My project calls for vented 3” composite floor deck. Is that available?
My project calls for vented 3” composite floor deck. Is that available?
Yes and no. In most cases the “vents” you will get are rolled in hanger tabs (described previously in an ASK D-MAC) that provide between 0.25% and 0.50% open area. Be sure to confirm the amount of venting that the deck will provide and compare that to the requirement from the designer before placing your order.
If you want to know why your project may have this requirement, continue reading.
Venting for Lightweight Insulating Concrete
Metal decks were first vented in order to allow excess mix water that is present in lightweight insulating concrete to drain out of the non-structural slabs used in some roof systems. These vents are simply slot shaped punctures running the length of each valley of the deck. In general, the venting area that is provided for this purpose on form decks and 1.5” Type B (Wide Rib) roof deck is between 0.5% and 1.5%, which is adequate for removing excess water (not water vapor). The water content in lightweight insulating concrete is much higher than that of normal weight or lightweight structural concrete.
Venting for Structural Floor Systems
In recent years, there have been issues with newer formulations of adhesives for floor coverings installed over concrete and metal deck slabs since they seem to be more sensitive to the moisture levels in the slab. This led to the recommendation that floor decks be vented in order to allow water vapor to escape and theoretically improve the performance of the adhesives.
Venting can only do so much…
Unfortunately, the diffusion of water vapor is directly proportionate to the open area in the vapor barrier (in this case, the metal deck). The vented 3″ composite steel deck with 0.50% open area will only increase the rate of diffusion by 0.50% (i.e. zilch, nada). The Steel Deck Institute has published a Position Statement on this topic that also recommends more impactful ways to reduce the moisture content of a concrete slab on metal deck. The Position Statements can be found here as a free download. A concrete pour stop should be used to keep the concrete from flowing into the metal deck. View our pour stop detail chart to determine which should be used.
As always, please ASK D-MAC if your project has metal deck requirements that you think are unusual. There’s a good chance that we’ll agree with you!
