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Roof Pitch Calculator & Guide to Measuring Roof Slope

Whether you are planning a roofing replacement, installing solar panels, or designing an addition, accurately determining your roof's pitch is crucial. The roof pitch (or slope) determines what materials you can use, how fast water drains, and the overall aesthetic of the structure. This calculator will help you easily find the pitch ratio, angle in degrees, and the hypotenuse (rafter length) based on simple measurements.

What is Roof Pitch?

Roof pitch is a numerical measure of the steepness of a roof. It is calculated as the ratio of the vertical "rise" to the horizontal "run".

  • Rise: The vertical change in height over a specific horizontal distance.
  • Run: The horizontal distance corresponding to the rise. In standard US construction, the run is often normalized to 12 inches (or 1 foot) to express the pitch as "x in 12".

Roof Pitch Calculator

Enter consistent units (e.g., both in feet or both in inches).

function calculateRoofPitch() { // Get inputs var riseInputStr = document.getElementById("roofRise").value; var runInputStr = document.getElementById("roofRun").value; var resultsDiv = document.getElementById("pitchResults"); var errorDiv = document.getElementById("pitchError"); // Reset previous results/errors resultsDiv.style.display = "none"; resultsDiv.innerHTML = ""; errorDiv.style.display = "none"; errorDiv.innerHTML = ""; // Parse inputs var rise = parseFloat(riseInputStr); var run = parseFloat(runInputStr); // Validation if (isNaN(rise) || isNaN(run) || rise <= 0 || run <= 0) { errorDiv.innerHTML = "Please enter valid, positive numerical values for both rise and run."; errorDiv.style.display = "block"; return; } // Calculations // 1. Standard Pitch Ratio (x/12) // We calculate how much rise occurs over 12 units of run. var pitchValue = (rise / run) * 12; // Round to nearest quarter for standard construction notation, but keep precise for display var standardizedPitch = pitchValue.toFixed(2) + "/12"; // 2. Angle in Degrees // tan(angle) = opposite/adjacent = rise/run // angle = arctan(rise/run) var angleRadians = Math.atan(rise / run); var angleDegrees = angleRadians * (180 / Math.PI); // 3. Rafter Length (Hypotenuse) // a^2 + b^2 = c^2 var rafterLength = Math.sqrt(Math.pow(rise, 2) + Math.pow(run, 2)); // Build Output HTML var outputHtml = '

Calculation Results

'; outputHtml += '
'; outputHtml += '
'; outputHtml += 'Pitch Ratio'; outputHtml += '' + standardizedPitch + ''; outputHtml += '
'; outputHtml += '
'; outputHtml += 'Slope Angle'; outputHtml += '' + angleDegrees.toFixed(1) + '°'; outputHtml += '
'; outputHtml += '
'; outputHtml += 'Rafter Length'; outputHtml += '' + rafterLength.toFixed(2) + ''; outputHtml += '(Same units as input)'; outputHtml += '
'; outputHtml += '
'; // End grid // Display results resultsDiv.innerHTML = outputHtml; resultsDiv.style.display = "block"; }

How to Measure for Roof Pitch

To use the calculator above, you need two measurements. You can measure these from inside the attic (the easiest way) or on the roof surface itself.

  1. Measuring the Run: Measure a horizontal distance from the edge of the roof towards the center. For simplicity when measuring inside an attic, you can measure a 12-inch horizontal run on a rafter using a level.
  2. Measuring the Rise: From the end of your horizontal run measurement, measure vertically straight up to the rafter.

For example, if you measure a horizontal run of 12 feet (the distance from the exterior wall to the center ridge beneath the peak), and the ridge is 6 feet higher than the exterior wall plate, your rise is 6 and your run is 12. The calculator would determine this is a 6/12 pitch.

Understanding Roof Slope Classifications

Roof pitches generally fall into categories that dictate the type of roofing materials required.

  • Flat Roofs (0/12 to 2/12): These are not truly "flat" but have a very slight slope for drainage. They require specialized membrane roofing systems (like TPO or EPDM) because shingles will not shed water effectively.
  • Low Slope Roofs (2/12 to 4/12): These require special considerations for underlayment (often double layers) if using asphalt shingles, to prevent water backup from ice dams or wind-driven rain.
  • Conventional Slope Roofs (4/12 to 9/12): This is the most common range for residential homes. Standard asphalt shingles, metal roofing, and tiles work well here. A 6/12 pitch is a very common and walkable slope.
  • Steep Slope Roofs (9/12 and above): Steep roofs shed water and snow very quickly. They are aesthetically pleasing but more difficult and dangerous to work on. A 12/12 pitch means the roof rises at a 45-degree angle.

Use the calculator above to ensure you know exactly what slopes you are working with before purchasing materials or beginning construction.

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