Shed Roof Rafter Calculator
Calculate shed roof rafter length, rise, run, roof angle, and overhang from your roof measurements.
Enter Shed Roof Measurements
Shed Roof Rafter Results
Your calculated measurements appear below.
How to Use the Shed Roof Rafter Calculator
Use this Shed Roof Rafter Calculator to estimate the rafter length for a single-slope shed roof. Enter the full horizontal roof run, roof pitch, and overhang to calculate the roof rise, main rafter length, roof angle, overhang rafter length, and total rafter length.
Enter the full horizontal distance from the low wall to the high wall.
For example, enter 4 for a 4:12 roof pitch.
Enter the horizontal overhang beyond the wall in inches.
Click Calculate Rafter to view the results.
What Is a Shed Roof?
A shed roof is a simple roof design with a single sloping surface. It is commonly used on sheds, lean-to structures, workshops, porches, and other small buildings.
Unlike a traditional gable roof, a shed roof does not have two sloping sides meeting at a central ridge. This makes the basic rafter calculation straightforward because the full horizontal roof distance can be used as the rafter run.
The roof pitch determines how quickly the roof rises as the horizontal run increases.
Shed Roof Rafter Formula
A basic shed roof rafter calculation uses right-triangle geometry. The roof rise is determined from the horizontal run and roof pitch, and the rafter length is calculated from the run and rise.
For example, with a 10-foot run and a 4:12 pitch, the calculated rise is 3 ft 4 in and the main rafter length is approximately 10 ft 6.49 in.
What This Calculator Calculates
Rafter Run
The full horizontal distance across the shed roof.
Rise
The vertical height created by the selected roof pitch.
Main Rafter Length
The sloped rafter length between the main roof supports.
Roof Angle
The roof angle calculated from the selected pitch.
Overhang Rafter
The sloped length created by the horizontal overhang.
Total Rafter Length
The main rafter length plus the sloped overhang length.
Shed Roof Rafter Calculation Example
Suppose a shed has a 10-foot horizontal run, a 4:12 roof pitch, and a 12-inch horizontal overhang.
The rise is calculated as:
This equals 3 ft 4 in. The main rafter length is approximately 10 ft 6.49 in.
The 12-inch horizontal overhang produces an overhang rafter length of approximately 1 ft 0.65 in.
Therefore, the total rafter length is approximately 11 ft 7.14 in.
How Roof Pitch Affects Rafter Length
Roof pitch directly affects the rise, roof angle, and rafter length. For the same horizontal run, a steeper roof pitch creates a greater rise and therefore a longer sloped rafter.
Increasing the pitch while keeping the run the same will increase the rafter length.
Shed Roof vs. Gable Roof Rafter Calculation
Shed roofs and gable roofs have different basic layouts, so their rafter calculations are not the same.
| Feature | Shed Roof | Gable Roof |
|---|---|---|
| Roof slopes | One | Two |
| Central ridge | No | Usually |
| Basic rafter run | Full roof run | Usually half the span |
| Roof rise | Based on full run | Based on one side |
This calculator is specifically designed for the basic geometry of a single-slope shed roof and uses the full horizontal roof run.
Overhang and Shed Roof Rafter Length
A roof overhang extends beyond the wall of the building. Because the overhang follows the roof slope, its actual rafter length is greater than its horizontal measurement.
This calculator converts the horizontal overhang into its sloped rafter length and adds it to the main rafter length to determine the Total Rafter Length.
Measurements and Units
The calculator uses feet for the shed roof run, inches for the overhang, and standard pitch notation such as 4:12.
Measure the horizontal distance accurately before entering your dimensions into the calculator.
Frequently Asked Questions
How do you calculate a shed roof rafter?
Calculate the roof rise from the horizontal run and pitch, then use the run and rise to calculate the sloped rafter length.
What is a common shed roof pitch?
Shed roofs can use different pitches depending on the building design, drainage requirements, roofing material, local conditions, and applicable building requirements.
Does overhang affect rafter length?
Yes. A horizontal overhang adds additional sloped length to the rafter.
Is a shed roof the same as a lean-to roof?
The terms are often used for similar single-slope roof designs, although the exact construction and building configuration can vary.
Does this calculator determine lumber size?
No. This calculator determines basic roof geometry and rafter measurements. It does not determine lumber size, grade, spacing, or structural load capacity.
Important Information
This calculator provides geometric estimates based on the measurements you enter. Actual construction may require additional considerations such as lumber size, rafter spacing, snow and wind loads, roof materials, connections, local building codes, and structural requirements.
Always verify construction measurements and requirements before cutting or installing structural components.
Related Calculators
Use our other roofing calculators for related roof and rafter calculations.
Explore More Roofing Calculators
Explore our collection of free roofing calculators to calculate rafter length, roof pitch, roof slope, roof angle, rise, run, and other useful roof measurements.