Calculate roof slope, angle, and rafter length by entering the rise and run.
Roof Pitch (x:12 Ratio):
Slope Angle (Degrees):
Pitch Percentage:
Rafter Length (Diagonal):
Understanding Roof Pitch: A Comprehensive Guide
Roof pitch is a critical measurement in architecture and roofing that describes the steepness of a roof. It is a ratio that compares the vertical "rise" to the horizontal "run." Understanding your roof pitch is essential for choosing the right shingles, calculating materials, and ensuring proper water drainage.
How to Calculate Roof Pitch Manually
The standard formula for calculating roof pitch is expressed as Rise / Run. In the United States, this is typically normalized to a 12-inch run. For example, if a roof rises 4 inches for every 12 inches of horizontal distance, it is called a "4:12 pitch."
Rise: The vertical distance from the top of the wall to the ridge of the roof.
Run: The horizontal distance from the outside of the wall to a point directly below the ridge.
Span: The total width of the building from wall to wall (Span is usually twice the Run).
The Pitch to Degree Conversion
While roofers use the x:12 ratio, engineers often work with degrees. You can convert pitch to degrees using the arctangent function: Angle = arctan(Rise / Run). A 12:12 pitch, where the rise and run are equal, results in a perfect 45-degree angle.
Pitch Ratio
Angle (Degrees)
Steepness Category
1:12 – 3:12
4.76° – 14.04°
Low Slope
4:12 – 9:12
18.43° – 36.87°
Conventional Slope
10:12 – 12:12
39.81° – 45.00°
Steep Slope
12:12+
> 45.00°
Extreme Slope
Why Roof Pitch Matters
The steepness of your roof dictates what materials you can use. Asphalt shingles generally require a minimum pitch of 2:12 to prevent water from backing up under the shingles. If your roof is lower than 2:12, it is considered a "flat roof" and requires specialized seamless membranes like EPDM or TPO to remain watertight.
Calculating Rafter Length
Our calculator also provides the rafter length using the Pythagorean theorem (a² + b² = c²). This is the diagonal length from the ridge to the eave. Knowing this measurement helps in ordering lumber and determining the total surface area of the roof for shingle estimation.
function calculateRoofPitch() {
var rise = parseFloat(document.getElementById('rise').value);
var run = parseFloat(document.getElementById('run').value);
var resultsDiv = document.getElementById('results');
if (isNaN(rise) || isNaN(run) || run <= 0) {
alert("Please enter valid positive numbers for both Rise and Run.");
return;
}
// 1. Pitch Ratio (Normalized to 12)
var pitchValue = (rise / run) * 12;
var formattedPitch = pitchValue.toFixed(2).replace(/\.00$/, '') + " / 12";
// 2. Slope Angle in Degrees
var radians = Math.atan(rise / run);
var degrees = radians * (180 / Math.PI);
// 3. Pitch Percentage
var percentage = (rise / run) * 100;
// 4. Rafter Length (Hypotenuse)
var rafter = Math.sqrt(Math.pow(rise, 2) + Math.pow(run, 2));
// Display Results
document.getElementById('pitchRatio').innerHTML = formattedPitch;
document.getElementById('slopeAngle').innerHTML = degrees.toFixed(2) + "°";
document.getElementById('pitchPercent').innerHTML = percentage.toFixed(2) + "%";
document.getElementById('rafterLength').innerHTML = rafter.toFixed(2) + " inches";
resultsDiv.style.display = "block";
}