Gable Roof Pitch Calculator: Find Slope in Degrees & Ratio

This gable roof pitch calculator works out the slope of a roof from two measurements you can take on site: the rise (vertical height) and the run (horizontal span). Enter those two numbers, and you get the pitch as a ratio (like 6/12), an angle in degrees, and the rafter length needed for that slope. The result is the standard roof pitch value used by builders and roofing suppliers, so you can order materials or check a plan without doing the trigonometry by hand.

Formula

The roof pitch is the ratio of the vertical rise to the horizontal run, expressed as “X in 12.� The formula is:

Pitch = (Rise ÷ Run) × 12

Where:

• Rise – vertical height from the top of the wall plate to the underside of the ridge, in inches (or any consistent unit).
• Run – horizontal distance from the outside edge of the wall to the center of the ridge, in the same unit as rise.
• Pitch – the number of inches the roof rises for every 12 inches of horizontal run (e.g., 6/12).

The angle in degrees is found with the arctangent: Angle = atan(Rise ÷ Run), where atan is the inverse tangent function. The rafter length per foot of run is the hypotenuse of a right triangle: Rafter length = √(Rise² + Run²).

Worked Example

Using the default values from the calculator: rise = 6 inches, run = 12 inches.

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

So a 6/12 roof rises 6 inches for every 12 inches of run, slopes at about 26.6 degrees, and each foot of horizontal run needs about 13.42 inches of rafter. The calculator returns exactly these values.

How to Measure Rise and Run on an Existing Roof

You don’t need to climb onto the roof to get these numbers. From inside the attic:

• Run: Measure horizontally from the inside face of the exterior wall (where the rafter sits) to the center of the ridge beam. This is half the building width minus any overhang.
• Rise: Measure vertically from the top of the wall plate to the underside of the ridge, at the same point where you measured the run.
• Alternative: If you have a level and a tape, place the level against the underside of a rafter, hold it level, and measure from the 12-inch mark straight down to the rafter. That vertical distance is the rise per 12 inches of run.

For a new build, use the architectural plans: the run is usually half the building width (less the ridge thickness), and the rise is listed on the roof section drawing.

Common Mistakes and How to Avoid Them

• Mixing units: If you measure rise in inches and run in feet, the ratio will be wrong. Convert everything to the same unit first.
• Using full width instead of run: The run is only half the building width (from wall to ridge), not the full span. Using full width halves the pitch.
• Forgetting the ridge thickness: The ridge board has its own width, so the actual run is (building width – ridge width) ÷ 2. Ignoring it adds a small error, usually less than 1 inch, but it matters on tight tolerances.
• Confusing pitch with angle: Pitch is a ratio (e.g., 6/12), not an angle. A 6/12 roof is about 26.6°, not 6°. Many people ask “what angle is 6/12?â€� – the answer is 26.6°.

Why Roof Pitch Matters

Pitch affects more than just looks. It determines:

• Water shedding: Steeper pitches (above 4/12) shed water quickly and are suitable for asphalt shingles. Low pitches (2/12 to 4/12) need special roofing materials like roll roofing or standing-seam metal.
• Snow load: In snowy regions, steeper pitches let snow slide off, reducing weight on the structure. Flatter roofs hold snow and require stronger framing.
• Attic space: A 6/12 or higher pitch gives you usable headroom for a finished attic. A 3/12 roof makes the attic crawl-space only.
• Material quantities: The steeper the pitch, the larger the roof surface area for the same building footprint, so you need more shingles, underlayment, and flashing.

How to Choose a Roof Pitch

If you’re designing a new roof, the choice depends on climate, style, and budget:

• 2/12 to 4/12: Low slope – common on modern and commercial buildings. Needs special low-slope roofing. Cheaper to frame but more prone to leaks if not detailed well.
• 4/12 to 6/12: Standard residential – works with asphalt shingles, good balance of cost and water shedding. Most North American homes fall here.
• 6/12 to 9/12: Steep – gives dramatic look and attic space, but more material and labor. Common on colonial and craftsman styles.
• 9/12 and up: Very steep – often used for aesthetic or snow-shedding reasons. Requires special safety equipment for roofing work.

Check local building codes – some areas have minimum pitch requirements for certain materials, and maximum pitch limits for walkability without special equipment.

Safety and Accuracy Notes

Measuring a roof involves working at heights. Always use a sturdy ladder, have someone hold the base, and never walk on a steep or wet roof without a harness and proper footwear. For attic measurements, watch your step on joists and wear a dust mask if insulation is present. The calculator gives you the pitch and rafter length, but it does not account for rafter overhang, ridge thickness, or the extra length needed for birds-mouth cuts. Always add the overhang length to the rafter length and consult a framing square or rafter table for the exact cut angles. If you’re building, verify your measurements against the actual structure before cutting.

Frequently Asked Questions

What is the standard roof pitch for a house? Most residential roofs are between 4/12 and 6/12. A 4/12 is the minimum for asphalt shingles, and 6/12 is a common default that works well in most climates. Steeper pitches are used for architectural style or heavy snow areas.

How do I convert roof pitch to degrees? Use the arctangent of the rise divided by the run. For a 6/12 pitch, that’s atan(6/12) = 26.6°. For a 4/12, it’s atan(4/12) = 18.4°. For a 12/12, it’s 45°.

What does “4/12� mean on a roof plan? It means the roof rises 4 inches for every 12 inches of horizontal run. It’s a ratio, not a measurement of the roof’s length. The actual roof surface is longer than the horizontal span because of the slope.

Can I use this calculator for a shed or garage roof? Yes, the same math applies to any gable roof, regardless of size. Just measure the rise and run for that structure. For a lean-to or single-slope roof, the run is the full horizontal distance from wall to wall, not half.

What is the minimum pitch for metal roofing? It depends on the panel type and how the seams are sealed. Standing-seam metal can go as low as 1/12, while corrugated panels typically need at least 3/12. Always follow the manufacturer’s specifications for your specific product.

How do I find the rafter length from the pitch? Multiply the run by the square root of (pitch² + 144) divided by 12, or simply use the calculator’s rafter length output. For a 6/12 pitch with a 10-foot run, the rafter length is about 11.18 feet before adding overhang.

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in rise | Rise (vertical) | in | 6 | 0
in run | Run (horizontal) | in | 12 | 0
out ratio | Pitch (rise/run) | :12 | (rise / run) * 12 | 1
out angle | Angle | deg | atand(rise / run) | 1
out len | Rafter length per foot of run | in | sqrt(rise^2 + run^2) | 2