b'T&R UpdateLight Weight Structural Concrete Roof Decks, What to doMatt Dupuis, PhD PEWriting this article finds me returning from the 65th annualout the water vapor pressure much like we do on a light weight Midwest Roofing Contractors Association (MRCA) Conventioninsulating concrete roof. and Tradeshow in Dallas, Texas. During this show membersTo date the roofing industry has no official solution or of the Technology and Research Committee (T&R) conversedsilver bullet solution to deal with a LWSC roof deck. But, from with many of the MRCA membership. One of the recurringarticles written in roofing periodicals and attempted solutions technical concerns membership is reporting is Light Weightmy firm has reviewed or investigated, the overwhelming choice Structural Concrete (LWSC) and how to handle it.for dealing with LWSC roof decks has been a fully adhered The concern with LWSC is far from a new one. In 2011 Renevapor retarder on the deck; basically stand and fight. Dupuis and Mark Graham reported at the 2011 InternationalMake note that some manufacturers are beginning to take Symposium on Roofing Technology in Washington D.C., thatissue with LWSC in technical documents. One manufacturer there were problems emerging with this class of concrete.has issued a very strong stance. They state that their polymer-That year, 2011, the MRCA released MRCA T & R ADVISORYmodified bitumen system are not acceptable over either RWSC BULLETIN 1/2011STRUCTURAL LIGHTWEIGHT CONCRETEor LWSC. In addition their single-ply membranes are not ROOF DECKS (Available from www.mrca.org). In this advisoryacceptable over concrete roof decks without a vapor retarder. MRCA membership was alerted to the latent moistureIn this same document they concur and reiterate the problems LWSC brings with it compared to Regular WeightAsphalt Roofing Manufacturers Associations (ARMA) stance Structural Concrete (RWSC). In 2012 the National Roofingon LWSC The selection of the deck material and its suitability Contractors Association (NRCA) released an Industry Issuefor use is the responsibility of the designer of record, who Update : Moisture in Lightweight Structural Concrete Roofmust make appropriate design accommodations to address Decks authored by Mark Graham (Available from www. high moisture content encountered in lightweight structural nrca.net). In that same year, the Single-Ply Roofing Instituteconcrete decks. released an industry information bulletin, SPRI 2-13 MoistureAt this point in history there are enough technical Concerns in Roofing Systems Applied Over Lightweightadvisories, trade publication articles and technical Structural Concrete Roof Decks (available at www.spri.org).presentations on the issue of LWSC that the Designer of Record SPRI echoed the same cautions and concerns as the MRCAcannot claim that it is the roofing contractors responsibility. and NRCA. Based on the questions and continued issues myThis flows from the age old specification dogma that once the firm, Structural Research Inc. continues to hear reports of androofer begins work on the deck, they have accepted the deck as forensically investigate, it is apparent the message is beingsuitable for the new roof. However this concept is in error with heard by the roofing community but not by architects andLWSC roof decks, as the industry is learning that a LWSC roof general contractors.deck will never be acceptable with its latent moisture hidden The exact differences between LWSC and RWSC arebelow the surface. For example, I have measured moisture covered in depth by the MRCA and NRCA documents. Thesein LWSC roof decks that were 5 years old, and still returning documents serve as warnings to members of the roofing99% relative Humidity by ASTM F-2170 probes. This particular community and generally aimed at roofing contractors. Thereroof only lasted that long, in my opinion, because of a heavy have been enough articles written and presentations mademopping of asphalt used to adhere the base layer of insulation. on the topic of roofing and LWSC that most members of thePin holes in that mopping slowed the advance of the LWSC roofing community appear to be aware of its existence. Themoisture into the roof system and allowed the roof system to current problem, as mentioned, is that architects and generalperform for those 5 years, until the moisture caused membrane contractors are still not informed or are ignoring the warnings.to fail from facer delamination.Therefore we still have roofing contractors being presented With this type of moisture problem, what can a roofing with LWSC roof decks that need to be roofed.designer or contractor do to construct a functional roof system In the June, 2013 issue of MR magazine I authored an articlewhen LWSC is discovered on a project? As stated there are in which I opined two methods of dealing with a LWSC deck.no solid answers available. These general bullet points may One way was to stand and fight; keep the moisture down inprovide some guidance.the LWSC slab. The other method was to be passive, and vent 20 www.mrca.orgMidwest Roofer'