Roofs That Last ... And Last ... And Last
- By Michael Fickes
- February 1st, 1999
At one major midwestern university, a routine roof inspection of a cafeteria turned up enough furniture to fill a classroom. Desks, chairs and even a couch lay strewn about the roof. The low-lying building, it seems, proved too much of a temptation for students living on the upper floors of the neighboring high-rise.
"Do you think we can hit that roof with Joe’s desk?"
"Let’s give it the old college try."
"Okay: off the side ledge, over to the vent, off the air conditioning unit and then nothing but roof."
Those darn students.
Then again, you can’t always blame students. At Oklahoma University, football fans from the community toss bottles, coolers and other debris from the top row of seats in the stadium onto the roof of a low-slung building below.
In both cases, however, maintenance required nothing more than a clean-up. Both roofs survive the regular onslaughts of debris thanks to a construction technique called protected membrane roofing or PMR.
An Idea Whose Time Has Come - Again
PMR is not new. According to David Roodvoets of DLR Consultants in Westerville, Ohio, the concept has been around for hundreds of years. One 20th-century example of PMR shows up at Harrod’s Department Store in London, England. The store’s concrete top deck forms the base surface for an asphalt roof covered by sod, grass and landscaped gardens, which protect the asphalt layer from the elements. "Harrod’s was built in the 1930s and, as far as I know, has never required a roof repair," Roodvoets says.
The Dow Chemical Company of Midland, Mich., developed the first modern PMR system in the 1950s as the result of an experiment conducted by researcher John Best. The Dow Chemical complex used coal tar roofs, which, when heated by the summer sun, would soften and drip into the building interiors. Best had been working with a new product called Styrofoam, and he decided to cover the asphalt on his building with it, to see if it would fend off the sun’s heat. He put down a layer of the lightweight insulating material and covered it with masonry cement to provide ballast.
"Twenty years later, Dow Chemical undertook a major roof maintenance program and tore all the roofs off," Roodvoets says. "When the construction crews came to Best’s building, they found a roof that apparently required no maintenance." The crews took up the cement and Styrofoam and found a soft, pliable asphalt layer that looked as if it had just been installed. Dow Chemical and Best filed for a patent and called the new roofing concept IRMA (insulated roof membrane assembly).
That was the original PMR concept, and Dow Chemical continues to market IRMA, with variations stemming the need for thicker insulation and more stable ballasting systems as a result of the energy shortages of the 1970s.
Modern IRMA systems feature a fabric rafting construction concept, according to Roodvoets. The assembly consists of two-foot by four-foot or eight-foot Styrofoam boards laid loosely over a waterproofing membrane and covered and secured at the perimeter by fabric, creating a sturdy raft. Ten pounds of gravel per square foot provides adequate ballast for an R-20 roof with four inches of foam insulation.
Another approach developed by Dow Chemical and marketed today by T. Clear Corp. of Hamilton, Ohio, consists of two-foot by four-foot tongue and groove planks of insulating material with a coat of latex-modified concrete on the top side for ballast. It weighs about 4.5 pounds per square foot and overlays the waterproofing membrane. Perimeter fasteners provide stability. T. Clear also markets PMR systems using ballasts of gravel or stone pavers.
Fallacies and Facts
James Sheahan of J. P. Sheahan Associates, Inc., a Midland, Mich., consulting firm, says that, while some colleges and universities have begun specifying PMR systems, such roofs remain relatively uncommon in academic institutions.
Facilities managers forego PMR systems for four reasons, some valid and some not, according to Sheahan and Roodvoets.
First, many assume that aesthetics, the most visible advantage of a PMR system, is the only advantage. At Oklahoma University, for instance, the roof beside the stadium was done in colorful pavers illustrating the school’s "OU" logo. During football games, the roof looks great from the blimp. The roof’s ability to withstand the damaging effects of debris tossed over the stadium wall gets lost next to the attractive aesthetic treatment. "Normally, no one will see a roof, and so aesthetic treatments simply don’t come up when a roofing system is being specified," Sheahan says.
The second reason to forego PMR involves up-front costs. "Conventional roofs typically use isocyanurate foam insulation under the membrane," Roodvoets says. "If you replace isocyanurate foam with extruded polystyrene (the generic term for Styrofoam-like material) beneath a membrane, the costs would be a wash. However, when you go above the membrane, you have to upgrade the extruded polystyrene to give it more weather resistance. This can add five percent to 10 percent to the cost per square foot. Ballasting techniques to hold loose lying insulation and membrane in place are usually competitive with methods used to adhere conventional membranes to a deck. But when you go to concrete ballast products, you can pay a premium of 10 percent to 50 percent per square foot depending on the number of penetrations in the roof."
If the up-front cost is considered as part of the lifetime cost of a roof, however, the cost of a PMR system measures up well against the cost of a conventional system. Then again, the temptation to reduce current construction costs by turning to a conventional roofing system often proves irresistible, according to both Roodvoets and Sheahan.
Third, PMR systems are generally limited to flat roofs. "Typically, you wouldn’t use a PMR system on slopes greater than two inches in 12, and even that would be pushing it," Roodvoets says. "With a proper design, you can do this, but it’s usually not done."
Psychology offers a fourth argument against PMR. "Because flat roofs traditionally use exposed waterproofing membranes, facilities managers have the feeling that you can find leaks easier on an exposed roof than on a protected membrane roof where the membrane is hidden underneath gravel or cement," Roodvoets says. "In reality, once you put insulation and gravel or cement over a membrane, leaks in the field of the roof are extremely rare. You can have a leak develop around a penetration, but even that is rare and reasonably easy to find."
Sheahan and Roodvoets note a number of benefits to PMR systems. "Schools generally build long-term structures with concrete roof decks, which are ideal for PMR systems," Sheahan says. "In addition, institutional buildings are usually large and usually have a lot of equipment on the roof. When maintenance teams work up on the roof, they won’t hurt the membrane because it’s hidden and protected. Some buildings may also need a useful or aesthetic surface on the roof. PMR systems can offer this in the form of gardens or paver designs."
Roodvoets adds that a PMR system protects the waterproofing membrane against the effects of ultraviolet radiation in sunlight, which will eventually break down any unprotected roofing membrane.
In short, PMR systems can stand up to foot traffic, weather and other problems that threaten ordinary roofs - like furniture-tossing contests among students - but may appear preferable to conventional roofing systems only if you look beyond up-front costs.
Michael Fickes is a Baltimore-based freelance writer with experience in higher education issues.