Gambrel Truss Calculator

A gambrel roof is defined by two pitches per side and one total rise, and all three determine the break point. Solve the break run x₁ from (p1/12)·x₁ = rise − (p2/12)·(half-span − x₁), which rearranges to x₁ = (12·rise − p2·half-span) ÷ (p1 + p2). The lower pitch angle is arctan(p1/12) and the upper is arctan(p2/12) — a classic 16/12-over-4/12 barn roof breaks at 53.1° and 18.4° from horizontal. The calculator above runs this algebra for any span, slopes, and rise, and flags combinations where no valid break point exists.

The classic American barn uses a 16/12 lower slope and a 4/12 upper slope with total rise equal to half the span, which produces the familiar ogee silhouette with near-vertical lower walls. A slightly tamer residential variant is 12/12 over 6/12. What matters structurally is the ratio: the lower slope two to four times the upper slope gives the look people want, while keeping the upper slope above 2/12 preserves shingle warranty minimums on the upper deck.

At identical span and ridge height, a gambrel has slightly more roof deck — typically 3–6% more — because the two-slope path bends outward. What it buys is interior volume where you want it: the lower slope stands steep and tall like a wall, so the usable second-floor width is far larger than a gable's floor at the same height. The calculator's comparison bar quantifies the deck penalty for your exact geometry so you can weigh deck area against headroom.

Yes — the worked example on this page walks a full 12 ft span, 16/12-over-4/12, 6 ft rise gambrel from break point to member lengths: break at 4 ft out and 5 ft 4 in up, lower slope 6 ft 8 in, upper slope 2 ft 2 in, and 17 ft 9 in of total deck width versus 17 ft 0 in for the equal-height gable. Every number is reproducible in the calculator above and the hand formulas in the two-slope method section.

The break-point joint is the gambrel's signature detail: it kinks the top chord, so the two upper-chord segments meet at an angle and the joint must resolve the bending change. Structurally that means the break joint carries a moment-like demand that plate designers treat specially, and the web layout (typically a pair of diagonals meeting the bottom chord near the quarter points) differs from a plain triangle truss. Snow also redistributes — the shallow upper slope can trap drift at the break — so local snow-load rules apply to each slope separately.

Yes. The 'Open the preloaded gambrel model' button carries an encoded link that rebuilds a complete 32 ft gambrel truss — two-slope top chord, quarter-point webs, and 15 kip of roof loading — inside the Structural Analysis tab. The solver reports every axial force and confirms the frame is statically determinate at 8 joints and 13 members.