Better Understand Why the Combination of Moisture and Concrete Roof Decks Is Troublesome

Unfortunately, many in the industry mistakenly equate this 28-day period with the time required for the concrete to be considered “dry”. This is false. There is no known correlation between this 28-day curing period and the concrete’s dryness. Of course, as stated earlier in this article, pressure on the industry to meet construction schedules helps promote this thinking.

Although most concrete decks may look and feel dry after 28 days, there is substantial evidence they are not sufficiently dry enough beneath the surface to accept a roof membrane.


A second method involves dripping or mopping hot bitumen onto the concrete deck surface. If the concrete is too wet, the hot bitumen causes water on the concrete surface to bubble, turn to steam and splatter. If the hot bitumen does not splatter, the concrete deck is believed to be sufficiently dry to accept a roof membrane.

Although the hot bitumen may indicate the presence of surface moisture, this method does not provide a way to know if a 1/2 inch or more below the surface the concrete is actually wet. NRCA no longer accepts this method as reliable.


A third method is standardized as ASTM D4263, “Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method”. This protocol calls for taping a small piece of trans- parent plastic or glass pane to the concrete deck surface for 24 hours. If, after 24 hours, there is condensation on the bottom of the plastic or glass, then the roof deck is considered too wet for the application of a roof membrane.

NRCA no longer considers this method reliable either. This method can result in false readings. One reason for inaccurate results is that it’s difficult to achieve an airtight seal at the test panel’s edges. Secondly, unless the temperatures on the top and bottom sides of the concrete deck are nearly the same, the resulting lack of a vapor pressure difference can give a false “dry” indication.

To illustrate, if the test is performed on a warm day, then no condensation may appear because the plastic will be warmer than the concrete slab. Even if the test is performed in the shade in- stead of the sun, similar results may occur.


Still another way for measuring the concrete deck’s level of moisture content is to use the method the flooring industry has found to be reliable: ASTM F2170, “Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes.” This meth- od calls for small holes to be drilled in the concrete slab, then small moisture probes are inserted and sealed in the drilled openings for a minimum of 72 hours. Each probe reads the concrete’s internal temperature and RH.

Although NRCA considers this method to have some merit, it also realizes ASTM F2170 must overcome the following challenges before becoming a viable test method:

  • First, the roofing industry, unlike the flooring industry, has not yet “established any benchmarks or accept- able levels” for moisture in concrete. Therefore, no one can say with any assurance at what RH reading a roof membrane can be reliably installed.
  • Second, no practical method has been established to get a reliable number even if an acceptable benchmark is established. The moisture probes used in the flooring industry are de- signed to work in an interior environment under a fixed temperature and fixed moisture level. That’s not the case when the probes are used on a concrete roof deck. They are exposed to changes in temperature and RH, as well as changes in weather. For in- stance, what happens if current technology moisture probes are installed in a roof slab and it rains? The test results would be skewed.

In addition, there currently is not an acceptable RH value for concrete roof decks. To attain the 75 percent value accepted by the flooring industry, normal-weight structural concrete would take close to 90 days to dry under ideal, controlled laboratory conditions (no rewetting). Meanwhile, lightweight structural concrete would take about six months to reach the same 75 percent value.

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About David F. Cook

David F. Cook is the principal architect for the Structural & Architectural Evaluation Group for CTLGroup, Skokie, Ill. He has more than 30 years’ architectural experience in commercial and residential construction, project design, structural evaluation and project management.

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