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) 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
- Area of a Roof
- Pitch of a Roof
- Number of Trusses Needed for a Roof
- 4x8s needed for a roof
- Shingles needed for a roof
- Number and Length of Metal Roof Panels
- Number and Length of Purlins
- Number and Length of Rafters
- Number and Length of Rafter and Collar ties
- Number and Length of wall top and bottom wall plates
- Number of Studs for a Room
- Number of Studs in a Wall
- Room Interior Area (wall and ceiling)
- Cubic Feet in a Room (volume)
- Number of 4x8 or 4x12 Sheets for a Room
- Number and Length of Floor Joists
- Number of Joist Hangers
- Compute the Cost of Floor Joists
- Compute iJoists for larger floors
- Compute the Number of 4x8 Sheets to cover the Deck or Floor
- Compute the number of Decking Boards to cover the deck or floor
Roof Types
