Common Rafter: A rafter that runs perpendicular (90° when looking straight down on the roof) from the top of the wall plate to the roof ridge forming the hypotenuse or diagonal of a right triangle.
Valley Rafter: A rafter that runs from the top of the wall plate to the roof ridge at the intersection of the gable extension with the main roof.
Valley Jack Rafter: A rafter that runs from the ridge to the valley, perpendicular to the ridge.
Hip Rafter: A rafter that runs from the top of the wall plate to the roof ridge diagonally.
Hip Jack Rafter: A rafter that runs 90° from the top of the wall plate to the hip rafter.
Cripple Jack Rafter: A rafter that runs from the hip to the valley perpendicular to the ridge.
Dormer Rafter: A rafter that sets on top of the main roof without cutting into the main roof. (For example, when remodeling or adding on.)
USING THE RAFTER SQUARE
The use of the Rafter Square is based on two simple building measurements; (1) the rafter run, and the (2) the rafter rise. These can be obtained from either building blueprints, drawings, or actual measurements. The tables used are also based on these two simple measurements.
Rafter Run: Run is the horizontal distance, measured in feet, that the rafter will span.
Rafter Rise: Rise is the vertical distance, measured in feet, between the highest and lowest point of the rafter.
Inch Rise: The rise measured in inches per foot run.
The Inch Rise gives you the corresponding number on the scale to be used, and can be obtained with the following formula:
Inch Rise, run and the tables in the back of this book are all that are needed to layout rafter lengths and cuts for common, hip, valley and jack rafters.
Obtain Rafter Run: Measure or calculate the horizontal distance the rafter will span, starting at the outside of the wall on which it rests, including any boarding on the wall if it extends to the wall top plate.
Obtain Roof Rise: Measure, calculate or obtain from the blueprints, the distance in feet the ridge will be from the top of the wall.
TABLE A : Change inches to feet
1'' = .08' 2'' = .16' 3'' = .25' 4'' = .33' 5'' = .42' 6'' = .50' 7'' = .58' 8'' = .67' 9'' = .75' 10'' = .83' 11'' = .92'
Calculate Inch Rise: Example; Run = 11 ft., Rise = 5 ft., 8 in. First, using Table A, convert rise to feet in decimal form. Therefore, rise now equals 5.67 ft. Using the formula for Inch Rise, we see that:
Round off Inch Rise to the nearest inch. Rounding off the Inch Rise will make the actual roof rise slightly higher or lower, and will not matter in most cases. Rounding off this figure has a more dramatic effect in buildings with longer rafter runs or higher roof rises. If Inch Rise is given on a blueprint, the previous calculation is not needed. Just use the Inch Rise given.
We are now ready to layout each type of rafter.