part and hand guns typically hold 1,500-ml cartridges.

Cartridges are particularly useful on smaller roofs, hard-to-reach areas, and on roof systems with many penetrations and cut-outs. Cartridge guns are lightweight, reliable, and easy to handle and maneuver.

For larger projects where the roof is more wide open, carts typically are preferred. Some carts have hoses as long as 30 feet, increasing mobility and production, so moving around the roof and providing coverage around penetrations do not pose problems.

Generally speaking, carts are adhesive-specific and should not be used for different brands of adhesives. This is primarily because adhesives differ in viscosity, and each cart is designed to dispense a specific brand. Because the adhesive is not atomized as it is with larger spray units, PPE typically is limited to eye protection and gloves while using carts, and the risk of adhesive overspray being carried by the wind or air currents to unwanted surfaces such as walls and cars is eliminated.


Current two-component insulation adhesives are designed for use on virtually every common roof deck. Most materials available on the market adhere to steel, wood, concrete, gypsum, lightweight concrete, cementitious wood fiber, smooth-surfaced BUR, smooth- and granular-surfaced polymer-modified bitumen, asphaltic vapor barriers and fleece-backed vapor barriers.

In addition, they are compatible with polyisocyanurate, expanded polystyrene, extruded polystyrene, and wood fiber, perlite and hard boards such as DensDeck,® Securerock® and high-density polyisocyanurate cover boards.

Unlike mechanical fasteners, two-part polyurethane insulation adhesives offer a nonpenetrating application method and do not provide a thermal short.


In 2005, an industry standard outlining field test procedures was introduced for determining the bond strength of insulation adhesives over various substrates. The standard—ANSI/SPRI IA-1 2005, "Standard Field Test Procedure for Determining the Mechanical Uplift Resistance of Insulation Adhesives over Various Substrates"—was developed by SPRI and canvassed through the American National Standards Institute (ANSI), becoming a national standard. Following ANSI protocol, SPRI reaffirmed the standard in 2010 through the review and ballot process. The test procedure encompasses various types of insulation adhesives, substrates and insulations.

To conduct a pull test for an adhesive, the ANSI/SPRI standard calls for a specific and detailed process. The specific procedure is available free on SPRI's website,

Economics of insulation adhesives

Insulation adhesives generally are more expensive than mechanical fasteners. However, on decks that require pre-drilling, the adhesive will provide a reduced installed cost for the insulation attachment. This is because more squares of material can generally be installed per man hour, which provides greater production per shift with the same number of roof technicians.

A simple example is to fasten a 4- by 8-foot insulation board to a concrete deck with a typical 16-fastener-per-board density. It will take about 10 minutes to drill the 16 holes and install the fasteners. However, as the fatigue of repetitive bending to drill and fasten increases, the time frame can expand to more than 15 minutes. To fasten the same 32 square feet of insulation using 4- by 4-foot boards with a two-part polyurethane adhesive typically takes less than one minute, and fatigue and ergonomic issues are not factors. Based on feedback my company has received from contractors, this labor saving and productivity increase easily outweigh the increased material cost.

Although mechanical fastening is the preferred method for many roof system installations, adhesive fastening is an option for many specialty applications. During the past few decades, adhesive fastening technology has evolved to the point there are many options and solutions for every building and deck type.

Stan Choiniere is technical director of OMG Roofing Products, Agawam, Mass.


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