b'Technical Articlehour), and it can often be furtherStandard test method for the dy-definedasACH50orACH75,namicwindupliftresistanceof withthenumericalreferringtomembrane-roofing systems.the prescribed air pressure differ- Byusingthesetools,thede-ence across the envelope in Pascals.signercandeterminewhatlevel A building that leaks all of its airof uplift resistance they need in a volumewithinthebuildinginroof assembly and its various zones, one hour at a pressure differenceandwiththisinformationthey of 50 Pascals would be labeled ascan source proven and tested roof-1ACH50; the same building thatingassembliesofferedbyroofing leakedtwiceitstotalvolumeofmanufacturers who participate in a air in one hour would be labeledCSA A123.21 testing and verifica-2ACH50,andsoon.Generallytionprogram.speaking, buildings with lower airAswindspeedsincreaseacross leakage rates of one to two ACHa low-slope roofing assembly, so do willperformmuchbetterthanthe uplift forces. As expected, a me-highervolumesfromacomfort,chanically attached membrane will energyefficiencyanddurabilitytend to billow and deform during standpoint. these wind events, while a fully ad-We can also measure air leakagehered membrane will be more likely acrossasectionoftheenvelope.to stay in place. When a membrane Figure 4: Roof parapet with transition membrane available for wall connection. Courtesy of Siplast Inc. This measurement is expressed asbillows, it is a result of uplift on the L/s/m 2at 75 Pa, or litres per sec- topsideovercomingadhesionor ond per square metre of surface atother downward forces on the bot-75 Pascals. This is useful in com- tom side. So what forces are keeping paring wall types or walls to roofs.a membrane down?Consider that a roof is not a standaloneOnewouldexpectthatacom- Ballast or weights in the form of mercial multi-unit residential wallstones or pavers are a popular choice riddledwithwindowsandventsfor inverted membranesa system andpipesmighthaveahigherof insulation, fabric and ballast that element of your building, but an integral partleakage rate per square metre thanholdsawaterproofmembranein acommercialflatroofassembly;place.Thiscanbeagoodoption this is good information to havewhereadditionalweightonthe when attempting to reduce overallstructure is not a concern.of the building envelope that deserves thebuilding air leakage. currentin- Mechanical fasteners and plates Sohowdoesourprovideasolidconnectiontothe ventoryofbuildingsstackup?structural deck but are intermittent We can do better! According to aand impose point loads and thermal same consideration we pay to the walls and2017 study by the Canada Mort- bridges onto the membrane, which gageandHousingCorporationcan wear and weaken the materials (CMHC), detached homes in On- over time.tario have an average ACH of 7.5;When fully spread onto the field other critical parts of the building. in the U.S., commercial buildingsofsingle-plymembranesorwhen have an average ACH of 13.1, ac- ribbonsareclosetogether,adhe-cording to a 2014 study releasedsivesprovideanexcellentconnec-by the National Institute of Stan- tion to the structure. This method dards and Technology (NIST). is gaining popularity with not only Mechanical effect is imposed onmembranesbutentireassemblies the envelope by the mechanical air- Air and the Roof of insulation and cover boards. Its handling systems of a building. InHow does wind and air move- primarybenefitiseaseofuse,re-ordertoproperlybalancetheair- mentaffectmyroof?Thisisaduction of puncture risk (especially handling system and ensure propersimple question with a very com- inre-roofs),reductionofthermal flow per design requirements, theseplex answerprefaced by it de- bridging and its high bond strength. mechanical systems take in more airpends and how much time doManyformsofmodified-bitumen thantheyexhaust.Bydoingthis,youhave?Simplyput,aswindroofingemployeitherbuilt-inad-theyslightlyincreasethepressurerushes around and over the surfacehesives (self-adhesives or torchable within the building envelope and tryof a low-slope roof, it creates windadhesives) and spreadable adhesives to account for any air leakage thatuplift as a result of a phenomena(hot or cold asphalts).mayaffectthemechanicalsystemscalled the Bernoulli effectba- Fullyadheredanddetailedair operation (Figure 2). sically the same physics that allowandvapourbarriersatthestruc-airplane wings to create lift. tural deck level are another method UnderstandingThe National Research Coun- of ensuring the primary water and cilofCanada(NRC)hasdevel- air barrier (the membrane) stays in LeakageRates oped an online tool to assist de- place during wind events. Imagine The air leakage rate in buildingssignersindetermininghowthisforamomentthatamembrane referstothevolumeofairleak- applies to their specific intendedwantstobillowduetotheuplift ingfromeitherthewholebuild- building design. The Wind Upliftforcesofthewind;whatifthere ing or across a measured section ofResistance Calculator determineswas enough air suction below the thebuildingenvelope.Asawholethe resistance of tested low-slopemembrane to keep it in place? The building measure, air leakage can bemembrane roofing systems in ac- introductionandcarefuldetailing expressed in ACH (air changes percordancewithCSAA123.21,of an air barrier at this level ensures 22ONTARIO ROOFING NEWSISSUE 4 2023/24 THE ONLY SOURCE FOR PROFESSIONAL ICI ROOFING CONTRACTORS IN ONTARIO ORN'