In a previous "On the Roof" column, the focus was on the top surface of roof insulation and other underlayment boards. It was pointed out that anything that adversely affected adhesion could result in shrinkage, peel, blister or blow-off problems.

Other interfaces are just as important. Roof decks are the primary stabilizer for roof membrane systems. They provide anchorage against shear and uplift forces. When bituminous membranes were attached directly to cast-in-place or precast concrete roof decks, we focused on proper cure of the roof deck (28 days), and a dry surface free of 'fins'. These decks also required the use of an asphalt primer.

A recent column expressed concerns about ASTM changing the permitted viscosity to comply with VOC enforcement zones. Some manufacturers in these zones are allowing the primer step to be omitted, others suggesting that the contractor preheat the primer or (gasp), to cut it with gasoline (when no one is looking), to make it more sprayable.

Problems with steel decks With steel roof decks, we generally use mechanically attached roof systems. What many have forgotten is that fire-rated cold adhesives were used with steel decking back in the 1950s, replacing hot asphalt after the famous General Motors fire in Livonia, MI. Roof system shrinkage, wrinkling, splitting and blow-offs associated with these cold 'vapor-barrier' adhesives quickly replaced fire losses as the major problem with roofs over steel decks.

In the 1970s, ARMA funded a study conducted by Factory Mutual (now FM Global) to figure out what was wrong. They adhered thermal insulation directly to steel decks, using hot asphalt, and various cold adhesives. Set-up time was found to range from less than one day for hot asphalt to 2-3 weeks for the other adhesives.

On a roof, membrane temperatures will plunge from day to night. With the unset adhesive incapable of resisting the membrane's contractive forces, displacement will occur. Wrinkles appear, flashings (if installed) will pull away from the walls and curbs, and wind gusts have been observed to actually lift the membrane and insulation off the deck. As a result of the ARMA research, coupled with FM's field experience, cold adhesives were removed from the FM Approval Guide.

While FM continued to approve restricted quantities of asphalt (12-15 lbs) on steel decks for several more years, problems were found with hot as well. The most serious were related to rapid gelling of hot asphalt when applied to a chilled roof deck. Work by Walt Rossiter at NIST and Rene Dupuis at SRI showed that the bitumen could lose its tack in just a couple of seconds when the deck was cold-a time period so short that roofers could not possibly get insulation boards properly imbedded. In 1984, FM delisted hot asphalt application to steel as well, leaving only mechanical fasteners for Class 1 approvals.

Cold adhesives are especially vulnerable to workmanship errors. Because the adhesives are expensive, they are applied in narrow ribbons to just the top flanges of the steel deck. When mechanical spreaders are used, the installer wheels the spreader around to start the next run. This leaves inadequate adhesive at the most vulnerable spots-perimeters and corners. Another problem with adhesive application is that it is typically a first-in-the-morning operation. With cold adhesives, the contractor does not have to wait for the kettle to get hot, or for frost or dew to evaporate. Applying adhesives to a frozen or wet deck guarantees failure.

Why cold adhesives? Environmental and safety issues have prompted the roofing industry to take another look at cold adhesives for direct-to-deck attachment. In particular, new cross-linking foam adhesives seem to have addressed the issues of VOCs and slow set-up.

For steel decks, a mechanically installed first layer of roof insulation or gypsum board is still required, but this layer helps resolve the questions about substrate quality and dryness. The foam is used to anchor subsequent insulation layers, as well as fleece-backed single-ply systems.

Because of cost, most of these foam adhesives are applied in ribbons. Proper training and inspection is essential with these high-tech adhesives, as the set-time, skinning and poor work practice issues will still exist, as well as concerns for application to unsuitable substrates.