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Moisture Issues in Concrete Roof Decks

Moisture in concrete roof decks that migrates into the roofing system can be a serious problem for the system’s durability and performance. The National Roofing Contractors Association (NRCA) has stated that they have received, and continue to do so, a significant number of reports regarding problems created by moisture in concrete roof decks.

Both normal-weight structural concrete and lightweight structural concrete contain additional water that is added to ensure the concrete fills all gaps and voids in the deck. However, lightweight structural concrete, despite its name, contains more water than normal-weight structural concrete. As the concrete dries during the curing period, excess moisture is released into the roofing system causing potential mold growth, adhesive loss, deterioration, and corrosion.

The problem can be traced to the tests used to determine the concrete roof deck’s dryness. One method simply assumes the concrete is dry after a 28-day time period. Another requires a plastic sheet be placed around the deck and to check for condensation buildup. Both are ineffective to accurately determine the amount of moisture within the concrete roof deck.

NRCA has recommended that designers not specify lightweight structural concrete for roof decks. Further, NRCA maintains that the ASTM F2170 probe testing is still the most reliable method of determining whether a concrete roof deck is dry. NRCA recommends a maximum 75 percent RH value as a threshold for determining a concrete roof deck’s dryness.

Author’s note: The information contained in this article is for general educational information only. This information does not constitute legal advice, is not intended to constitute legal advice, nor should it be relied upon as legal advice for your specific factual pattern or situation. Regulations and laws may vary depending on your location. Consult with a licensed attorney in your area if you wish to obtain legal advice and/or counsel for a particular legal issue.