Moisture in concrete roof decks

Normal-weight and lightweight structural concrete cause some concern

NRCA continues to receive a significant number of reports of moisture-related problems associated with concrete roof decks. Following is some background information and NRCA's latest recommendations for addressing the issue.

What's happened

The issue of moisture in concrete roof decks is not new. Since 2009, NRCA has received numerous reports of moisture-related problems with roof systems installed on concrete roof decks—both lightweight structural and normal-weight structural concrete. Reported problems include roof system moisture accumulation, adhesion loss, adhesive issues with water-based and low-volatile organic compound adhesives, metal and fastener corrosion, insulation R-value loss and microbial growth.

Since the 2001 publication of The NRCA Roofing and Waterproofing Manual, Fifth Edition, NRCA no longer considers the plastic sheet test method as a viable assessment to determine a concrete roof deck's dryness before roof system application. Also, there is little to no correlation between concrete's 28-day curing period and its true "dryness."