Roof framing desciption
A roof must have a water-shedding surface air and vapour barriers to prevent moisture accumulation in the roof assembly, thermal insulation and ventilation to remove moisture that has escaped from the conditioned space. In Canada, most houses are framed with prefabricated wood trusses and have an unheated, ventilated attic space. Cathedral ceilings and flat roofs do not have attic spaces, but there must still be a vented space above the insulation to remove moisture.
Steeper roofs tend to shed water better than flat or low-slope roofs. The slope of a roof is expressed as a rise-to-run ratio with the vertical component, or rise, always shown first. There are two conventions for expressing the slope of a roof: imperial and ratio.
The imperial convention is based on the use of a framing square, and the run is always expressed as 12, based on 12 inches to a foot. For example, a roof with a slope of 45° is expressed as a 12⁄12 slope. A roof with a 4⁄12 slope has a rise of 4 inches for every 12 inches of run.
When expressed as a ratio, for slopes less than 45°, the first number (the vertical component) is always shown as one. A ratio of 1:5, for example, indicates a vertical of 1 unit for every 5 units of horizontal dimension. For slopes steeper than 45°, the second number (the horizontal component) is always shown as one. A ratio of 5:1, for example, indicates a vertical of 5 units for every one unit of horizontal dimension. The use of mixed units such as 1 mm in 10 m should be avoided.
Expressed as a ratio, the imperial designation of 4⁄12 is 1:3. In special cases where a high degree of accuracy is required, angular expressions of slope are acceptable.
Pitched roofs vary in slope from 1:6 to 1:1 or more. Roof slope affects such factors as the minimum slope required for various roofing materials to reduce the risk of water backing up through the roof membrane. The minimum allowable slope for asphalt shingles is 1:6, and there are different installation requirements for roof slopes of 1:3 or greater and slopes less than 1:3. There are minimum allowable slopes for each type of roofing material.
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