Gambrel Roof Calculator

Free Gambrel Roof Calculator for Rafter Lengths and Angles

This gambrel roof calculator works out everything you need to frame a barn-style, two-pitch roof: the rafter length for the steep lower slope, the rafter length for the shallow upper slope, both plumb cut angles, the total rise to the ridge, and even how much usable floor space you'll end up with inside the loft. Type in your building width, your two roof pitches, and where you want the break point, and every number updates instantly along with a live diagram.

A gambrel roof is that classic barn shape with two different slopes on each side instead of one straight pitch: a steep lower section near the eaves and a flatter upper section near the ridge. It's the roof people usually picture on a red barn, a Dutch colonial house, or a modern loft-style shed, and it's popular for one simple reason — it opens up a lot more usable headroom inside the attic or loft than a plain gable roof does at the same overall height.

What Is a Gambrel Roof?

A gambrel roof (sometimes called a barn roof or a Dutch gambrel) has two roof slopes on each side of the ridge, meeting at a point partway up called the break point. The lower slope, closer to the walls, is steep, sometimes close to vertical. The upper slope, closer to the ridge, is much shallower and does most of the work of shedding rain and snow off the top of the building.

Because the lower slope is so steep, it eats up very little horizontal space while still climbing a good amount of height. That's what frees up so much extra room inside compared to a single-pitch gable roof, which has to use one consistent angle the whole way from wall to ridge. This is exactly why gambrel roofs show up so often on barns that need a hay loft, on garages that double as workshops with storage above, and on homes that want a full extra floor tucked under the roofline.

The Break Point: Where the Two Slopes Meet

The break point is the single most important design decision on a gambrel roof, because it controls how the whole shape looks and how much usable loft space you end up with. Set it too close to the wall and the steep lower slope barely climbs before it flattens out, losing you interior height. Set it too far out and the steep slope eats up too much horizontal room, cutting into your usable floor width lower down.

This calculator lets you set the break point two ways. The simplest is as a percentage of the half-span, measured out from the wall — a common starting point for a classic barn look is somewhere around 30% to 40%, and this calculator defaults to 35%. If you already have a specific horizontal distance in mind, from an existing set of plans or a truss manufacturer's spec sheet, you can switch to entering that run directly in feet instead.

How This Calculator Works — Purlin Plate Framing

There are a couple of ways carpenters frame the point where the two slopes meet. This calculator uses the most common traditional method: a horizontal purlin plate (sometimes called a knee plate) sits at the break point, running the length of the building. The lower, steep rafters land on the wall's top plate at the bottom and on this purlin plate at the top. The upper, shallow rafters then start where the lower rafters left off, running from the same purlin plate up to the ridge board.

Because both rafter sections bear on a flat horizontal plate rather than joining each other directly at an angle, each one behaves exactly like a simple single-pitch rafter with its own plumb cut angle. That's genuinely how a lot of real gambrel barns are framed, and it keeps the math straightforward and the on-site cutting simple, without needing an oddly angled compound joint where two different slopes would otherwise try to meet in mid-air.

• Step 1 — Find each slope's angle: θ = arctangent(rise ÷ 12) for a ratio pitch, or enter the angle directly in degrees.
• Step 2 — Split the half-span at the break point into a lower run and an upper run.
• Step 3 — Find the rise of each slope: Rise = Run × tangent(θ).
• Step 4 — Find the rafter length of each slope: Rafter Length = Run ÷ cosine(θ).
• Step 5 — Add the two rises together for the total height from the wall top to the ridge.
• Step 6 — If a wall height was entered, add it to the total rise for the overall building height.

Worked Example: A 24-Foot-Wide Barn

Say you're framing a 24-foot-wide barn with an 8-foot side wall, a steep 24/12 lower pitch, a shallow 6/12 upper pitch, and a break point set at 35% of the half-span. The half-span is 12 feet, so the lower run works out to 12 × 0.35, or about 4.2 feet, leaving an upper run of about 7.8 feet.

The lower slope angle, arctangent(24 ÷ 12), is about 63.43 degrees, giving a lower rise of roughly 8.4 feet and a lower rafter length of about 9.39 feet. The upper slope angle, arctangent(6 ÷ 12), is about 26.57 degrees, giving an upper rise of about 3.9 feet and an upper rafter length of about 8.72 feet. Adding the two rises together gives a total rise of about 12.3 feet, so with the 8-foot wall the ridge sits roughly 20.3 feet above the ground — a proportion that will look instantly familiar as a classic barn silhouette.

Why the Loft Width Feature Matters

Rafter lengths and cut angles are only half the story on a gambrel roof. The other half is figuring out how much of that loft or attic space you can actually stand up and walk around in, which depends on both slopes and the break point together. That's what the usable loft width feature and chart on this page are for.

Enter a headroom height, like 5.5 feet or 6 feet, and the calculator traces out from the ridge centerline along the roofline until it hits that height, then doubles the distance to give you the usable floor width at that headroom across the whole loft. The chart shows this relationship across a full range of headroom heights at once, so you can see the trade-off clearly: demand more headroom and the usable width shrinks, especially once you cross the break point into the steep lower slope's zone.

Gambrel Roof vs. Gable Roof vs. Mansard Roof

A standard gable roof uses one single pitch on each side, which is simpler to frame but gives the attic a triangular cross-section that loses usable width quickly as you move away from the center. A gambrel roof breaks that single slope into two, trading a bit of extra framing complexity for noticeably more standing headroom across a wider stretch of the loft floor.

A mansard roof takes the same basic idea even further, using four slopes instead of two — steep on all four sides of the building rather than just the two long sides — which maximizes usable floor space on every wall but adds real framing and waterproofing complexity at the corners. For most barns, garages, sheds, and Dutch colonial-style homes, a two-slope gambrel hits the sweet spot between extra interior space and straightforward construction.

Who Uses This Calculator

This tool comes in handy across a range of gambrel roof projects:

• Barn builders and pole-barn contractors use it to work out rafter lengths and cut angles for both slopes before ordering lumber.
• Shed and garage builders use it to plan a gambrel roofline that leaves room for a usable loft or storage area above.
• DIYers and homeowners use it to sanity-check a set of purchased barn plans, or to sketch out their own gambrel roof from scratch.
• Designers and hobbyist framers use the loft width chart to compare different break points and pitch combinations before settling on a final roofline.