b'Concrete DecksStructural Concrete Decks,Vapour Retarders and MoistureRethinking what we know By Helene Hardy Pierce and Joan P. Crowe, AIAThis article originally appeared in theFebruary2020issueofIIBEC Interface. Learn more about the In-ternational Institute of Building En-closure Consultants (IIBEC) by visit-ing their website at IIBEC.org.Introduction andCurrent PracticesEver since we have been install-ing roofing systems on structural concrete decks, roofing profession-als have known that an important factor in how well an installation willperformovertimedepends on the concrete deck being a suit-able substrate for the roofing sys-tem. Generally, we use terms such as clean, smooth, even and dry to describe a concrete deck that is suitable for receiving a new roofing system. When a concrete deckcannotmeetthesedescrip- isdependenton(amongothervided to differentiate between theaggregate that can hold more ini- about what the concerns are and tors,thenwemustaddressthethings) the adhesion of the insula- different densities of poured con- tial water than traditional hardhowonemightmitigatethose shortcoming(s) in an effort to en- tion to the concrete deck, whethercrete, including lightweight insu- rockaggregate,combinedwithconcerns.iiisure that the installed roofing sys- adhered with hot asphalt, low-riselating concrete. As change has oc- the use of metal forms that areGenerally, mitigation includes tem will perform. foamorotheradhesives.Ifthecurred, our industry has reactedleftinplace,haveresultedinarecommendations such as roof-Consider a generic installationsurface is not clean, then the at- when issues became known. Twosignificant likelihood of having toingshallnotcommenceuntil thatismembraneagnosticatachment could very well be inter- significantchangestonotearedeal with what should be consid- the deck is dry or in the event roofing system installed with thefered with and substandard; if thetheuseoflightweightaggregateered as a wet deck. And recentofmoisturewithinthedeck,or insulationadheredtoacast-in- surface is not smooth and even,in structural concrete and the useworkiindicates that even normal- it is the designers responsibility place structural concrete deck (asa board insulation will probablyofnon-removableforms.Fortyweight structural concrete pouredto select the deck and determine opposed to a lightweight insulat- have less surface area attachment;years ago, it was not common toover non-removable forms retainsits suitability for use. Even when ing concrete fill) with an adheredand if the deck is not dry, mois- think of structural concrete thatasignificantamountofwaterwe are offering recommendations membraneovertheinsulation.ture may well interfere with prop- consisted of a proper pour overwithin the concrete. as a manufacturer, a roofing con-Many would agree that this is aer adhesion of the insulation. removableformsasapotentialOurindustryhasaddressedtractor or a roof consultant, often commoninstallationoverthisTherehavebeenchangesinsourceforasignificantamountbothofthesechangesinmate- werelyuponwhathasworked type of roof deck and the need forthe materials and methods usedof water that roofing system de- rialsandmethodsinjointbul-clean, smooth, even and dry isin the construction of structuralsignersneededtounderstandletinsandtechnicaladvisories ii ,inotherroofsystemconfigura-easily understood, given that theconcretedecksoverthepast30and account for in their designs.whichgenerallycautionabouttions, such as installing a vapour attachment of the roofing systemyears. For clarity, Table 1 is pro- However, the use of lightweightthepotentialformoistureandretarder to the deck if it is surface the deleterious effects of trappeddry or ensuring that a vapour re-moisture. Table 2 is a summary oftarder with a permeability of X DECK TYPE DENSITY OF THE CONCRETE some of those concerns. is installed.In addition to the referencedCertainly,thesearegoodrec-advisories,manymanufacturersommendations.Thedifficulty Normal-weight structural concrete 150 lb/ft3 have also issued their own tech- comes with the recommendation nicalbulletinsthatreiteratethebehindtherecommendation. Lightweight structural concrete 85120 lb/ft3 concernsandhaveprovidedaMuchlikethewell-understood means to caution building ownersquestionbehindthequestion, Lightweight insulating concrete 2040 lb/ft3 and designers about the potentialtherecommendationbehindthe for long-term performance issues.recommendation is where our in-Table 1: The density of poured concrete across three different deck types. Over the past 15-20 years, theredustry should perhaps be focusing havebeennumerousarticlesattention and providing solutions.8ONTARIO ROOFING NEWSISSUE 2 2020 THE ONLY SOURCE FOR PROFESSIONAL ICI ROOFING CONTRACTORS IN ONTARIO ORN'