Moisture in new concrete roof decks

Other possibilities
The concrete industry uses admixtures for myriad reasons. Some purport one benefit is to reduce the amount of water vapor from concrete decks. Laboratory tests and field experience suggest they have little effect. Therefore, use of concrete admixtures does not appear to be a panacea for the effects of latent moisture in concrete roof decks.

There is discussion the color of the roof surface has some effect on the issue of wet concrete. A black/low-reflectance/high-solar-gain membrane, because its temperature is warmer than a white/highly reflective roof, will drive moisture out of a roof system. However, the color of the roof surface has not been definitively deemed as a problem or a solution. What is known is a compressed construction schedule that pushes roof installation shortly after the installation of a concrete deck does not allow for adequate drying time.

Last year, NRCA, Chicago Roofing Contractors Association, Canadian Roofing Contractors Association, and a pair of roofing manufacturers began sponsoring research that includes outdoor exposure, indoor/lab exposure, lab-based hydrothermal measurements, instrumentation, and modeling (using WUFI software). The research seeks to analyze normal-weight versus LWSC with respect to:

  • the surface finish;
  • rewetting;
  • drying capacity (one-way and two-way drying);
  • moisture over time; and
  • time for instruments to measure moisture levels.

The work is ongoing, and it will continue into 2018 with the validation of the computer modeling.

It is incumbent on specifiers and designers of roof systems that will be installed over new concrete decks to understand the issues with normal- and lightweight structural concrete. General contractors should recognize compressing construction schedules exacerbates these issues because of inadequate drying time, and it may be appropriate for these contractors to use protective tarps or tents to prevent rewetting of concrete decks.

Roofing contractors have been installing new roofs over new concrete roof decks for decades. However, litigation initiated by property owners over roof system failures caused by concrete deck-sourced moisture can no longer be ignored.

By 2011, several research papers had been introduced at the International Roofing Symposium concerning roofing over new concrete decks.

“Currently, we have a pile of legal cases in our files. Even one that was initiated in 2012 and is now going to trial in 2017,” says Matt Dupuis, PhD, PE, principal of Middleton, Wisconsin-based Structural Research Inc. (SRI). “These cases are leaving all the parties involved in the construction process ‘on the hook’ from a legal perspective.”

His father Rene Dupuis, PhD, PE, helped found SRI in 1978. Since that time, the company has been a highly active research firm in the commercial roofing industry. At this writing, the company is attempting to quantify through current research factors contributing to concrete roof deck failures in new construction applications.

The decision to use a concrete roof deck begins with the structural engineer or architect, who makes a conscious choice based on his or her priorities. These can run the gamut from pursuing points under the Leadership in Energy and Environmental Design (LEED) program to construction in seismic zones. In the latter case, use of a lightweight concrete deck will give the designer benefits by being able to build higher while limiting seismic sway.

“It’s important specifiers understand the difference between lightweight concrete decks curing and lightweight concrete decks drying,” emphasizes Dupuis. “These are two completely different events.”

Specifically, curing relates to a chemical process wherein the concrete goes from a liquid to a solid state and gains strength. This chemical process proceeds at a well-documented rate. The drying of concrete is less understood by those working with the building envelope.

“The tests we have in our inventory are only testing for the presence of moisture at the surface of the concrete,” says Dupuis. “Typically, we are talking well less than [25 mm] 1 in. deep. This doesn’t help much when we roof over a deck and leave it for years. It takes time, but the moisture will move up through the concrete and make its presence known within the roof system.”

Until the roofing industry can determine a safe drying time for concrete decks, Dupuis recommends using a “heavy” vapor retarder with a perm rating of 0.01. Specifiers can also opt for doubling-up popular peel-and-stick membranes for a similar perm rating.

“Roof system manufacturers are responding and coming out with solutions in terms of systems and product recommendations—you just need to ask for them,” he says. “Putting a few dimes in per square foot for a vapor retarder is worth avoiding a potentially catastrophic roof failure and thousands or millions of dollars in legal fees.”

James R. Kirby, AIA, is GAF’s building and roofing science architect for the East Coast. He has a master’s degree in architectural structures, is a licensed architect, and has nearly 25 years of experience in the roofing industry. Kirby’s work has ranged from forensic investigation to technical writing, speaking, and education. He is a member of the American Institute of Architects (AIA), ASTM International, International Code Council (ICC), Midwest Roofing Contractors Association (MRCA), National Roofing Contractors Association (NRCA), RCI International, and the U.S. Green Building Council (USGBC). Kirby can be reached at

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