Insights to cement your success with concrete roof decks

Moisture from concrete roof decks can migrate into the roofing system and cause a host of problems. AIA partner GAF explores these moisture concerns and steps architects can take to accommodate a potential roof failure.

Concrete decks are one of the more common types of low-slope roof decks for commercial buildings.  While there are many advantages to concrete decks, there are two primary reasons to consider them—desired fire ratings and structural requirements.

Concrete can have a wide range of densities depending on the type and amount of aggregate used, the amount of water used in the mixture, and whether air is introduced as a foam via the use of surfactants and the like.

Structural concrete decks are cast-in-place and become an integral part of the structure. They use normal weight concrete and are designed to carry heavy loads. Structural concrete composite decks are based on a steel panel deck system that is overlaid with normal weight or lightweight structural concrete. Loads are carried by the combination of the steel deck and concrete, which act as a single component. Typically, the steel panels are embossed to ensure mechanical coupling to the concrete.

Roof decks that are cast-in-place contain a high level of water when poured. This is true for normal weight structural concrete and, importantly, the amount of water is greatly increased with the use of lightweight structural concrete. Normal weight structural concrete uses regular aggregates (i.e., hard rocks) with a low moisture absorption rate, while lightweight concrete uses shales and clays that are expanded with air to make them less dense, but they have a higher moisture absorption rate.  

On a jobsite, regardless of concrete type, additional water is often added to the mix to reduce the viscosity to ensure concrete easily flows so there are no gaps or voids in the final product. Aggregate for lightweight concrete is often intentionally loaded with water because of the voids. This is necessary so the water needed for concrete curing is not pulled into the aggregate.

Typically the roofing industry has required a 28‐day curing period prior to testing the roof deck for “dryness” and suitability for roofing. This recommendation was garnered from the concrete industry’s recommendation for the appropriate amount of time for concrete to cure and develop adequate compressive strength; however, the 28-day timeframe is not related to the amount of water in the concrete, only to the cure time. The roofing industry now understands that concrete continues to cure and release excess moisture (i.e., dry out) for a significant time. It’s important to note that lightweight concrete’s higher moisture content (relative to normal weight) means a much longer drying time is needed.

When a concrete deck is poured, some of the mix water is used up by the curing process and some evaporates, but the rate of evaporation is slow, so large quantities of water remain stored within the structure of the concrete for extended periods of time. Moisture retention is exacerbated by construction methods that install concrete over non‐removable, non-vented metal forms (or other impermeable substrates). While the concrete itself is generally not damaged by this moisture, the moisture typically migrates into the roofing system where it is absorbed by materials that are more sensitive to moisture.

There are several technical advisories that have been issued to raise awareness about the potential for moisture issues associated with concrete roof decks. These include the National Roofing Contractors Association, the Asphalt Roofing Manufacturers Association, the Single-Ply Roofing Institute, the Polyisocyanurate Insulation Manufacturers Association, and the International Institute of Building Enclosure Consultants.

Allowing concrete to thoroughly dry is most appropriate; however, it is reasonably impractical to go beyond 28 days. More realistic ideas must be implemented to accommodate a potentially wet substrate.

• Installation of a vapor retarder on the top surface of the concrete deck. The National Roofing Contractors Association (NRCA) advises that a vapor retarder of less than 0.01 perm is necessary over new concrete roof decks. From a practical standpoint, a vapor retarder of less than 0.01 perm is effectively a vapor barrier—almost no moisture passes through into the roof assembly.  For more on the use of vapor retarders, see this guide.
• Use a venting base sheet in conjunction with vents and venting edge details (at edges, parapets, and penetrations) to provide a pathway to allow the moisture in a wet concrete deck to escape slowly over time.  
• Lightweight concrete should always be installed over a vented steel deck to allow downward drying.  

Care must be taken to ensure that moisture in poured concrete decks is prevented from moving upward into the roof assembly. This is typically achieved by the installation of a vapor retarder over the deck. Although more creative solutions can be found in this article.

For more information about moisture in concrete decks, see this article to clear up misinformation in the industry and this article on the advantages of concrete over steel decks.

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