Rafters Calculator
Calculate rafter length and roof height based on building span, roof pitch, and overhang.
Calculation Results
The total required length of the rafter including the sloped overhang.
The vertical height of the roof ridge above the wall plate level.
How This Calculator Works
To use the Rafters Calculator:
- Enter the total building span (the outer wall-to-wall width).
- Select the roof pitch from the dropdown menu (e.g., 4/12 pitch rises 4 inches for every 12 inches of run).
- Enter the desired horizontal overhang distance (usually 12 to 18 inches).
- The calculator immediately displays the exact sloped rafter length and the vertical ridge height.
Formula & Calculations
Rafter length is calculated using the Pythagorean theorem. First, we find the horizontal run, which is half of the building span:
Run = Span / 2
The total horizontal run including the overhang is:
Total Run = Run + Overhang
We then calculate the slope factor based on the pitch (expressed as rise over 12):
Slope Multiplier = √(1 + (Pitch / 12)²)
The sloped rafter length is the total run multiplied by the slope factor:
Rafter Length = Total Run × Slope Multiplier
The vertical rise of the roof (height of the ridge above the wall top plate) is calculated by multiplying the run by the pitch ratio:
Roof Height = Run × (Pitch / 12)
Step-by-Step Calculation Examples
Standard 24ft Span with 4/12 Pitch
Calculating rafter requirements for a typical 24-foot building span with 12-inch overhangs.
- span: 24
- pitch: 4
- overhang: 12
- rafter_length: 13.70 ft
- roof_height: 4.00 ft
Steeper 30ft Span with 8/12 Pitch
Calculating rafter requirements for a larger 30-foot building span with a steeper pitch and 18-inch overhangs.
- span: 30
- pitch: 8
- overhang: 18
- rafter_length: 19.83 ft
- roof_height: 10.00 ft
Frequently Asked Questions
What is building span?
Building span is the total horizontal distance between the outer surfaces of the supporting walls of a roof. It represents the total width the rafters must cover.
Is rafter overhang sloped or horizontal?
Overhang can be measured horizontally or along the slope of the rafter. In general architectural drawings, overhang is specified as a horizontal distance (e.g., a 12-inch overhang extends 12 inches past the wall plate horizontally). This calculator uses horizontal overhang.
Does rafter length include the birdsmouth cut?
Yes, the geometric rafter length calculated is from the ridge to the end of the overhang. When building, you will mark this length and cut a notch called a "birdsmouth" where the rafter rests on the top wall plate.
How do you calculate rafter spacing?
Rafter spacing is typically 16 inches or 24 inches on-center, depending on local building codes, structural loads, and the size of the rafter lumber.
What is the difference between a rafter and a truss?
Rafters are stick-built on-site from dimensional lumber, leaving the attic space open. Trusses are pre-fabricated, engineered triangular structures consisting of webs and chords, which are stronger but occupy the attic space.