function calculateRoofPitch() {
var rise = parseFloat(document.getElementById('roof-rise').value);
var run = parseFloat(document.getElementById('roof-run').value);
var resultArea = document.getElementById('pitch-result-area');
if (isNaN(rise) || isNaN(run) || run <= 0 || rise < 0) {
alert("Please enter valid positive numbers for Rise and Run.");
return;
}
// Calculation Logic
var angleRad = Math.atan(rise / run);
var angleDeg = angleRad * (180 / Math.PI);
var percentage = (rise / run) * 100;
// Standardizing to x:12 format
var factor = 12 / run;
var standardRise = rise * factor;
// Display results
document.getElementById('res-ratio').innerText = standardRise.toFixed(2) + " / 12";
document.getElementById('res-degrees').innerText = angleDeg.toFixed(2) + "°";
document.getElementById('res-percent').innerText = percentage.toFixed(2) + "%";
// Classification Logic
var classification = "";
if (standardRise <= 2) {
classification = "Flat or Low Slope";
} else if (standardRise <= 4) {
classification = "Low Slope (Requires specific roofing)";
} else if (standardRise <= 9) {
classification = "Conventional Pitch";
} else {
classification = "Steep Slope";
}
document.getElementById('res-class').innerText = classification;
resultArea.style.display = 'block';
}
How to Calculate Roof Pitch: A Comprehensive Guide
Understanding roof pitch is critical for selecting the right roofing materials, calculating water runoff, and determining the total surface area of your roof. Roof pitch represents the steepness of a roof, expressed as the vertical "rise" over a horizontal "run."
The Roof Pitch Formula
In the United States, roof pitch is most commonly expressed as a ratio of inches of rise for every 12 inches of run. The formula is:
Pitch = Rise / Run
To find the angle in degrees, we use trigonometry (the arctangent function):
Angle (Degrees) = arctan(Rise / Run) × (180 / π)
Standard Roof Pitch Classifications
Flat Roofs: 0/12 to 2/12 pitch. These are rarely perfectly flat and usually have a slight slope for drainage.
Low Slope: 2/12 to 4/12 pitch. These require specialized materials like membrane roofing or metal panels to prevent leaks.
Conventional Roofs: 4/12 to 9/12 pitch. The most common for residential homes, suitable for standard asphalt shingles.
Steep Slope: Over 9/12 pitch. These often require extra safety equipment for installers and are common in Victorian or Tudor-style homes.
Example Calculation
Scenario: You measure a roof and find that for every 12 inches of horizontal distance, the roof rises 6 inches.
Rise: 6
Run: 12
Pitch: 6/12
Angle: ~26.57 degrees
Grade: 50%
How to Measure Pitch on Site
You can measure roof pitch from the attic or from the outside. Use a 12-inch level. Hold the level perfectly horizontal (run). Measure the vertical distance from the 12-inch mark on the level down to the roof surface (rise). If that distance is 4 inches, you have a 4/12 pitch.
Disclaimer: Always use caution when working on ladders or roofs. For accurate structural assessments, consult a professional roofing contractor.