Enter Roof Measurements
Enter the outside-to-outside building width.
Enter the rise for every 12 inches of horizontal run. Example: 6 means a 6:12 roof pitch.
Enter the horizontal overhang beyond the wall.
Rafter Length Results
Main rafter length including overhang.
Rafter length is calculated from the horizontal run, roof pitch, and overhang.
How to Use the Rafter Length Calculator
Our Rafter Length Calculator helps you quickly determine the length of a roof rafter using the building span, roof pitch, and overhang. Enter your measurements and the calculator automatically works out the rafter run, rise, roof angle, main rafter length, overhang rafter length, and total rafter length.
- Enter the Building Width / Span: Enter the total outside-to-outside width of the building in feet.
- Enter the Roof Pitch: Enter the roof rise for every 12 inches of horizontal run. For example, enter 6 for a 6:12 roof pitch.
- Enter the Overhang: Enter the horizontal overhang or eave measurement in inches.
- Calculate: Click the Calculate Rafter Length button to see your results.
What Is Rafter Length?
Rafter length is the sloping distance from the upper part of a roof to the point where the rafter reaches the wall or overhang. It depends mainly on the horizontal rafter run and the vertical rise created by the roof pitch.
For a simple symmetrical roof, the building span is divided in half to determine the horizontal run of one rafter. The roof pitch is then used to determine the corresponding rise. These two measurements form a right triangle, which can be used to calculate the rafter length.
If an overhang is included, the additional sloping length of the overhang can be calculated separately and added to the main rafter length.
Rafter Length Formula
Rafter length can be calculated using the Pythagorean theorem. First determine the horizontal rafter run and vertical rise.
For a symmetrical roof, the rafter run is generally half of the building span. For example, a 24-foot building span has a 12-foot horizontal run for one side of the roof.
When an overhang is included, the overhang has its own horizontal run and rise. Its sloping length is calculated in the same way and added to the main rafter length.
What This Calculator Calculates
The Rafter Length Calculator provides several measurements that help you understand the geometry of a roof rafter.
Rafter Length
Calculates the total sloping rafter length including the entered overhang.
Rafter Run
Calculates the horizontal distance from the wall toward the roof ridge for one side of the roof.
Rise
Calculates the vertical height created by the selected roof pitch.
Roof Angle
Converts the roof pitch geometry into the corresponding roof angle in degrees.
Main Rafter Length
Shows the sloping length before the overhang is added.
Overhang Rafter
Calculates the additional sloping length created by the entered overhang.
How Roof Pitch Affects Rafter Length
Roof pitch has a direct effect on rafter length. When the building span remains the same, a steeper roof creates more vertical rise. This increases the length of the sloping rafter.
For example, consider the same building span with two different roof pitches. A lower pitch produces less rise, while a higher pitch produces more rise. Because the rafter length is based on both run and rise, the steeper roof requires a longer rafter.
This relationship is important when estimating rafter material because changing the roof pitch can change the required rafter length even when the building width stays unchanged.
Rafter Length Calculation Example
Suppose a building has a 24-foot span, a 6:12 roof pitch, and a 12-inch horizontal overhang.
The exact result can vary slightly depending on rounding. The calculator performs the calculation using the entered measurements and displays the resulting dimensions.
Rafter Length vs. Rafter Run
Rafter run and rafter length are different measurements. Understanding the difference is important when working with roof dimensions.
Rafter Run
The horizontal distance from the outside wall toward the roof ridge for one side of a roof.
Rafter Length
The actual sloping distance along the rafter from the lower point toward the roof ridge, including the calculated overhang when one is entered.
The rafter length is always longer than the horizontal run when the roof has a positive pitch because the rafter follows the slope of the roof rather than a horizontal line.
Measurements and Units
This calculator uses feet for the building width or span, the standard rise-per-12 format for roof pitch, and inches for the overhang.
Measure carefully and keep your units consistent with the calculator fields. Small measurement differences can affect the calculated rafter length.
Frequently Asked Questions
How do I calculate rafter length?
Determine the horizontal rafter run and vertical rise, then use the formula √(Run² + Rise²). If the roof has an overhang, calculate its sloping length and add it to the main rafter length.
What information do I need to calculate rafter length?
You need the building span, roof pitch, and overhang measurement. This calculator uses those measurements to determine the rafter dimensions.
Does roof pitch affect rafter length?
Yes. For the same building span, a steeper roof pitch creates more rise and therefore increases the sloping rafter length.
Does this calculator include the overhang?
Yes. Enter the horizontal overhang in inches and the calculator calculates the overhang rafter length and includes it in the main Rafter Length result.
What is a 6:12 roof pitch?
A 6:12 roof pitch means the roof rises 6 inches for every 12 inches of horizontal run.
Important Information
This calculator is provided for general calculation and informational purposes. The results are based on the measurements and roof geometry entered by the user.
Actual roof construction may require additional considerations such as ridge board or ridge beam dimensions, birdsmouth cuts, seat cuts, lumber dimensions, local building codes, structural loads, framing methods, and other construction requirements.
Always verify measurements and construction requirements before using calculated dimensions for a real building project. For structural or code-related decisions, consult a qualified building professional or the appropriate local authority.
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