Structural Lightweight Concrete Roof Decks

Over the past couple of years, we have seen a sizable increase in the specification and use of structural lightweight concrete roof decks. We felt it important enough that we share with you the concerns the roofing industry has about these decks, and to outline roofing assemblies that can perform the best when installed over these structural lightweight concrete decks and minimize the risk for roofing system warranty issues in the future.


The main difference between standard structural concrete and structural lightweight concrete is the aggregate that is used in the mix. With standard structural concrete the aggregate used is sand and stone. With structural lightweight concrete a mix of shale, slate, slag and clay are used and, with this mix, the aggregate is fully saturated prior to mixing. The end result of the structural lightweight mix is a deck that can be 20-25% lighter than standard structural concrete. However, on the down side, this structural lightweight deck can retain moisture for long periods of time. There have been studies done that show the relative humidity in these structural lightweight decks can be very high even after 4 to 5 years of being in place. This high level of RH can adversely affect any adhesion that is attempted to this roof deck resulting in adhesion failures. Because the cause of the failure was not the roofing material, but the roof deck, the roofing system warranty does not cover this failure.


Our recommendations when structural lightweight concrete decks are going to be installed, is to either install a ballasted roofing assembly or to install a vapor barrier directly over the structural lightweight deck and then mechanically fasten at least one layer of roof insulation to the deck. We would also recommend that any structural concrete deck, standard or lightweight, be tested using ASTM F2170 in Situ RH Test to ensure the concrete deck’s relative humidity is less than 80% prior to beginning any roofing work, particularly if adhering directly to the deck.


As always, if you have any questions, please contact your local Carlisle Rep to discuss in further detail.