There are multiple choices of roof membrane systems available, including traditional bituminous, polymer-modified bituminous, thermoplastic, elastomeric, metal, and sprayed-in-place polyurethane foam. Variables include the roof deck, type of air or vapor retarder, surfacing, thermal insulation, etc. Traditional attachment methods include hot-applied systems, mechanical fasteners, or even ballasted systems. Yet, cold-applied adhesives were one of the earliest and most versatile systems around. Their heydays were the years following the 1953 fire at General Motors’ transmission plant in Livonia, MI. In that event, a steel, “fireproof” building was constructed using hot asphalt to attach a bituminous vapor retarder to a steel deck, and to attach the thermal insulation to the retarder.
An under-deck fire occurred, and the steel deck transferred the heat load to the bituminous materials directly above. The asphalt melted and vaporized, entering the building through the various seams and weld-holes of the metal deck, feeding the fire. A total of 34.5 acres of building was reduced to rubble. In studying this loss, the combustible nature of the bitumen was a major contributor. (Other factors were the absence of sprinklers and firewalls, the fact that the GM fire brigade attempted to fight the fire without promptly calling the fire department, etc.)
The roofing industry responded to this disaster quickly, introducing thin (4-millimeter) PVC vapor retarders installed in solvent-based cold adhesives. The quantities of adhesive used in these systems was restricted by fire and cost considerations. They were applied in ribbons roughly 0.25-inches thick, usually on 6-inch centers, to ensure contact with the vapor retarder (or the following layer of thermal insulation). Once a layer of approved thermal insulation was in place, practice reverted to solid moppings of hot asphalt to install the rest of the roof system.
While this practice of using fire-resistant vapor retarders and reduced quantities of adhesive solved the fire problem, new problems appeared. Blow-offs were attributed to inattentive application, where the installer skipped application of adhesive at the perimeter. Since the adhesive application cart has to turn around, the end runs of the adhesive spreader were starved as half the adhesive dropped into the ribs of the deck.
Many steel decks of that era also have supplemental V-grooves in the top flange of the deck. In the process of cold-rolling the steel deck, the top steel flanges dished somewhat, resulting in the adhesive flowing toward the depressed Vs. This left insufficient adhesive to wet-out the bottom of the insulation boards. Still another factor was that the adhesives were slow curing, especially in cold weather on a cold steel deck (e.g. if the building is unheated during construction of the envelope). With roofing becoming a year-round (i.e. fast-track) profession, contraction of the roof membrane during the first few cold nights before the adhesive has set up sometimes resulted in flashing pulling off the walls or even splitting the brand-new membrane.
Since the adhesive had not yet set up, construction traffic on the newly embedded retarders and insulations sometimes results in the insulation boards popping up as the flexible deck recovers from a construction deflection. (With hot asphalt, the adhesive is set up in minutes.)
For these reasons, FM Global eventually banned the use of solvent-based adhesives, setting the stage for the mechanically fastened systems that dominate today.
Why Try Adhesives Again?
There is a new generation of adhesives, generally called low-rise or slow-rise polyurethane foam. As contrasted with the earlier solvent-based adhesives, these products cross-link fairly quickly, not requiring 30 (or more) days to reach full strength. Secondly, the rise of the foam provides a thicker layer of adhesive, assisting in making contact between the various layers of material. As with the earlier materials, there is very little material per square, so fire contribution from the adhesive is negligible.
A great deal of information on these new adhesives can be found by searching for low-rise foam adhesives on the Internet.
Pros and Cons of Low-Rise Foams
- The cured adhesive is inert, and generally would not be affected by moisture penetration into the roof system. (This was true of hot-applied asphalt and solvent-based asphalt adhesives as well.)
- The system can be cost effective, compared to mechanical attachment.
- Mechanical fasteners cause holes in the roof deck that could be a problem in the future when the building is reroofed.
- The system requires no kettles and fumes are far less then when using hot (or solvent-based asphalt applications.
- Weather. Generally require the deck to be dry with surface temperatures of 50 degrees F. or higher.
- Workmanship. With fasteners, they are either there or not. This can be verified when the underside of the steel deck is visible. In addition, the fastener is unaffected even if the deck is cold or frozen. Applying foam to a damp or frozen deck is doomed to failure.
- Conditions change from start-up to midday, and this may require changes in application techniques requiring a higher degree of operator skill.
- Substrate flexibility. A flexible deck may deflect under the weight of passing loads, springing back before the adhesive sets.
FM Global is being conservative in approving these new foam adhesives when considering systems over steel decks. FM recognizes that its uplift tests are conducted indoors under warm, dry, wind-free conditions. Cold and wet conditions are difficult to duplicate in the laboratory, but happen all the time in the field. Moreover, installing insulation and/or vapor retarders is generally the first job of a roofer’s workday. Waiting for the deck to dry out and to warm sufficiently from a rising sun is not likely to happen. (One way to verify adhesion would be for quality assurance observers to require that field uplift tests be conducted each morning and midday.)
Further information on roof securement can be found in FM Global’s Property Loss Prevention Data Sheet 1-29, Roof Deck Securement and Above-Deck Roof Components (updated August 2007). One paragraph from this updated data sheet follows:
22.214.171.124.2: For components adhered with urethane-based adhesives, asphalt, or other adhesives in ribbons, spots, etc., reduce the spacing between ribbons or spots over the FM-Approved field-of-roof spacing as noted. In the roof perimeter, not more than 60 percent of the field-of-roof spacing between rows or area. In the roof corners, not more than 40 percent of the field-of-roof spacing. Example: A particular roof cover as FM Approved is adhered with a urethane-based adhesive in ribbons at 1 feet on center in the field of the roof. The perimeter would need a ribbon spacing of 6 inches and the corners would need a ribbon spacing equal to 4 inches on center.
This FM Data sheet can be downloaded from (www.roofnav.com).
Listings of slow-rise foams can be found using FM Approvals’ RoofNav product directory. (There is no charge to register to use the RoofNav program.)