Engineer-Reviewed Tool

Roof Pitch Calculator

Convert roof rise-over-run to degrees, slope percentage, and rafter multiplier instantly. Built for contractors in the US, UK, and Australia.

Enter Roof Dimensions

Measurement Unit

Rise (in)

Run (in)

Note: Standard US pitch uses a run of 12 inches. For metric, a run of 100 or 1000 is often used.

Results

Live

Pitch Ratio

-- : 12

Angle

-- °

Slope

-- %

Rafter Multiplier

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Technical Guide

Understanding Roof Pitch & Geometry

Roof pitch is the most fundamental measurement on any roofing project. It dictates the materials you can use, the amount of material you must order, and how well the roof will shed water and resist wind uplift.

However, communicating pitch across different regions can be confusing. In the United States, roofers speak in ratios (e.g., "6:12"). In the UK and Australia, architects and builders speak in degrees (e.g., "26.57°"). This calculator is designed to bridge that gap, providing instant conversions between all standard formats.

The Holy Trinity: Rise, Run, and Pitch

Every pitched roof is a right-angled triangle. To calculate the pitch, you need two dimensions:

Rise: The vertical distance from the top of the wall plate to the highest point of the roof (the ridge).
Run: The horizontal distance from the outside of the wall plate to the center of the ridge.

Once you have these two numbers, the geometry is fixed. In the US, we standardise the run to 12 inches. So, a roof that rises 6 inches over a 12-inch run has a 6:12 pitch.

The Danger of Confusing Slope % and Angle

A common and expensive mistake in roofing estimation is confusing the Slope Percentage with the Angle in Degrees, or using the Slope Percentage when you should be using the Rafter Multiplier.

Let's take a 6:12 pitch as an example:

The Angle is 26.57°. (This is the arctangent of 6/12).
The Slope Percentage is 50%. (Rise divided by Run, multiplied by 100).
The Rafter Multiplier is 1.118. (The secant of the angle).

If you have a 1,000 sq ft footprint and you use the slope percentage (50%) to estimate roof area, you will order 1,500 sq ft of shingles. But the true surface area is only 1,118 sq ft. You would over-order by nearly 400 sq ft — which is hundreds of dollars wasted on material that will likely be thrown away.

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Global Code Differences: IRC vs BS 5534

The pitch of your roof is not just a design choice; it is a regulated safety and performance requirement.

In the United States, the International Residential Code (IRC 2021) dictates that asphalt shingles cannot be installed on a slope lower than 2:12 (about 9.46°). Between 2:12 and 4:12, a double layer of underlayment is required.

In the United Kingdom, BS 5534 and the manufacturer's instructions for the specific tiles dictate the minimum pitch. Standard clay plain tiles typically require a minimum pitch of 35°. However, wind-driven rain exposure can force even steeper pitches or the use of specialized breathable underlays.

In Australia, while the National Construction Code (NCC) governs drainage, the most critical factor is often wind classification (AS 4055). Roofs in cyclonic regions require vastly different tie-downs and fasteners, regardless of the pitch.

Choosing the Right Roofing Material

Once the geometry is set, the choice of material dictates the longevity, maintenance, and structural load of the roof. Your pitch will directly determine which of these you can use:

Asphalt Shingles: The dominant choice in the US due to low cost and ease of installation. They require a minimum pitch of 2:12 and typically last 15–30 years.
Metal Roofing (Standing Seam or Corrugated): Highly durable (40–70 years) and excellent at shedding snow and rain. Metal can handle lower pitches (as low as 1:12 for some profiles) but requires specific fastening systems.
Clay and Concrete Tiles: Extremely common in the UK and Australia. They offer excellent thermal mass and longevity but are incredibly heavy. They usually require a steeper pitch (often 30° or 35° minimum) and a structure specifically engineered to bear the dead load.
Slate: A premium natural stone that can last over 100 years. It requires specialist installation and a very steep pitch (usually 35° or more) to shed water effectively.

Wind Uplift and Fastening

Water is not the only enemy of a roof; wind is equally destructive. When wind blows over a pitched roof, it creates negative pressure (suction) on the leeward side, attempting to lift the roof off the building.

In the United States, ASCE 7 dictates wind exposure categories, which in turn dictate the nailing schedule for shingles. In Australia, wind classification is governed by AS 4055, and in cyclonic regions (like Far North Queensland), the tie-down requirements connecting the roof to the walls are incredibly strict. In the UK, BS EN 1991-1-4 dictates wind actions, and areas of high exposure require mechanical fixings for tiles rather than traditional mortar.

A failure to match the fastener type, length, and spacing to the local wind zone is a leading cause of roof failure during storms. Never rely solely on a generic pitch calculation without consulting the local wind loading requirements.

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The Bottom Line

Roof pitch is a simple triangle, but it governs complex real-world decisions—from the materials you can legally buy to how much wind your roof can survive. By verifying your rise and run, and using the correct multiplier for area calculations, you ensure your material orders are accurate and your structure is compliant.

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Frequently Asked Questions

Roof Pitch FAQs

01 How do you convert a 6:12 pitch to degrees?

Divide the rise by the run (6 ÷ 12 = 0.5). Then, calculate the inverse tangent (arctan) of 0.5, which equals 26.565°. This means a roof that rises 6 inches for every 12 inches of run has an angle of approximately 26.57 degrees.

02 What is the difference between roof pitch and roof slope?

Roof pitch is typically expressed as a ratio of rise-over-run (e.g., 6:12), primarily used in the US. Roof slope is often expressed as a percentage (e.g., 50%) or an angle in degrees (e.g., 26.57°), used in the UK, Australia, and civil engineering. They describe the same geometry but are not interchangeable mathematically; a 50% slope is not the same as a 50° angle.

03 What is the minimum roof pitch for asphalt shingles?

Under the International Residential Code (IRC 2021), the minimum pitch for standard asphalt shingles is 2:12 (about 9.46°). However, roofs between 2:12 and 4:12 require a double layer of underlayment to prevent water infiltration. Below 2:12, standard shingles cannot be used, and a flat-roof membrane is required.

04 What is the roof pitch multiplier?

The roof pitch multiplier (or rafter multiplier) is a number used to calculate the true surface area of a pitched roof. It is equal to the secant of the roof angle (1 / cos θ). To find your actual roof area, multiply the flat footprint area by this multiplier. For example, a 6:12 pitch has a multiplier of 1.118.

05 How do I calculate rafter length from pitch?

To calculate rafter length, you can use the Pythagorean theorem (a² + b² = c²) where 'a' is the rise and 'b' is the run. Alternatively, divide the run by the cosine of the pitch angle. For a 6:12 pitch, the multiplier is 1.118, so a 12-foot run would require a rafter length of 12 × 1.118 = 13.416 feet (before adding for overhang).

06 Does a steeper roof pitch cost more?

Yes, generally. First, the actual surface area of the roof increases, requiring more materials. Second, steeper roofs are more difficult and dangerous to walk on, increasing labor costs and often requiring additional safety equipment like roof jacks or harnesses.

07 What is a walkable roof pitch?

A standard roof is generally considered walkable without special fall protection up to a 6:12 pitch (26.57°). Pitches steeper than 6:12 are typically considered too dangerous to walk on without roof jacks, harnesses, or scaffolding. A 4:12 pitch is generally comfortable to walk on.

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About This Tool

BuildMetric Roofing Engineering

Calculators & Estimating Tools

This calculator uses standard trigonometric formulas (arctangent) to convert rise-over-run into degrees and slope. It has been reviewed by our structural engineering team to ensure accuracy across US (IRC), UK (BS 5534), and Australian (AS 1684) standards.

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