There are two basic types of roofs — pitched and flat — and each type has many variations.

The slope of a roof is expressed as a ratio of rise-to-run with the vertical component, or rise, always being shown first. There are two conventions for expressing the slope of a roof: imperial and metric.

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 450 is expressed as a 12/12 pitch. A roof with a 4/12 pitch has a rise of 4 inches for every 12 inches of run.

Using the metric convention, for slopes less than 45°, the first number should always be shown as one. A ratio of 1:5, for instance, indicates a rise of 1 mm for every 5 mm of hori zontal dimension, or 1 m for every 5 m. For slopes steeper than 45°, the second number (that is, the horizontal component) should always be one to facilitate easy verification. A ratio of 5:1 expresses a rise of 5 mm for a horizontal dimension of 1 mm, or 5 m for each 1 m. The use of mixed units, such as 1 mm in 10 m, should be avoided.

Expressed as a ratio, the standard slope reference of 4 in 12 (or 400 mm in 1200 mm) becomes 1:3; similarly, 3 in 12 becomes 1:4. In special cases, where a high degree of accuracy is required, angular expressions of slope are acceptable.

For purposes of definition, flat roofs might be classed as those having less than 1:6 slope. Pitched roofs vary in slope from 1:6 to 1:1 or more (for example, 2:1), depending on the roof covering and the use of attic space.

L-shape trussed roof.

See isometric below

L-shape trussed roof.

See isometric below ridge valley trusses roof sheathing gable end

girder truss common trusses double wall plates

Note: For clarity, some structural members of some of the trusses have been omitted, and roof sheathing appears continuous.

ridge valley trusses roof sheathing gable end girder truss common trusses double wall plates

Note: For clarity, some structural members of some of the trusses have been omitted, and roof sheathing appears continuous.

The dimensions of roof joists and rafters for the various grades and species of lumber and for the different live loads encountered are given in Tables 29 to 32.

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