To calculate the roof rafter length, determine the roof's pitch and the direction in which the rafters rise from the opposite exterior walls till the ridge, find out the measurements of the opposing shorter walls of the building, and use it and a roof-pitch table to the determine the rafter length. Finally, add the overhang measurement to this value.
Begin calculating the roof rafter length by determining the direction in which the rafters of the roof rise from the ridge till the opposite exterior walls. Normally, the construction plan for the roof contains this information.
Alternatively, measure all the exterior walls, and determine which two opposite walls are longer. The rafters' bottom ends should be attached to the longer walls.
To determine the roof's pitch, access the roof's top using a ladder, place a 12-inch long level on it so that it is perpendicular to the roof's edge, and adjust it to bring the bubble to the center. Then, measure from the level's base till the roof sheeting's top. This is the roof's rise. The roof's slope is the rise in inches for every 12 inches.
Next, divide the length of the opposite shorter exterior walls by two, refer to a rafter cutting guide, look for a suitable roof-pitch table depending on the roof's pitch, and use it to determine the length of the rafter corresponding to this value.
Finally, add the measurement of the eaves on the exterior walls to this result. This is the overall length of the rafter.