Online Gambrel Truss Calculator - Design Customized Gambrel Trusses and Rafters
Design your own customized gambrel trusses with our Online Gambrel Truss Calculator. This tool allows you to easily configure the unique dimensions and angles needed for gambrel truss construction, providing all the necessary cut list parts for precise assembly. Ideal for both architectural projects and practical outbuilding constructions, such as barns and sheds. View detailed diagrams, generate accurate cut lists, and print or download your plans as PDFs for streamlined construction and installation.
How to Use This Tool
Enter the building width
Type the span of the bearing walls. The classic gambrel profile draws immediately using the semicircle layout, with the lower and upper chord breaks placed automatically.
Adjust the break point and pitches
Drag the knee break or type custom pitches if you want a taller loft or a shallower upper roof than the classic 22.5-degree profile. The diagram updates every chord length and joint angle live.
Review the gusset joints
Check the angles at the peak, the knee breaks, and the heels. Each joint shows the bevel for both meeting members so you can cut plywood gussets that fit tight.
Export the cut list and plans
Download the complete cut list and truss diagram as a PDF, or print it. Build one truss flat, verify it, then duplicate - the cut list handles quantities for the whole run.
The semicircle method and gambrel angles
The classic barn gambrel comes from inscribing the truss in a semicircle whose diameter equals the building width. Mark points on the arc every 45 degrees and connect them with chords: the result is four equal-length roof segments, a steep lower slope at 67.5 degrees from horizontal, and a shallow upper slope at 22.5 degrees. Every joint between segments breaks at 22.5 degrees, so one saw setting cuts every bevel - the reason this layout survived a century of barn building.
Because all four chords are equal, each one has length 2R x sin(22.5 degrees), where R is half the building width. You are free to move the break point for more loft headroom; the tool then solves the non-classic angles for you, which no longer come out to a single tidy saw setting.
R = building width / 2
classic chord length (each of 4 segments) = 2 x R x sin(22.5 deg) = 0.7654 x R
lower slope angle = 67.5 deg from horizontal (about 28.9/12 pitch)
upper slope angle = 22.5 deg from horizontal (about 5/12 pitch)
peak height (classic profile) = R
every joint bevel (classic profile) = 22.5 deg
Worked example: 24 ft wide barn, classic gambrel
- Building width 24 ft, so the semicircle radius R = 12 ft and the peak sits 12 ft above the wall plates.
- Chord length: 2 x 12 x sin(22.5 deg) = 24 x 0.38268 = 9.18 ft. All four roof segments are identical at about 9 ft 2-1/4 inches.
- The knee break lands at 12 x cos(45 deg) = 8.49 ft out from the center of the span - that is 12 - 8.49 = 3.51 ft in from each wall - at a height of 12 x sin(45 deg) = 8.49 ft above the plate. Between the two breaks the loft is 2 x 8.49 = about 17 ft wide at that 8.49 ft height, and the full 24 ft is available at plate level.
- Set the saw once: 22.5 degrees. The heels, both knee breaks, and the peak all cut at the same bevel on the classic profile.
- Cut plywood gussets for the five joints, build the first truss on a flat surface over the printed full-size layout, and use it as the pattern. The exported cut list covers chords, gussets, and any web members for the full truss count.
Frequently Asked Questions
What are the standard angles for a gambrel roof?
The classic profile uses 67.5 degrees from horizontal on the lower slope and 22.5 degrees on the upper slope, which comes from laying the truss out on a semicircle with points every 45 degrees. Every joint bevel is 22.5 degrees, so one miter setting cuts the whole truss. In pitch terms that is roughly 29/12 over 5/12. You can deviate for more headroom, but you lose the single-setting convenience - the builder computes the custom angles either way.
Why use the semicircle method at all?
Three reasons. All four roof segments come out the same length, so you cut one chord length repeatedly with one bevel setting - fast and hard to get wrong. The proportions look right because the profile is geometrically regular, avoiding the pinched or bulbous look of arbitrary gambrels. And the shape approximates an arch, which handles load gracefully. It is the profile on most traditional American barns for exactly these practical reasons.
How much headroom does a gambrel loft give compared to a gable?
Substantially more. On a 24 ft wide building, a 6/12 gable gives you 6 ft of peak height with steeply tapering sides, while the classic gambrel gives a 12 ft peak and near-vertical lower walls at 67.5 degrees. You get usable floor width of roughly 14 to 16 ft with standing headroom, which is why gambrels dominate barns and storage sheds. The builder shows the interior clearance as you adjust the break point.
What size gussets should I use on a site-built gambrel truss?
A common shop practice for sheds and barns is 1/2-inch plywood gussets on both faces of each joint, sized to give roughly 12 inches of contact along each member, glued and nailed or screwed in a grid pattern. The knee joints carry the largest moment, so do not undersize them. For permitted residential structures, gusset and fastener sizing should come from an engineer or an approved design - verify what your local building department accepts.
Can gambrel trusses span the full width without interior walls?
That is the point of building them as trusses - the bottom chord ties the walls and the triangulated profile carries the roof, so barns get a clear-span loft. Practical site-built spans run to about 24 or 30 ft depending on snow load, lumber size, and gusset quality; beyond that, get an engineered design. Note that a gambrel framed as individual rafters with purlins, rather than trusses, does need interior support or engineered knee walls.
What pitch do I enter for shingle and underlayment requirements?
Treat each slope separately. The upper slope at 22.5 degrees is about 5/12, fine for standard asphalt shingle application. The lower slope at 67.5 degrees is far steeper than 21/12 - shingles there behave almost like siding and manufacturers have specific steep-slope fastening instructions, or you can side it with the wall material. IRC R905.2.2 sets minimum shingle slopes at the shallow end; steep-slope details come from the manufacturer. Verify locally.
Does the tool give me a cut list for the whole truss run?
Yes. Enter the truss count and the export includes every chord with its length and end bevels, the gusset dimensions, and quantities totaled across the run, plus a dimensioned diagram of the truss to full scale references. Print the PDF and snap the truss outline on your build deck so every truss gets assembled over the same layout - the fastest way to keep a long run of trusses identical.