Tips for Handling Moisture in Concrete Roof Decks

Brief 

Moisture in structural concrete roof decks has been a longtime problem for contractors, but they can address it by having engineers and other contractors determine “when a newly placed normal-weight or lightweight structural concrete substrate is ready to be covered with a new roof system,” writes Mark Graham, vice president of technical services at the National Roofing Contractors Association.

Insight

Newly poured and, in some instances, existing structural concrete roof decks present unique moisture migration problems for the roofing industry.

This issue is not new—the industry has been facing it for years, and several authors and I have written numerous articles in Professional Roofing and other publications about it. Over time, the roofing industry has advanced its knowledge of moisture in concrete roof decks and how to best address it. And NRCA has updated its best practice guidelines for successfully dealing with structural concrete roof decks’ moisture.

Concrete basics

There are three general types of concrete: normal-weight structural concrete, lightweight structural concrete and lightweight insulating concrete.

Normal-weight structural concrete is what most people think of as concrete; it has a density of about 150 pounds per cubic foot and, when properly designed, can carry structural loads. Lightweight structural concrete has similar structural load-carrying capabilities as normal-weight structural concrete, but it has a density in the range of about 85 to 120 pcf.

Both normal-weight and lightweight structural concrete are produced by mixing large and small aggregates, Portland cement, water and, in some instances, other additives such as fly ash and various chemical admixtures. Admixtures can add entrained air to the concrete, accelerate the concrete’s curing, retain the concrete’s excess moisture and/or lengthen concrete’s finishing time.

The use of admixtures typically is not visually identifiable in the field; microscopic analysis usually is necessary for post-application identification of admixtures.

The primary difference in composition of normal-weight structural concrete and lightweight structural concrete is the type of large aggregates used. Normal-weight structural concrete contains normal-weight aggregates such as stone or crushed gravel, which are dense and typically absorb no more moisture than about 2% by weight.