Selecting the Best Roofing Options

By Sanders, Russell | Journal of Property Management, September-October 1993 | Go to article overview

Selecting the Best Roofing Options


Sanders, Russell, Journal of Property Management


Reroofing may not be the most visible improvement property managers can make, but it is probably one of the most important.

With the ever-widening selection of roofing products available, however, choosing a reroofing system can be somewhat bewildering. Today's choices range from bituminous built-up roofs to the polymer-modified bitumen roofs to thermoplastic and thermoset roofs. Often the easiest alternative is to leave the decision-making entirely in the hands of your roofing contractor or consultant. But to make sure you are getting the best quality roof for your money, and not just one contractor's favorite, you may need to do a little research.

The key to reroofing decisions, according to Jim Carlson, technical specialist at the National Roofing Contractors Association (NRCA), is to remember that "no one membrane or roof system is applicable for all low-slope reroofing projects in all climates." Carlson says the layout and slope of the existing roof assembly and the existing nature of the surrounding climate are just a few of the conditions that may affect which system is best suited for a specific application.

Materials and installation methods

Three general types of roof membranes make up most of the low-slope commercial reroofing market in the United States. They are: asphalt built-up, single-ply EPDM, and modified bitumen roofs.

Built-up roofing. The built-up bituminous roof has been around the longest and is the most prevalent low-slope roofing system in the United States. It consists of several layers of saturated felts, coated felts, fabrics, or other types of reinforcing mats assembled in place between alternate layers of bitumen.

The layers are then topped with a surface of mineral aggregate and bitumen; liquid-applied, bituminous-based coating; or a granule-surfaced sheet to protect them from the degrading effects of ultraviolet light, weather conditions, and general impact. Reflective surfacings arc also helpful in minimizing heat gain and thermal cycling.

Most of the felts used today are made of glass fibers, while a small percentage are organic. There are two types of bitumen that can be used: asphalt, derived from petroleum refining; and coal-tar pitch, a byproduct of coke processing.

Single-ply roofing. These systems came into the picture in the United States in the 1960s. Unlike the built-up roofs, they are one layer of waterproof membrane that may be put down in one application.

There are two main categories of single-ply roofing: the thermoset and the thermoplastic systems.

Of the thermosets, the EPDM (ethylene propylene diene monomer) membrane makes up most of the single-ply market. It consists of a synthetic rubber based in ethylene, propylene, and a small amount of non-conjugated diene to provide sites for vulcanization.

The most commonly used thermoplastics are PVC (polyvinyl chloride) and PVC copolymers. These plastics get their elasticity from plasticizers.

There are three primary methods for single-ply installation: fully-adhered, ballasted, and mechanically-attached.

Using the fully-adhered method, the contractor generally fastens down a layer of insulation board and then adheres the membrane to the insulation with total coverage of contact adhesive.

In a hallasted application, every component is laid loose. The insulation may be set down loose, the membrane is then laid loose over the insulation, and then the ballast--either a paver or stone--weighs the system down.

The third method, less commonly used, requires mechanical attachment of an insulation board and top membrane. This system is usually attached with screws at the seams or in the field of the membrane.

Modified bitumen roofing. Polymer modified bitumen is a third type of roof membrane system that some classify as single-ply. But at the same time, this system has many of the characteristics of a built-up roof. …

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