Roofing options "Built-up roofing systems were prevalent in the 1970s, but the method got a black eye because of quality-control issues relating to application of the asphalt," says Brad Burdic, national manager of the preferred accounts group for Johns Manville Roofing Systems Group in Denver. However, there has been a resurgence in demand for the built-up systems because asphalt has a reputation as a very stable product and a good performer, he reports.

The biggest advantage of the built-up roofing system is its redundancy. The roofing is literally built up with layers of hot asphalt. Because of the layered effect, it's less likely to produce a roof with holes.

Soprema Inc. specializes in modified bitumen roofing material that utilizes a built-up background with styrene butadiene styrene (SBS) that can be mopped down with either a torch or cold-application process. The finished product is a modified asphalt sheet with a granular surface. "This material is quite tough," says Norbert Lash, a Midwest technical representative for Soprema Inc. in Wadsworth, Ohio. "It is stronger and has good elastomeric value." The modified bitumen roofing, which moves with the building, is also ideal for roofs with high foot traffic, because it is very puncture-resistant.

A drawback of the built-up roofs is that asphalt does not handle roof movements as effectively as the membranes, and such movements can potentially cause cracking. One cause of the cracking and splitting is thermal shock, which can occur during significant swings in temperature. For example, on a cold October morning the temperature may dip to 32 degrees before rising to a high of 62 degrees once the sun comes out. Asphalt typically does not respond well to rapid changes in temperature.

Single-ply technologies first hit the market around 1980, and the single-ply membranes now comprise about 40% of the total roofing business. There are three dominant categories of membranes: EPDM, PVC (polyvinyl chloride) and TPO. EPDM membranes are essentially rubber membranes, and they have been the workhorses of the single-ply industry since the early 1980s. PVC membranes are plastic membranes, while TPO membranes feature the combined properties of both EPDM and PVC products.

Although there probably will always be a place for built-up roofing, single-ply does offer distinct advantages for some owners. Foremost, single-ply membranes can be installed at a rate that is competitive with the built-up system. In addition, the single-ply offers the advantages of being more elastomeric in nature.

Another advantage is that the membrane material is cleaner and safer to work with compared to built-up roof systems, which require hot asphalt to be mopped onto the roof. The rolls of membrane are basically hauled onto the roof and unrolled. In addition, the single-ply membranes adhere better to the new styrene roof insulations that are becoming prevalent in the industry.

"The latest trend has been toward tougher membranes," Ducharme says. In the past, installation involved loose-laid membranes that were ballasted with rock. Now, the tougher, reinforced membranes are adhered directly to the insulation. One such reinforced membrane manufactured by Carlisle is the FleeceBACK membrane, which actually puts fabric on the back of the membrane to improve tear-resistance. The reinforced EPDM membranes are ideal for roofs that receive a high volume of foot traffic.

EPDM still represents about 75% of the single-ply market. However, TPO is a growing segment, currently accounting for about 18% of the single-ply membrane business, according to Tom Gallivan, marketing manager for Stevens Roofing Systems in Holyoke, Mass. The popularity of TPOs for use in roofing applications has increased tremendously in the United States during the past several years. This is largely due to a highly desirable set of performance characteristics that TPOs provide.