b'VentilationIn the experiments, no visual or imminent condensation was identi-fied in the flat-roof structures with a vapour barrier installed and a vent ratioof1/300.Inaddition,struc-tures with no installed vapour barri-ers, whether with a 1/300 or 1/100 vent ratio, and the structure with no ventilation all revealed imminent or some frost formations. Despite some overall encouraging initial findings, subsequent research was halted due to a lack of funding, and the 1/300 requirement was adopted by ensuing building codes. 1The Evolution of Roof Ventilation in the BuildingCodeThe continued use of the 1/300 vent ratio for sloped roofs and 1/150 ventratioforflatroofsinNorth Americanbuildingcoderequire-ments for ventilation of attic spaces is a somewhat arbitrary choice. Al-though the 1/300 vent ratio value was supported by the experiments ofRowleyandcolleaguesand subsequentlyverifiedthroughad-ditional testing for flat-roof struc-tures by the HHFA, the data from thoselimitedexperimentsisnot significant enough to be considered Figure 2: Various roof ventilation strategies are employed for similarascientificbasisforthebuilding roofs in the Notre-Dame-de-Grace borough of Montreal. coderequirements.In Canada, the requirement for In 1942, the US Federal Housingtally projected roof area for slopedwhich oversaw the FHA, conductedthe specimens, with the other twothe 1/300 vent ratio first appeared Authority(FHA)publishedProp- roofs. Rowleys findings were oftenresearch in order to verify the 1/300having either no ventilation or a ventin a 1959 revision inserted into the 1953NationalBuildingCodeof erty Standards and Minimum Con- assumed to be the source or basis forrequirement. Under the supervisionratio of 1/100. The structure with aCanada (NBCC), 4and the require-structionRequirementsforDwell- the 1/300 vent ratio; however, theof Ralph Britton (HHFA principal ings, 3which stated that ventilationFHA provided no references or cita- investigator), Penn State Universityvent ratio of 1/100 and one of thement (Article 3.6.2[b]) did not make should be provided for all roof cavi- tions in their publication. 1 conducted the tests, which used sixstructureswitha1/300ventratioanydistinctionbetweenstructures ties and that the minimum net ven- Five years later, the Housing andinsulated flat-roof structures. A ventwere studied without the installationwith flat and sloped roofs:tilation area be 1/300 of horizon- Home Financing Agency (HHFA),ratio of 1/300 was used for four ofof a vapour barrier. All unheated attic spaces shall be ventedtotheoutsideair.Thetotal unobstructed area of vents shall be not less than 1/300 of the insulated ceil-Simple to install ing area. Each vent shall be at least pre-grown modular60 square inches in net cross-sectional vegetated roof systems area and vents shall be spaced to pro-videtheoptimumcirculationand change of air.Adistinctioninrequiredvent ratios between flat- and sloped-roof structureswaslaterintroducedin the 1977 edition of the NBCC. 5Ar-ticle 9.19.1.3 states, Where insula-tion is placed below the roof sheath-ing, and the roof slope is less than 2 in 12 or the roof incorporates no at-tic space, the unobstructed vent area shall be not less than 1/150 of the insulated ceiling area, uniformly dis-tributed on all sides of the building. Only minor changes to these articles in the code have been made in later editions. In the latest version of the liveroof.ca NBCC (2020), 6the 1/150 vent ra-519-671-5777 tio for flat-roof structures remains. Article 9.19.1.2-2 states, Where the 16ONTARIO ROOFING NEWSISSUE 1 2024 THE ONLY SOURCE FOR PROFESSIONAL ICI ROOFING CONTRACTORS IN ONTARIO ORN'