Depending on what you are reading (and the source), the preferred low-slope roofing system for your building could be metal, spray-in-place polyurethane foam, EPDM, TPO, PVC, modified bitumen, conventional built-up roof, or even a green, protected, or vegetated roof. Many of the current claims are based upon some life-cycle analysis. The problem is that different analyses provide conflicting conclusions.
A complete life-cycle analysis, as defined in ASTM E-917 Standard Practice for Measuring Life-Cycle Costs of Buildings and Building Systems, would “encompass all relevant costs over a designated study period, including the costs of designing, purchasing/leasing, constructing/installing, operating, maintaining, repairing, replacing, and disposing of a particular building design or system.”
For roofing systems, part of the problem lies in the fact that we really don’t know the “life” of the systems we are trying to analyze. In addition, operating and maintenance costs can be highly variable depending upon roof traffic, inspection programs, and roof drainage.
One measure of “life” is when the decision is made to re-cover or re-roof the building. However, there are many variables associated with establishing that “life” has ended. One is the criticality of the occupancy. The U.S. Air Force defines this as the mission of that particular structure. A high-mission building might get a new roof after the first drop of water enters the structure, while a lower-rated structure might get a new pail and mop instead.
Another measure of life has to do with cash flow of the corporate entity. When times are bad, roofs may get patched daily as necessary, but when a cash flow is good, a major roof-replacement program may be undertaken regardless of the actual roof age.
Another measure of roof life becomes skewed by warranty life, which is mostly a marketing decision on the part of the guarantor to encourage upgraded (and more profitable) roof membrane systems.
For many modified bituminous (MB) and single-ply systems, the products are different and generally better than they were a decade ago. For example, elastomeric EPDM systems have made a number of important improvements over the years; and their expected life should reflect that. The flashings now consist of durable EPDM rubber, replacing the previously used uncured neoprene materials that aged poorly. Seam tapes and butyl adhesives are more durable than previous generations of wet adhesives. On the other hand, applications of reflective (generally white) coatings over single-ply roofs have not been around long enough to establish their “life” or to prove that they will be cost effective. As a result of these improvements, their “life” should be better than recent historical data might indicate.
Thermoplastics
Early on, life of thermoplastic (weldable) systems was characterized by loss of plasticizer, resulting in embrittlement and shrinkage. However, the poor performers have long disappeared from the marketplace. Sarnafil, a major PVC-membrane producer, has recently published data showing that many of their membranes have lasted at least 20 years and are still weldable (repairable) after all that time. What greater verification of sustainability? Competitors such as DuroLast (PVC) and FiberTite (KEE) have also established track records over at least 20 years of exposure.
Thermoplastic polyolefin (TPO) systems, as contrasted to EPDM and PVC roofing, do not have such a long track record. Some research papers on TPO have indicated that of all the different products marketed under the generic TPO name, the aging and welding properties could vary markedly.
MB Systems
Two-ply polymer-modified bituminous roof systems were slow to be accepted in the United States - the result of very bad experiences in the 1960s and 1970s, when the number of plies in built-up roof systems was reduced to just two plies of reinforcing. Much of the durability information on MB systems was gleaned from European experience.
Of the two generic types of MB, APP, and SBS, neither has been successful in driving the other from the marketplace, and both are proven in terms of durability. New peel-and-stick application systems have been introduced as alternatives to the customary torch- or mop-applied systems, but the track record of previous generations of self-adhering membranes has not been good.
Metal Roofing
Metal roof systems are of two distinct types. Hydrokinetic refers to systems that rely upon water-shedding to perform. Most of these start with a structural sub-deck, a waterproof or water-deflecting membrane (felt or film), and a metal roof surface. Forms include seamed sheets or imitations of tile or shakes. Longevity depends upon the system design, waterproofing attributes of the underlayments, quality of flashings, and durability of the organic-coating finishes. Cost is also affected by the gauge of the metal used and the number and type of fasteners/cleats used.
Most of the structural low-slope (known as hydrostatic) steel systems are coated with zinc-aluminum alloy coatings of proven durability. However, to comply with the emissivity issues of some new energy codes, colored organic polymeric coatings are being applied over the zinc or zinc-aluminum sub-coating. The life-cycle data on these new reflective, emissive coatings is yet to be established, as well as the time schedule for recoating.
Sprayed-in-Place Polyurethane Foam (and Coating)
As with the many systems just discussed, the SPF industry has also recently published a report on life-cycle benefits to SPF systems. Acrylic, urethane-elastomer, and silicone top-coatings have all proven to be very durable and repairable. Since SPF serves as both thermal insulation and membrane, direct LCC comparison with other systems must include the thermal insulation in the calculations as well. Sustainability relies upon recoating SPF systems when the coating begins to pinhole or erode, so these costs must be included in a life-cycle cost (LCC) analysis. SPF systems are frequently applied over older, weathered BUR systems, so cost analysis must consider alternatives which include total tear-off of the old BUR systems. This is also true of installing new structural systems with improved ventilation and drainage over existing, poorly designed roof systems.
Built-Up Roofing - Still an Option?
The hot-applied built up roof system is still around. It generally consists of three or more plies of asphalt-treated roofing felt laid in hot asphalt. As compared to the other systems, experience with BUR goes back centuries, not decades, and most experts feel comfortable predicting 15-plus years of life. As with single-ply roofing, improvements include conversion to glass mats from organic reinforcements, and the utilization of MB flashings on BUR systems.
Other Variables in LCC Analysis
Additional considerations include adequate drainage (i.e. slope, number and size of drains, etc.) and the nature of the substrate. In hurricane-prone environments, dense substrates such as gypsum board; lightweight, insulating concrete; and faced isocyanurate board seemed to perform better than lower-peel-resistant products such as wood fiber, glass fiber, or perlite boards. In hail belts, denser substrates are more resistant to impact than softer underlayment boards. Some single-ply products are offered in greater mil thickness (e.g. 90 mils instead of 45) just for this purpose.
Life-cycle is also very contractor dependent. Since roofs are field-applied, some contractors do a better job of controlling their installers than others. Roofing is a seasonal business and requires an experienced labor force. This can have more affect on roof life and performance than possibly any other factor. Use of qualified roof consultants both for design and observation may be the best investment a building owner can make to improve roof durability.
Other factors that are not well-defined include maintenance and overall roof management of the installed roof. While this intuitively suggests that maintained roofs last longer, the investment/return ratio is poorly defined and subjective in nature. Most trade associations have collaborated on inspection and repair manuals to better define how to inspect various roof types and on proper repair techniques for commonly experienced problems.
Perhaps the roofing industry should establish a common set of assumptions to provide a level playing field when making cost comparisons. One thing is certain - nature is a tough laboratory and long-term performance data should reveal a more true “life.”
Resources
MBCI-Metal Roof and Wall Systems’ Low Slope Life Cycle Cost (by Ducker Research)
Roofing Life-Cycle Costs Emerge by Drew Ballensky, Duro-Last
World Class Manufacturing’s What is Life Cycle Cost?
Spray Polyurethane Foam Alliance Life Cycle Cost Study (by Michelsen Technologies)
Sustainability Characteristics of SPF Roofing & Insulation Systems by Mason Knowles, Modern Materials magazine, Nov. 2003
