Shed Rafter Length Calculator

Calculator Form

Project Name

Unit

Calculation Mode

Shed Width / Run

Low Wall Height

High Wall Height

Pitch Rise Per 12

Overhang Projection

Birdsmouth Allowance

Waste Percentage

Rafter Spacing (inches)

Board Width

Rafter Quantity

Example Data Table

Sample shed roof framing inputs and outputs are listed below.

Case	Run	Rise	Overhang	Rafter Length	Angle
Small Garden Shed	8 ft	1.5 ft	0.75 ft	8.88 ft	10.62°
Workshop Shed	12 ft	2 ft	1 ft	13.15 ft	9.46°
Storage Shed	16 ft	3 ft	1.25 ft	17.52 ft	10.62°

Formula Used

Basic rafter length: √(run² + rise²)

Rise from pitch: (run × pitch rise) ÷ 12

Rafter with overhang: √((run + overhang)² + rise²)

Final cut length: rafter with overhang + birdsmouth allowance

Length with waste: final cut length × (1 + waste% ÷ 100)

This calculator applies the Pythagorean theorem to the right triangle formed by horizontal run and vertical rise. Optional fields extend the framing length for overhang, cutting allowance, and waste planning.

How to Use This Calculator

Select the measurement unit for your project.
Choose wall-height mode or pitch mode.
Enter shed width, which acts as the rafter run.
Add low wall height and high wall height, or pitch.
Enter overhang projection and any birdsmouth allowance.
Add a waste percentage for ordering lumber safely.
Set spacing, board width, and quantity if needed.
Press calculate to show results above the form.
Use the CSV and PDF buttons to export results.

Frequently Asked Questions

1) What is a shed rafter length?

It is the sloped length of a roof member running from the low wall to the high wall. It depends on horizontal run, vertical rise, and any extra tail or overhang included in the cut.

2) Why is overhang included separately?

Overhang extends the rafter beyond the wall line. Keeping it separate helps you compare structural span length and total cut length, which improves estimating and field layout accuracy.

3) Can I calculate from pitch instead of wall heights?

Yes. Choose pitch mode and enter the rise per 12. The calculator converts pitch into total rise using your shed width, then computes rafter length automatically.

4) What does birdsmouth allowance mean here?

It is a simple added cutting allowance for seat and heel adjustments. Actual framing geometry varies by code, bearing depth, and lumber size, so verify final field cuts before construction.

5) How is the slope angle calculated?

The slope angle comes from arctangent of rise divided by run. It shows the roof incline in degrees, which helps with saw settings and framing checks.

6) Does this calculator estimate how many rafters I need?

Yes. It gives an estimated number of rafter positions from shed width and on-center spacing. Treat it as a planning value and confirm layout from your actual framing design.

7) Should I add waste percentage?

Adding waste is helpful for practical purchasing. It covers trimming, defects, and cutting losses. Many builders add a modest percentage so material orders are less likely to come up short.

8) Is this suitable for permit drawings?

Use it for estimating and preliminary checks. Final structural design, allowable spans, connections, loads, and code compliance should be reviewed by a qualified professional or local building authority.