Hip Roof Calculator
e.g., for a 6/12 pitch, enter 6
e.g., for a 6/12 pitch, enter 12 (standard)
Recommended 10-15% for materials
Calculation Results:
Common Rafter Length:
Hip Rafter Length:
Jack Rafter Shortening (per spacing):
Total Roof Surface Area (incl. waste):
Total Fascia Length:
Total Soffit Area:
Approximate Total Rafters (all types):
Ridge Length:
Please enter valid positive numbers for all fields. Overhang and Waste Factor can be zero or positive.
'; return; } // Ensure buildingLength is always greater than or equal to buildingWidth for consistent calculations // This simplifies ridge and common rafter logic. var effectiveBuildingWidth = Math.min(buildingWidth, buildingLength); var effectiveBuildingLength = Math.max(buildingWidth, buildingLength); // Convert units to feet for consistent calculations var pitchRise_ft = pitchRise / 12; var pitchRun_ft = pitchRun / 12; var rafterSpacing_ft = rafterSpacing / 12; // Calculate Pitch Angle var pitchAngleRad = Math.atan(pitchRise_ft / pitchRun_ft); // True Run (half of the effective building width, to the wall plate) var trueRun = effectiveBuildingWidth / 2; // True Rise (vertical height from wall plate to the top of the common rafter/hip rafter) var trueRise = trueRun * (pitchRise_ft / pitchRun_ft); // 1. Common Rafter Length // Length from wall plate to the ridge/peak var commonRafterLengthToPlate = Math.sqrt(trueRun * trueRun + trueRise * trueRise); // Length of the overhang portion of the rafter var overhangRafterLength = overhangLength / Math.cos(pitchAngleRad); // Total common rafter length var totalCommonRafterLength = commonRafterLengthToPlate + overhangRafterLength; // 2. Hip Rafter Length // Horizontal run of the hip rafter (diagonal across the corner) var hipRafterRunToPlate = Math.sqrt(trueRun * trueRun + trueRun * trueRun); // Diagonal of a square with side 'trueRun' // Length from wall plate to the peak var hipRafterLengthToPlate = Math.sqrt(hipRafterRunToPlate * hipRafterRunToPlate + trueRise * trueRise); // Total hip rafter length var totalHipRafterLength = hipRafterLengthToPlate + overhangRafterLength; // Hip overhang is also 'overhangLength' horizontally // 3. Jack Rafter Shortening (per rafter spacing) // This is the amount each successive jack rafter shortens along the slope var jackRafterShortening = rafterSpacing_ft / Math.cos(pitchAngleRad); // 4. Total Roof Surface Area (including overhangs and waste) var totalProjectedArea = (effectiveBuildingLength + 2 * overhangLength) * (effectiveBuildingWidth + 2 * overhangLength); var totalRoofArea = totalProjectedArea / Math.cos(pitchAngleRad); var totalRoofAreaWithWaste = totalRoofArea * (1 + wasteFactor / 100); // 5. Total Fascia Length var totalFasciaLength = 2 * (effectiveBuildingLength + 2 * overhangLength) + 2 * (effectiveBuildingWidth + 2 * overhangLength); // 6. Total Soffit Area // This calculates the area of the flat soffit under the overhang var totalSoffitArea = 2 * (effectiveBuildingLength * overhangLength) + 2 * (effectiveBuildingWidth * overhangLength) + 4 * (overhangLength * overhangLength); // 7. Approximate Total Rafters (all types) var numCommonRafters = 0; var ridgeLength = 0; if (effectiveBuildingLength > effectiveBuildingWidth) { ridgeLength = effectiveBuildingLength – effectiveBuildingWidth; // Number of common rafters along the ridge (two sides) numCommonRafters = 2 * Math.ceil(ridgeLength / rafterSpacing_ft); } var numHipRafters = 4; // Number of jack rafters on one side of a hip (excluding the common rafter if present) var numJackRaftersPerSide = Math.max(0, Math.floor(trueRun / rafterSpacing_ft) – 1); var totalJackRafters = 4 * numJackRaftersPerSide * 2; // 4 hips, 2 sides per hip var approxTotalRafters = numCommonRafters + numHipRafters + totalJackRafters; // Display Results document.getElementById('resultCommonRafterLength').innerHTML = 'Common Rafter Length: ' + totalCommonRafterLength.toFixed(2) + ' ft'; document.getElementById('resultHipRafterLength').innerHTML = 'Hip Rafter Length: ' + totalHipRafterLength.toFixed(2) + ' ft'; document.getElementById('resultJackRafterShortening').innerHTML = 'Jack Rafter Shortening (per ' + rafterSpacing + 'in spacing): ' + jackRafterShortening.toFixed(2) + ' ft'; document.getElementById('resultTotalRoofArea').innerHTML = 'Total Roof Surface Area (incl. ' + wasteFactor + '% waste): ' + totalRoofAreaWithWaste.toFixed(2) + ' sq ft'; document.getElementById('resultTotalFasciaLength').innerHTML = 'Total Fascia Length: ' + totalFasciaLength.toFixed(2) + ' ft'; document.getElementById('resultTotalSoffitArea').innerHTML = 'Total Soffit Area: ' + totalSoffitArea.toFixed(2) + ' sq ft'; document.getElementById('resultApproxTotalRafters').innerHTML = 'Approximate Total Rafters (all types): ' + Math.round(approxTotalRafters) + ''; document.getElementById('resultRidgeLength').innerHTML = 'Ridge Length: ' + ridgeLength.toFixed(2) + ' ft'; }Understanding Your Hip Roof
A hip roof is a type of roof where all sides slope downwards to the walls, usually with a gentle slope. It has no vertical ends (gables). This design makes it very stable and resistant to wind, which is why it's popular in hurricane-prone areas. The slopes meet at "hips" (external angles) and sometimes at a central ridge if the building is rectangular and not square.
Key Components of a Hip Roof:
- Hip Rafters: These are the main structural members that run from the corners of the building up to the ridge or peak, forming the external angles of the roof.
- Common Rafters: These run perpendicular from the wall plate to the ridge board. In a rectangular hip roof, they are found along the longer sides, connecting to a central ridge. In a square hip roof (pyramid hip), there are no common rafters or ridge.
- Jack Rafters: These are shorter rafters that run from the wall plate to a hip rafter. They fill the space between the common rafters and the hip rafters.
- Ridge Board: A horizontal board at the very top of the roof where the common rafters meet. A true hip roof on a square building will not have a ridge board, as all four hip rafters meet at a single point (a pyramid hip).
- Overhang: The portion of the roof that extends beyond the exterior walls, providing shade and protection.
- Fascia: The board that runs horizontally along the edge of the roof, covering the ends of the rafters.
- Soffit: The underside of the eave overhang, often vented to allow air circulation into the attic.
How the Calculator Works:
This calculator helps you estimate the critical dimensions and material quantities for a hip roof based on your building's dimensions and desired roof pitch. It uses fundamental trigonometry and geometry to determine rafter lengths, roof surface area, and other key measurements.
- Building Width & Length: These define the footprint of your structure.
- Roof Pitch (Rise/Run): Describes the steepness of your roof. For example, a 6/12 pitch means the roof rises 6 inches vertically for every 12 inches of horizontal run.
- Eave Overhang Length: The horizontal distance the roof extends past the wall.
- Rafter Spacing: The on-center distance between your rafters, typically 16 or 24 inches.
- Waste Factor: An essential percentage added to material estimates to account for cuts, errors, and damaged pieces.
Example Calculation:
Let's say you have a building that is 24 ft wide and 36 ft long, with a 6/12 roof pitch, a 1.5 ft eave overhang, 16-inch rafter spacing, and a 10% waste factor.
Using the calculator with these inputs:
- Building Width: 24 ft
- Building Length: 36 ft
- Roof Pitch Rise: 6 inches
- Roof Pitch Run: 12 inches
- Eave Overhang Length: 1.5 ft
- Rafter Spacing: 16 inches
- Waste Factor: 10%
The calculator would provide results similar to these (values are approximate and rounded for example):
- Common Rafter Length: ~15.90 ft
- Hip Rafter Length: ~18.27 ft
- Jack Rafter Shortening (per 16in spacing): ~1.49 ft
- Total Roof Surface Area (incl. 10% waste): ~1782.00 sq ft
- Total Fascia Length: ~126.00 ft
- Total Soffit Area: ~189.00 sq ft
- Approximate Total Rafters (all types): ~86
- Ridge Length: ~12.00 ft
These figures help you determine the lumber needed for rafters, the amount of roofing material (shingles, underlayment), and the linear feet of fascia and square footage of soffit material required for your project.