## Cutting Common Rafters

Laying out a rafter is not a difficult problem if the workman understands the underlying principles. The student should, therefore, strive to understand principles rather than to rely upon certain rules and figures that may memorized.

The main jobs in laying out a rafter are (1) determining the angles of cut at the ends of the rafter and at the bird's mouth or seat, and (2) determining the length. This is easily done by means of the carpenter's steel square and is made possible by the fact that a rafter may be considered as the hypotenuse of a right triangle, the rise and run of the rafter being the other two sides or legs of the triangle.

121. Definitions.â€”The ivork line is a straight line laid off about midway between the edges of the rafter and parallel to them. It is used as a base line in measuring and marking out the rafters.

Work fine

Work fine

The run of a rafter is the horizontal distance measured from the outside edge of the plate to a point directly below the top end of the rafter (see Fig. 97). In the case of a plain gable roof with the ridge in the middle of the building, the run is half the width or span of the building.

The rise is the vertical distance from the plate to the upper end of the rafter (upper end of the work line).

The pitch of a rafter is a measure of its slope and is defined as the ratio of the rise of the rafter to twice its run. Expressed as a formula,

Pitch rise

### 2 X run 80

In the case of a gable rafter, the pitch is then the ratio of the rise of the rafter to the span of the building. For a rafter having a rise of 4 ft. and a run of 8 ft., the pitch is