Roof Pitch Calculator: Rafter Length & Slope

This roof pitch calculator works out the slope angle, rafter length, and rise for a roof when you enter the run (horizontal span) and the rise (vertical height). You get the pitch in degrees, as a ratio (like 6/12), and the rafter length needed for that side of the roof, all in one go. It uses the standard right‑triangle formulas that carpenters and roofers use every day.

Formula

The calculator uses three basic right‑triangle relationships. Let R be the horizontal run (in feet), H be the vertical rise (in feet), and L be the rafter length (in feet).

• Pitch angle (in degrees): θ = arctan(H / R)
• Pitch ratio (rise per 12 inches of run): (H / R) × 12
• Rafter length (hypotenuse): L = √(R² + H²)

All lengths are in the same unit (feet in the example, but you can use meters or inches as long as you are consistent). The angle is measured from the horizontal to the roof slope.

Worked Example

Using the default values – run = 12 ft, rise = 6 ft:

1. Pitch angle = arctan(6 / 12) = arctan(0.5) ≈ 26.6°
2. Pitch ratio = (6 / 12) × 12 = 6, so the pitch is 6/12 (six inches of rise per foot of run).
3. Rafter length = √(12² + 6²) = √(144 + 36) = √180 ≈ 13.42 ft.

The calculator shows 26.6°, 6.0 (as inches per foot), and 13.42 ft – exactly matching the hand calculation.

Why Roof Pitch Matters

Pitch is not just a cosmetic detail; it determines how water drains, how much snow load the roof can shed, and which roofing materials are suitable. Low‑slope roofs (under 2/12) need special membranes, while steep roofs (above 6/12) allow standard shingles and shed snow quickly. Local building codes often set minimum pitches for certain materials, and insurance companies may have their own rules for steep roofs. Knowing your pitch helps you plan framing, choose materials, and estimate material quantities.

How to Measure Your Roof’s Pitch

If you don’t have plans, you can measure an existing roof from inside the attic or from the ground with a level and a tape measure.

• From inside: Place a level against a rafter, hold it perfectly horizontal, and measure the vertical distance at a point 12 inches from the rafter’s bottom end. That vertical distance is the “rise per 12â€� – your pitch ratio.
• From outside: Use a pitch gauge or an app on your phone that uses the accelerometer. Hold it against the roof surface and read the angle.
• From a drawing: If you have building plans, look for the roof section view – it usually shows the rise and run dimensions.

Be careful when walking on a roof; always use proper safety equipment and a harness if the slope is steep.

Common Mistakes and How to Avoid Them

The most frequent error is mixing units – entering rise in inches but run in feet. The calculator assumes both are in the same unit, so convert first. Another mistake is forgetting that the rafter length from this formula is the theoretical length from the top plate to the ridge, not including the overhang or the ridge board thickness. You must add the tail (eave) length and subtract half the ridge thickness for a real cut. Also, the pitch ratio is often written as “X/12� – that X is the rise per 12 inches of run, not the angle. A 6/12 roof is about 26.6°, not 6°.

Choosing a Roof Pitch

For new construction, the pitch is usually decided by climate, architectural style, and budget. In snowy regions, a steeper pitch (6/12 or more) helps snow slide off and reduces the chance of ice dams. In windy coastal areas, a lower pitch (3/12 to 5/12) offers less wind resistance. Aesthetic preferences also play a role: traditional homes often use 6/12 to 9/12, while modern flat‑roof designs may use 1/12 or even 0.5/12. Remember that steeper roofs use more materials and are more dangerous to work on, which raises cost.

Safety Considerations

Working on a roof is one of the most dangerous home maintenance tasks. Always:

• Use a sturdy ladder that extends at least 3 feet above the roof edge.
• Wear shoes with good grip, and avoid working in wet or icy conditions.
• Use a safety harness anchored to a secure point if the pitch is above 4/12.
• Have someone on the ground to hand you tools and call for help if needed.

If you are not comfortable with heights or unsure about the structural integrity, hire a professional roofer.

Frequently Asked Questions

What does a 6/12 roof pitch mean?

It means the roof rises 6 inches vertically for every 12 inches (1 foot) of horizontal run. This is a moderate slope, common on many houses, and corresponds to an angle of about 26.6 degrees.

How do I calculate rafter length with an overhang?

First, calculate the rafter length for the main span using the formula above. Then add the length of the overhang (the horizontal distance from the wall to the eave edge) converted to the same unit. For example, if you have a 12‑ft run and a 2‑ft overhang, use a total run of 14 ft in the formula, then subtract half the ridge board thickness for the actual cut length.

What is the minimum pitch for metal roofing?

Most metal roofing manufacturers require a minimum pitch of 3/12 (about 14°) for standing seam panels and 2/12 for some exposed fastener panels, but always check the specific product’s specifications. Low pitches need special sealing and fastener patterns to prevent leaks.

Can I use this calculator for a hip roof?

Yes, for the common rafter that runs from the wall to the ridge. Hip rafters (that run diagonally to the corners) are longer and need a different calculation that accounts for the hip angle. This calculator gives you the common rafter length, which is the main piece you need for framing.

Does roof pitch affect solar panel installation?

Yes. The angle of your roof relative to the sun affects how much solar energy you can capture. A pitch of 30–45 degrees is often optimal for solar panels in many locations, but the ideal angle depends on your latitude. Panels can also be mounted on adjustable racks to compensate for a suboptimal roof pitch.

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in run | Run (horizontal) | ft | 12 | 0
in rise | Rise (vertical) | ft | 6 | 0
out pitch_deg | Pitch (angle) | degrees | atan(rise / run) * 180 / pi | 1
out pitch_ratio | Pitch (rise/run) | in per ft | rise * 12 / run | 1
out rafter | Rafter length | ft | sqrt(run^2 + rise^2) | 2