How to Calculate Roof Slope in Degrees

Roof Slope Calculator (Degrees)

function calculateRoofSlope() { var roofRise = parseFloat(document.getElementById('roofRise').value); var roofRun = parseFloat(document.getElementById('roofRun').value); var resultDiv = document.getElementById('result'); if (isNaN(roofRise) || isNaN(roofRun) || roofRise < 0 || roofRun < 0) { resultDiv.innerHTML = "Please enter valid positive numbers for Rise and Run."; resultDiv.style.backgroundColor = '#ffe0e0'; resultDiv.style.color = '#d32f2f'; return; } if (roofRun === 0) { resultDiv.innerHTML = "Roof Run cannot be zero. Please enter a positive value."; resultDiv.style.backgroundColor = '#ffe0e0'; resultDiv.style.color = '#d32f2f'; return; } var slopeRadians = Math.atan(roofRise / roofRun); var slopeDegrees = slopeRadians * (180 / Math.PI); resultDiv.innerHTML = "Roof Slope: " + slopeDegrees.toFixed(2) + " degrees"; resultDiv.style.backgroundColor = '#d4edda'; resultDiv.style.color = '#155724'; }

Understanding Roof Slope and Pitch

Roof slope, often referred to as roof pitch, is a critical measurement in construction and roofing. It defines the steepness of your roof, impacting everything from drainage efficiency and material selection to structural integrity and aesthetic appeal. While pitch is commonly expressed as a ratio (e.g., 6/12, meaning 6 inches of rise for every 12 inches of run), slope can also be expressed as an angle in degrees, which is what this calculator helps you determine.

What are Rise and Run?

• Roof Rise: This is the vertical distance a roof extends upwards from the top plate to the ridge. In simpler terms, it's how high the roof goes.
• Roof Run: This is the horizontal distance from the outside wall of the building to the center of the roof (the ridge). It's essentially half the span of the roof.

These two measurements form a right-angled triangle with the roof rafter, allowing us to use basic trigonometry to find the angle.

Why is Roof Slope Important?

Knowing your roof's slope in degrees is essential for several reasons:

• Drainage: A sufficient slope ensures proper water runoff, preventing pooling and potential leaks.
• Material Selection: Different roofing materials (shingles, tiles, metal, flat membranes) have minimum slope requirements for effective performance and warranty validity.
• Building Codes: Local building codes often specify minimum and sometimes maximum roof slopes for various structures.
• Safety: Steeper roofs can be more challenging and dangerous to work on, affecting labor costs and safety protocols.
• Aesthetics: The slope significantly contributes to the architectural style and visual impact of a building.

How to Measure Roof Rise and Run

If you're measuring an existing roof, you can often find the rise and run by:

1. From the Attic: Measure the vertical distance from the top of the wall plate to the underside of the ridge board (rise). Then measure the horizontal distance from the outside edge of the wall plate to the center of the ridge (run).
2. From the Exterior (less precise): Use a level and a tape measure. Hold the level horizontally against the underside of the roof overhang. Measure 12 inches horizontally along the level (this is your run). Then, measure the vertical distance from the 12-inch mark on the level down to the roof surface (this is your rise for a 12-inch run). You can then scale this up or down if your actual run is different.

For new construction or planning, these dimensions are typically found on architectural blueprints.

The Calculation Explained

The calculator uses the trigonometric function tangent (tan). In a right-angled triangle, the tangent of an angle is the ratio of the opposite side (rise) to the adjacent side (run). To find the angle itself, we use the inverse tangent function, also known as arctangent (atan).

Slope (radians) = arctan(Rise / Run)

Since most people understand angles in degrees, the result is then converted from radians to degrees using the conversion factor: 1 radian = 180/π degrees.

Examples of Roof Slope Calculations

Let's look at a few practical examples:

1. Standard Pitch: If your roof has a rise of 6 inches and a run of 12 inches (a common 6/12 pitch):
   arctan(6 / 12) = arctan(0.5) ≈ 0.4636 radians
0.4636 * (180 / π) ≈ 26.57 degrees
   A 6/12 pitch is approximately 26.57 degrees.
2. Steeper Roof: Consider a roof with a rise of 12 inches and a run of 12 inches (a 12/12 pitch):
   arctan(12 / 12) = arctan(1) ≈ 0.7854 radians
0.7854 * (180 / π) = 45 degrees
   A 12/12 pitch is exactly 45 degrees.
3. Low Slope Roof: For a roof with a rise of 2 inches and a run of 12 inches (a 2/12 pitch):
   arctan(2 / 12) = arctan(0.1667) ≈ 0.1651 radians
0.1651 * (180 / π) ≈ 9.46 degrees
   A 2/12 pitch is approximately 9.46 degrees.

Use the calculator above to quickly determine the slope of your roof in degrees for any given rise and run measurements.