1652325240836 Figure511

Roofing’s Weakest Link

June 22, 2006

Field experience is a hard way to gain knowledge. If roof systems would only fail faster, we could gain that experience more quickly and at less cost. It is when a system is pretty good - but not good enough - that it really hurts.

Examples abound in the low-slope commercial roofing system business. Case in point - do you remember the oil embargo and very high inflation during the Nixon Administration (1971-1973)? The Administration attempted to freeze the prices of construction materials (among other things). Back then, built-up bituminous roofing was dominant, with the major products being 43-pound, coated, organic base sheets and #15 asphalt-saturated, organic-ply sheets. Prices of both were frozen and the roofing manufacturers were not able to pass on the increased cost in raw materials such as asphalt. However, new products avoided these price caps, so the roofing industry rushed to introduce #20 saturated felts (Twin-Twenty®) and two-ply, coated, felt systems (Bond-Ply® or Dual-Eighty®) - systems that only used two-coated felts instead of the traditional 4-ply BUR systems. Even the slogans were clever: 1 + 1 = 4.

A Painful Lesson
These systems were not really that bad - just not good enough. Unfortunately, they did not fail that
 quickly, during which time virtually every membrane manufacturer jumped on board; only then did the splits and blisters appear in abundance. Manufacturers looked to their meager and hurried research for explanations, tending to blame workmanship rather than their products since their test roofs didn’t seem that bad. These 2-ply failures severely damaged the credibility of the BUR industry and helped set the stage for the acceptance of single-ply and modified bituminous (MB) roof systems. Looking back, it’s easy to understand the reluctance of the U.S. roofing industry to endorse MB roofs until they had performed for many years in Europe. Yes, these are mostly 1 + 1, but they really do appear equal to the traditional 4-ply BUR. Performance criteria for these MB systems were developed and ASTM standards published. Even today, we are still learning about roof slippage with MB, as well as addressing problems with cold adhesive systems. The self-adhered systems also have weak links that will ultimately have to be addressed. Part of the trouble with research on membrane roofing is the multiple interactions of roof deck, vapor control, attachment methods, membrane itself, climate variables, roof drainage (slope), workmanship techniques, and maintenance.

EPDM roof systems are fortunate in that they have many strong points - especially durability. As problems showed up, such as premature aging of uncured neoprene flashings, water sensitivity of neoprene adhesives, and sheet shrinkage, solutions evolved with strong research behind them. Taped seams, restraining bars, and thicker sheets have worked well, and EPDM performance is now excellent. Ballasting of EPDM roofs led to some interesting challenges; fortunately, the technical efforts by Single Ply Roofing Industry (SPRI) and its Technical Director Dave Roodvoets managed to publish ANSI-RP-4, which addresses ballast size, weight, and eave requirements.

At the March 2006 Roof Consultants Institute’s annual convention, a very well-organized presentation by Dr. Bas Baskaran of the National Research Council of Canada addressed an issue with mechanically fastened roof systems, whether EPDM, PVC, or TPO. The questions posed were:

  1. By modifying the wind uplift resistance for one of the components, how much does the wind-uplift rating of the assembly improve?
  2. Does the presence of a vapor retarder in a system modify the force resistance chain? If so, to what degree does the wind-uplift rating change?
  3. Does a difference in steel deck thickness affect the force resistance chain; if so, how much?

(In addition, during the course of the research, an evaluation was made of single-sided seam welds for thermoplastic systems vs. double-sided welds.)

Dr. Baskaran’s illustration of wind effects and the links of the “chain” are illustrated in Fig. 51-1. Note that the “conventionally attached” mechanically fastened system met 90psf uplift resistance, while the step-wise improvements went from 90 to 135 to 158 and, ultimately, to 180psf with only incremental cost increases. Each step was earned by understanding, identifying, and improving the weakest link.

A Special Interest Group for Dynamic Evaluation of Roofing Systems (SIGDERS) is carrying out this fine work. The group includes materials manufacturers, building owners, and industry associations. The entire paper is available from RCI, SPRI, or NRCC. Also of interest may be A Guide for the Wind Design of Mechanically Attached Flexible Membrane Roofs, a 107-page book that is available on the Institute for Research in Construction’s website (http://irc.nrc-cnrc.gc.ca/pubs/catalogue/nrcc47652_e.html).

Asphalt Roofing Manufacturers Association (www.asphaltroofing.org)
Single Ply Roofing Industry (SPRI) (www.spri.org)
National Roofing Contractors Association (www.nrca.net)
National Research Council Canada (www.NRC.ca)
The Association for Roofing, Waterproofing, and Building Envelope Professionals (RCI Inc.) 

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