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


The roofing industry is looking at various ways to remedy or minimize the problems surrounding concrete roof deck moisture. Aside from ASTM F2170 and the promise that it may one day help determine a concrete slab’s acceptable level of dryness, let’s look at some other ways of solving these problems.

One of the most obvious solutions is to avoid using lightweight structural concrete. It holds significantly more water than normal-weight structural concrete and requires at least twice as long to sufficiently dry out.

Another thing to avoid is using a metal deck. Doing so will not allow the concrete slab to dry to the bottom.

Another approach is to change the mix by including additives and admixtures that change the water/cement ratio. This mix would contain less water and dry out faster, yet provide a good, high-strength concrete.

There’s also work being done on using different coatings to coat the roof deck. These coatings are designed to seal the moisture in and prevent it from causing problems with the moisture-sensitive materials. While this may sound promising, the cost of many of these coatings are relatively expensive compared to the overall roof system, so, at the present time, this is not a good solution.

Still another approach is to use moisture-vapor-transmission calculations. Moisture migrates through roofing system components over time. The construction industry has been investigating moisture vapor flow for many years. Sophisticated computer programs have been developed to predict moisture vapor flow and are used by building envelope consultants. These programs have inherent limitations. They are typically one-dimensional analytic tools that focus on vapor diffusion and do not account for bulk air or water leakage. Therefore, if the roof membrane and air barrier malfunction, the results will be inaccurate. Another limitation is that hygrothermal analysis program results require knowing the maximum amount of moisture that roofing materials can safely tolerate and these levels have not been established by manufacturers.


Manufacturers’ installation requirements often try to place the responsibility on roofing contractors when it comes to ensuring a structural concrete deck’s suitability for installing roof- ing materials, project contract and specification.

NRCA, however, says it is not the contractor’s responsibility and given current technology, it is beyond his or her controllability to assess properly. Pointing to all the variables involved in this decision, such as concrete mix design, placement, curing and drying, NRCA claims roofing contractors do not have this information or simply lack the knowledge to make an informed decision.

Normally, a roofing contractor can make a visual or tactile assessment of whether the concrete’s uppermost surface is dry, but he or she cannot accurately assess what level of moisture content lies within the concrete and its likely release.


According to NRCA, the decision to cover a newly placed concrete deck with a new roof system should be made by the building’s structural engineer, general contractor, concrete supplier and concrete placement contractor. The reason is that each of them should have considerably more knowledge than the roofing contractor about the particular concrete’s curing and moisture-release rates. If additional assistance would be helpful, NRCA recommends consulting the building’s roof system designer and roof system manufacturer.

NRCA contends its position on this issue is similar to the flooring industry. That is, floor-covering manufacturers of resilient tile and textile floor coverings and coatings typically require quantitative moisture testing be performed before floor covering installation on concrete. They use ASTM F2170 for this purpose.

In new construction, NRCA recommends designers should not specify—and construction managers and general contractors should not use—lightweight structural concrete for roof decks or as toppings for roof decks. It is NRCA’s opinion that the risks of moisture-related problems associated with lightweight structural concrete roof decks outweigh the benefits.

Should lightweight structural concrete be used, NRCA recommends designers clearly state the concrete’s drying parameters and ASTM F2170 should be used for this purpose. Until such time that pass-fail criteria be established for determining concrete’s internal humidity, NRCA suggests a maximum of 75 percent RH value be used. And, whenever organic-based materials are used for roof system components, such as wood fiberboard, perlite board and some insulation facer sheets, then lower RH values may be required.


For the near term, it appears that the roofing industry will continue to face costly and troublesome problems with moisture and concrete decks. Until the roofing industry finds and adopts an acceptable moisture test method or finds a system that can accommodate the release of moisture from the underlying structure, the project designer, general contractor, concrete contractor and their suppliers—not the roofing contractor—should make the decision of when a concrete deck may be roofed. With concrete specifications and the project environment at their fingertips, including the heated interior and additional high-moisture interior components and other factors that could drive moisture out of the concrete, these other professionals are more knowledgeable of the potential water/moisture migration and potential vapor pressures than roofing contractors.

Once the design and management group believe the roof deck is sufficiently dry for it to be safe to install the roof assembly, then, and only then, should they authorize the roofing contractor to proceed with installation of the roof.

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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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