Rafter Length & Roof Framing Calculator
Common-rafter geometry and material quantities for a simple symmetrical gable roof.
Geometry estimate, not structural design. Common rafters only: no hips, valleys, jack rafters, unequal slopes or shed roofs.
Your common rafters
Rafters to buy: pieces
Shortest listed stock length
This is the shortest listed length at least as long as the calculated line. No trimming reserve is included. Confirm availability and extra length for cuts, layout and finishing before ordering; this is not a finished cut length.
- Horizontal run to ridge face
- ft
- Roof rise to ridge-face geometry
- ft
- Roof angle from horizontal
- °
- Rafter line from wall to ridge face
- in
- Horizontal eave overhang
- in
- Sloped tail / overhang line length
- in
- Total rafter line length
- ft
- Calculated common rafters
- pieces
- Rafter-pair locations
- Number of spaces
- Entered maximum on-center spacing
- in
- Actual evenly distributed spacing
- in
- Purchasing allowance
- %
- Base calculated rafter line footage
- ft
- Purchase-planning rafter line footage
- ft
- Purchased stock footage
- ft
- Estimated rafter lumber cost
A rafter pair has one common rafter on each side of the ridge. The count assumes common rafters at both building ends. Actual spacing evenly distributes locations without exceeding your entered maximum.
Rafter plan
Geometry
How rafter length is calculated
For a simple gable roof, half the building span establishes the basic horizontal run. Half the entered ridge-board thickness is removed from each side’s run. The entered pitch determines rise. The rafter line is the hypotenuse.
Overhang is not added directly
A 12-inch horizontal overhang requires more than 12 inches measured along a sloped rafter. We convert the horizontal projection into its corresponding sloped length before adding it to the wall-to-ridge line.
Why ridge thickness matters
The two rafters terminate at opposite faces of the ridge board, so each side’s horizontal run is reduced by half the ridge thickness, not the full thickness.
We count spaces along building length by rounding up length divided by maximum spacing, then add one location for the other end. Each location has two common rafters. Purchasing allowance applies once to this piece count.
Stock guidance uses the common list 8, 10, 12, 14, 16, 18, 20, 22 and 24 ft. Your lumberyard may not stock every length. Calculated line footage differs from purchased board footage. Optional cost covers only the boards priced, excluding ridge board, fascia, sheathing, hardware, delivery, tax, labor and other materials.
Geometry estimate, not structural design
RockUtility calculates common-rafter geometry and material quantities from the dimensions, pitch and spacing you enter. It does not determine whether those values are structurally appropriate.
Actual framing requires consideration of lumber species and grade, rafter size, span, dead/live loads, snow, wind, bearing, connections, birdsmouth geometry, ridge beam versus ridge board, ceiling/rafter ties, collar ties, openings, hips/valleys, and local code and engineering requirements.
This tool does not size lumber, select allowable spacing, design birdsmouth cuts or connections, size ridge beams, specify ties, approve structural adequacy or design spliced rafters. Confirm geometry and continuous board length against your framing plan before cutting.