What You Should Know About Your Cement Roof Deck?

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The roofing industry has seen some major innovations in the construction of concrete roof decks. Primarily, most cement decks are built with lightweight concrete, rather than normal weight concrete. Lightweight concrete is more affordable. That said, because of this shift in construction materials, many cement decks were built or maintained without considering the different requirements of implementing normal and lightweight concrete; and furthermore, lightweight concrete has its own host of issues. Because of this, it’s possible that your flat cement roof may need work. 

Let’s take a look at the differences between lightweight concrete and normal weight concrete, and in doing so, explore all of the most important things you need to know in order to identify if your concrete roof deck needs some maintenance or repair. 


These days, many concrete roof decks are built with lightweight concrete. This is because lightweight concrete is less dense and comparably as strong as normal weight concrete, meaning that it costs less per cubic foot. Take a look at the difference in their weights below:

  • Normal Weight Aggregate Concrete = 150 pounds per cubic foot
  • Structural Lightweight Aggregate Concrete = 90 to 120 pounds per cubic foot


While lightweight concrete is more affordable and comparably strong, tt tends to absorb water, which isn’t necessarily an issue, but if your lightweight concrete wasn’t properly installed, it could deteriorate more quickly. Let’s compare water absorption in normal weight concrete and lightweight concrete to take a closer look: 

  • Normal weight structural concrete aggregate does not absorb significant water (<2% by weight); the material is dampened, so that the water to cement ratio is not upset by including dry aggregate. The water to cement ratio is the key to insure complete chemical hydration of the cement powder to concrete with full strength.
  • Lightweight structural aggregate reportedly can absorb anywhere from 5 to 25% of its mass with water. It is a porous clinker type material filled with air voids, giving it the lightweight features. Think of the lava rock in your gas grill. Cement paste will not fill the voids, but if the lightweight aggregate is not water soaked, ponded or subjected to a continuous water spray in the bulk pile, the lightweight material will rob free water from the concrete mix, drastically upsetting the handling properties and ultimate compressive strength of the batch.

Dry Time:

  • Normal weight structural concrete will achieve a 75% internal relative humidity in less than three months.

To achieve the same 75% internal relative humidity, lightweight structural aggregate takes twice as long—almost six months, according to Portland Cement Association (PCA) Engineering Bulletin 119. When structural lightweight concrete is then poured, the aggregate voids are loaded with water.

Why Does Your Cement Deck Need an Inspection?

If a roof is installed over the top surface, and the deck was cast into a non-vented metal form deck, the logical conclusion is that the dry down mechanism is, at best, severely slowed. Cement deck roof systems installed over structural lightweight concrete in the tight time-driven sequence of construction will likely incur a very heavy upward water vapor drive.

Heavy moisture vapor accumulating within concrete roof decks lead to problems, which could include: loss of insulating value, face degradation, physical damage to the insulation, fasteners and adhesives.

Roof attachment may be lost locally, depending on the system design and materials used. Any organic or water based material or adhesive subjected to moisture condensation will suffer a loss of physical property with the roof system. This damage will occur with single ply, modified bitumen and built-up roof systems not using a well-designed vapor retarder. A complete vapor retarder seal is needed; the structural lightweight should be poured on a vented steel deck to accommodate down venting.

The flooring industry was confronted with this same issue some time ago. A moisture probe test was developed in 2002 called: ASTM F2170, “Standard Test Method for Determining Humidity in Concrete Floor Slab Using In-Site Probes.” The probes are sealed in place for 72 hours and read electronically. Acceptable relative humidity levels within the structural lightweight range from 65-85%, depending on material and flooring manufacturer.

The roofing industry has yet to establish any benchmarks or acceptance levels. Recent F2170 probe tests on 2-year and 7-year-old structural lightweight cement decks yielded relative humidity readings in the 90 – 98% range. These roofs used bead adhesives to construct the roof assembly. No vapor retarders were used. What did they find? A tear off and rebuild was necessary due to heavy moisture intrusion up into the roof insulation.

In Layman’s Terms: 

If you have experienced ongoing leaks and deterioration on your cement deck roof, AND you know the deck to be comprised of concrete; your problems may be a result of the roof system being installed over a deck that wasn’t allowed enough dry time, or it wasn’t effectively sealed to prevent the moisture from escaping the concrete—especially true if your flat cement roof implements lightweight concrete. This can lead to leaking, mold/mildew, and deterioration between the deck and membrane.

Contact NIR Roof Care for a Cement Deck Inspection

If you suspect this to be a problem at your building, contact a professional roofer to further investigate. There are currently numerous lawsuits pending right here in the Midwest, and NIR: Roof Care may be able to determine if you’re eligible for a payout.

To schedule an appointment, simply contact us. We’ve been providing the communities of Rockford, Chicagoland, Milwaukee, greater Madison, Beloit, and Joliet, with expert and affordable commercial roofing maintenance services and emergency repairs, since 1987, and we look forward to helping you save!

Connect with NIR Today!

Get in touch with us now to discuss your roofing needs. Our flat roofing specialists are ready to answer any questions and recommend a course of care to protect your roof from any problems. We look forward to hearing from you!