Calculator
Example data table
| Inputs | Run | Rise | Pitch | Angle | Rafter length |
|---|---|---|---|---|---|
| Rise & Run | 12 ft | 4 ft | 4 : 12 | 18.43° | 12.65 ft |
| Pitch & Run | 10 ft | 3.33 ft | 4 : 12 | 18.43° | 10.54 ft |
| Wall heights | 14 ft | 2 ft | 1.71 : 12 | 8.13° | 14.14 ft |
Formula used
- slope = rise / run
- pitch (X:12) = 12 × slope
- angle (deg) = arctan(slope) × 180/π
- slope (%) = 100 × slope
- rafter = √(run² + rise²)
- drop over overhang = slope × overhang
- rafter with overhang = √((run+overhang)² + rise²)
How to use this calculator
- Select a calculation mode based on what you measured.
- Pick your length unit, then enter all lengths in that unit.
- Optional: add an overhang to estimate extra rafter length.
- Press Calculate to show results above the form.
- Use the CSV/PDF buttons to save your build notes.
Pitch selection for garden sheds
A shed roof pitch expresses how much the roof rises for a given horizontal run. A 4:12 pitch rises 4 units for every 12 units of run, which equals about 18.43° and 33.33% slope. In garden settings, pitches around 2:12 to 6:12 are common because they balance runoff, headroom, and straightforward framing. Use the calculator to compare options and keep the pitch consistent.
Drainage and cladding considerations
Slope influences drainage speed and how long water lingers on panels, shingles, or membranes. A higher slope reduces ponding risk and helps debris slide off, especially under trees. A lower slope can work, but it needs careful detailing at laps, penetrations, and flashing. Convert pitch to degrees and percent slope to communicate clearly and to align gutter fall with runoff direction. For gardens, direct runoff away from pathways to reduce mud and protect mulched planting areas after storms.
Rafter length and material optimization
Rafter length is the hypotenuse of the rise and run triangle: √(run² + rise²). This estimate supports lumber selection, cut waste control, and blocking layout. In a compact potting shed, small pitch changes still add noticeable length across multiple rafters. Use the rounding selector to match your cutting tolerance and generate a repeatable cut list.
Overhang planning for weather control
Overhangs protect siding, doors, and stored tools from splashback. The calculator treats overhang as a horizontal extension, then reports the extra rafter length and the vertical drop across that overhang. With a 4:12 pitch, every 12 inches of overhang drops about 4 inches. Use this to size fascia, set drip edges, and confirm walkway or bed clearance.
Field measurement checklist
Measure run horizontally, not along the rafter. Verify level references with a straightedge or laser, then record wall-to-wall run and the rise between high and low points. If you only know wall heights, enter both heights and the run to derive rise. Confirm your roofing product minimum slope and local weather needs before final cuts.
FAQs
What is the difference between pitch and slope?
Pitch is expressed as X:12 (rise per 12 units of run). Slope is the rise divided by run, often shown as a percent. The calculator reports both, plus the equivalent angle in degrees.
How do I measure run correctly on an existing shed?
Measure horizontally between the high-side wall line and the low-side wall line. Do not follow the rafter or roof surface. A level, tape, and plumb reference make the measurement more reliable.
Can I use wall heights instead of measuring rise?
Yes. Enter the front and back wall heights and the horizontal run. The calculator uses the absolute height difference as rise, then computes pitch, angle, and rafter length.
How should I choose an overhang value?
Use the planned horizontal projection from wall to fascia. Bigger overhangs improve splash protection but increase rafter length and vertical drop at the eave. The overhang outputs help you check clearance and trim sizing.
Why does the angle matter if I already have a pitch ratio?
Angles help when setting saw bevels, checking layout with an angle finder, or discussing slope on drawings. Converting to degrees also makes it easier to compare different pitches quickly.
Does this calculator replace structural design?
No. It converts geometry into pitch, angle, and length estimates. For rafter sizing, spacing, connections, and loading from wind or snow, follow local requirements and manufacturer guidance or consult a qualified professional.