Tempo:
the ceiling area more than one and one-half joist spaces, two roof joists are nailed together to form a header. One outside rafter header is then added and end-nailed to the lookout rafters and roof joists. This serves as a nailing support for the roof sheathing, fascia board and soffit covering.
ROOF SPACE VENTILATION Always provide adequate ventilation of the roof space above the insulation. Accumulated water vapour in attic spaces and under low-slope roofs could condense during cold weather in sufficient quantity to cause damage. Since most types of roof membranes are highly resistant to vapour transmission, vapour does not pass through them and ventilation is needed to remove vapour that enters the roof space. During cold weather, heat lost through the ceiling insulation combined with sun exposure may provide enough heat to melt the snow on a roof but not on the projecting eaves. Water from the melting snow can freeze and form ice dams
at the eavestrough and roof overhang, which are exposed to outdoor temperatures on all sides and are therefore colder than the ceiling. This may cause water to back up at the eaves, penetrate through the roof and leak into the walls and ceilings (see Figure 100). Adequate airtightening in combination with insulation and ventilation beneath the roof sheathing will keep attic temperatures low during winter and help prevent snow on the roof from melting. A space above the insulation between the exterior walls and the underside of the roof sheathing allows ventilation from the eaves to the attic space. When no air ventilation baffles are used, the clearance must be at least 63 mm (21⁄2 in.). If baffles are used, the clearance can be reduced to 25 mm (1 in.) (see Figure 24 on p.56). Roofs with slopes of 1:6 and greater are required to have an unobstructed vent area at least 1⁄300 of the insulated ceiling area, measured on a horizontal plane. For roofs with slopes less than 1:6, the area must be at least 1⁄150 of the ceiling area. Roof-, gable- or eave-type vents can be used (see Figures 92 and 93). Vents must be uniformly
92 Soffit roof ventilation
baffle
perforated soffit
pre-finished fascia airflow
ROOF SPACE VENTILATION Always provide adequate ventilation of the roof space above the insulation. Accumulated water vapour in attic spaces and under low-slope roofs could condense during cold weather in sufficient quantity to cause damage. Since most types of roof membranes are highly resistant to vapour transmission, vapour does not pass through them and ventilation is needed to remove vapour that enters the roof space. During cold weather, heat lost through the ceiling insulation combined with sun exposure may provide enough heat to melt the snow on a roof but not on the projecting eaves. Water from the melting snow can freeze and form ice dams
at the eavestrough and roof overhang, which are exposed to outdoor temperatures on all sides and are therefore colder than the ceiling. This may cause water to back up at the eaves, penetrate through the roof and leak into the walls and ceilings (see Figure 100). Adequate airtightening in combination with insulation and ventilation beneath the roof sheathing will keep attic temperatures low during winter and help prevent snow on the roof from melting. A space above the insulation between the exterior walls and the underside of the roof sheathing allows ventilation from the eaves to the attic space. When no air ventilation baffles are used, the clearance must be at least 63 mm (21⁄2 in.). If baffles are used, the clearance can be reduced to 25 mm (1 in.) (see Figure 24 on p.56). Roofs with slopes of 1:6 and greater are required to have an unobstructed vent area at least 1⁄300 of the insulated ceiling area, measured on a horizontal plane. For roofs with slopes less than 1:6, the area must be at least 1⁄150 of the ceiling area. Roof-, gable- or eave-type vents can be used (see Figures 92 and 93). Vents must be uniformly
92 Soffit roof ventilation
baffle
perforated soffit
pre-finished fascia airflow
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