The Roof Rafter Count calculator computes the total number of rafter boards (roof joists) for a gabled roof based on the ridge length, pitch and span. It also returns the length of the ridge board, and the length and number of rafter ties and collar ties.
INSTRUCTIONS: Choose units and enter the following:
Rafter Boards (RB): The calculator computes the following rafter framing materials:
The Roof Rafter calculator is meant to help with material estimates. In all cases, confirm your estimates with a professional estimate and adhering to local building codes.
The rules in this calculator are as follows:
Rafter ties or Ceiling Joists keep the rafter boards from kicking out under the downward load on the roof. Collar ties are suggested for roofs with a rise greater than forty eight inches (48"). There is one collar tie per rafter. Note: some builder use a collar tie on every other rafter. The collar tie length is a function of the pitch of the roof and the position of the collar tie. This algorithm puts the collar tie two feet below the ridge, but only if the ridge is 48" above the rafter ties.