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Free Gambrel Roof Truss Calculator. Easily find rafter lengths, angles, rise, and span for your gambrel roof. Perfect for DIYers and pros.
A gambrel roof gives a barn-style look. It offers more attic space without raising the whole roof. But figuring out the right truss sizes and angles can feel like a puzzle. That’s where our Gambrel Roof Truss Calculator steps in. It makes the math quick and easy so you can focus on building.
Manual roof math can be tricky. You’ll deal with runs, rises, slopes, and angles. One small mistake can throw off your cuts. Our calculator solves this. It gives you instant, accurate numbers for:
With the right inputs, you’ll get results you can trust.
The calculator uses basic geometry. It treats each slope as a right-angled triangle. From there, it applies Pythagoras’ theorem and simple slope formulas.
Here’s the math in plain text:
Half run = span ÷ 2
Upper run = half run − lower run
Slope factor = pitch ÷ 12
Rise = run × slope factor
Rafter length = √(run² + rise²)
Angle = atan(slope factor) × (180 ÷ π)
Interface angle = |lower angle − upper angle|
You can add overhang by increasing the run before the rafter length step. You can subtract birdsmouth depth after the length step.
Enter your total span. Add the lower run, lower pitch, and upper pitch. If you have overhang or birdsmouth depth, enter those too. Hit calculate.
You’ll see your half run, rises, lengths, and angles. It’s that simple. No guesswork. No complex drawings.
Some builders use the golden ratio for beauty. In roof terms, this means the lower slope is steeper and longer. The upper slope is shorter and flatter. The golden ratio is about 1.618:1. You can set your lower run and upper run in that ratio if you want a classic look.
A gambrel truss can span 12–40 feet, sometimes more. The exact span depends on the wood, loads, and truss design. Always follow your local building codes or talk to an engineer.
First, use the calculator to get rafter lengths. Multiply each slope length by the span to find the area for each side. Add them together. Then divide by the coverage per bundle to see how many bundles you’ll need.
It can be less stable in heavy wind.
Snow can pile on the upper slope.
It needs more joints, which means more points for leaks if not sealed well.
A gambrel roof looks great and saves space. But the math can scare off beginners. Our Gambrel Roof Truss Calculator makes it easy. Enter your numbers and get instant results. You’ll cut with confidence, save time, and avoid costly mistakes.
Extra attic or storage space without a taller building.
Yes, as long as you input accurate dimensions.
Yes, if you’re building a structural truss. The calculator gives geometry, not load analysis.