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:
- (L) Ridge Length
- (S) Roof Span
- (P) Roof Pitch
- (oH) Overhang (default: 12")
- (oC) On-center rafter spacing (default: 24")
Rafter Boards (RB): The calculator computes the following rafter framing materials:
- Length of Ridge Board
- Length and number of Rafter Boards
- Length and number of Rafter Ties
- Length and number of Collar Ties
The Math / Science
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:
- The length of the Ridge is the length of the ridge plate
- The choice of rafter boards is 2x4 for rafters shorter than six feet (6'), 2x6s for rafters between 6 feet and 10 feet, and 2x8s for up to 16 foot long rafters. Consult your local codes for the required material specifications.
- Collar ties are included when the total rise is above 48".
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.
Building Calculators
- Total Building Materials Calculator
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- Shingles needed for a roof - Calculates the number of standard bundles (33.3 ft2) of shingles for a roof.
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- Screws for a Metal Roof - Computes the number of screws needed for a metal roof.
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- Number and Length of Rafters - Computes the number and length of rafter boards based on roof dimensions and spacing.
- Number and Length of Rafter and Collar ties - Computes the number and length of rafter ties and collar ties based on roof dimensions and spacing.