What You Should Know About Your Cement Roof Deck

When it comes to roof decks, you probably don’t pay much attention to what is underneath your roof membrane. Just like roof membranes, there are a variety of different types of decks that provide support and stability to the roof system as a whole; metal, concrete, gypsum and even wood. Some come precasted, while others are constructed/poured on-site.  Hopefully, your architect/designer of the building took this into account before choosing what type of roof deck was best suited for how your building based on how it is used.

The roofing industry has been hit with some new facts regarding the use of Concrete Roof Decks.  There are several types of concrete decks; lightweight, regular and insulated lightweight.  In this article, we’ll focus on lightweight concrete and what you need to know.

New construction has moved from using normal weight aggregate to structural lightweight aggregate. This can be enticing, as the weight savings is considerable while the structural properties are similar.


Normal Weight Aggregate Concrete = 150 pounds per cubic foot


Structural Lightweight Aggregate Concrete = 90 to 120 pounds per cubic foot


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 according to studies.


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.

If a roof is installed over the top surface and the deck was cast into non-vented metal form deck, the logical conclusion is that the dry down mechanism is at best severely retarded.  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 the roof system leads 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 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 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 concrete roof 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.

What does this all mean to you?

If you have experienced ongoing leaks and deterioration on your 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. therefore creating leaking, mold/mildew and deterioration between the deck and membrane.

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.

Posted in NIR Blog.