Calculator Inputs
Use actual project dimensions. Enter the horizontal shed roof run, not the sloped rafter length.
Example Data Table
These sample values show how different shed roof conditions can affect rafter length and estimated span demand.
| Example | Run | Pitch | Spacing | Load | Member | Typical Use |
|---|---|---|---|---|---|---|
| Light garden shed | 8 ft | 3 / 12 | 16 in | 25 psf | 2 x 6 | Small storage roof |
| Workshop shed | 10 ft | 4 / 12 | 16 in | 30 psf | 2 x 8 | Standard backyard structure |
| Snow area shed | 12 ft | 6 / 12 | 12 in | 50 psf | 2 x 10 | Higher load condition |
| Lean-to roof | 7 ft | 2 / 12 | 24 in | 20 psf | 2 x 6 | Simple covered bay |
Formula Used
Slope factor: sqrt(1 + (pitch / 12)^2)
Rise: horizontal run × pitch / 12
Simple rafter span: horizontal run × slope factor
Total cut length: (horizontal run + overhangs) × slope factor
Total roof load: dead load + live load + snow load
Line load: total load × spacing ÷ slope factor
Section modulus: S = b × d² / 6
Moment of inertia: I = b × d³ / 12
Maximum moment: M = wL² / 8
Deflection: Δ = 5wL⁴ / 384EI
Allowable bending span: L = sqrt(8 × Fb × S / w)
How to Use This Calculator
- Measure the horizontal distance from the low support wall to the high support wall.
- Enter the roof pitch as inches of rise for every 12 inches of run.
- Add low side and high side overhangs if rafters extend past the walls.
- Enter rafter spacing, roof length, and expected roof loads.
- Select a lumber species and grade, or enter custom design values.
- Choose a deflection limit. Use stricter limits for better stiffness.
- Press calculate and review the span, cut length, load, and utilization checks.
- Export the result as CSV or PDF for job notes and material planning.
Shed Roof Rafter Planning Guide
Why Rafter Span Matters
A shed roof looks simple. It still needs careful framing. The rafter span controls strength, stiffness, and material cost. A longer span creates more bending. A heavier roof creates more load. Wider rafter spacing also increases load on each rafter. This calculator helps you compare those factors before cutting lumber.
Understanding Shed Roof Geometry
A shed roof has one slope. The low wall and high wall form the roof rise. The horizontal distance between them is the run. Pitch describes how much the roof rises over twelve inches of run. A steeper pitch increases the sloped rafter length. It also changes roof angle and overhang length.
Loads and Member Size
Rafters carry dead load, live load, and snow load. Dead load includes roofing, sheathing, underlayment, and ceiling materials. Live load covers temporary service loads. Snow load can be large in cold regions. Member depth has a strong effect on span. A deeper rafter is usually much stiffer than a shallow rafter.
Strength Checks
The calculator estimates bending, shear, and deflection. Bending checks the rafter against midspan stress. Shear checks force near supports. Deflection checks visible sag. A design can be strong but still too flexible. That is why all checks matter.
Practical Framing Notes
Real sheds need more than rafter sizing. Check bearing length, hurricane ties, blocking, birdsmouth cuts, and fastener schedules. Use dry and straight lumber where possible. Protect cut ends from moisture. Confirm local rules before building. For large spans, heavy snow, or occupied buildings, ask a qualified professional to review the design.
FAQs
1. What is a shed roof rafter span?
It is the sloped distance a rafter travels between supports. The calculator also shows total cut length when overhangs are included.
2. Should I enter roof width or rafter length?
Enter the horizontal run between the low and high support walls. The calculator converts that run into sloped rafter span.
3. What does pitch mean?
Pitch is the roof rise over twelve inches of horizontal run. A 4 / 12 pitch rises four inches for every twelve inches.
4. Does this replace local span tables?
No. It is a planning tool. Always compare results with local code span tables, lumber stamps, and professional guidance.
5. Why does spacing affect span?
Wider spacing gives each rafter more roof area to carry. That increases line load and can reduce the safe span.
6. What is deflection?
Deflection is rafter bending movement under load. Too much deflection can cause sag, cracked finishes, and poor roof performance.
7. Why is overhang separate from span?
The simple span is between supports. Overhang adds cut length and creates a cantilever area that needs separate field judgment.
8. Which rafter size should I choose?
Use a member that passes span, bending, shear, and deflection checks. Then confirm it with your local code requirements.