Gambrel Roof Calculator
Plan a barn-style gambrel roof. Enter the building size, the steep lower slope, the shallow upper slope and where the roof breaks, and get the roof height, lower and upper rafter lengths, knee-joint cut, roof area, roofing squares, shingles, sheathing and loft headroom — with a slope check, a live cross-section, comparison tables and a downloadable PDF.
| Upper pitch | Height | Upper rafter | Squares | Loft at 7 ft | Shingles |
|---|---|---|---|---|---|
| Enter the building size and slopes to compare | |||||
| Width | Height | Lower rafter | Upper rafter | Squares | Bundles |
|---|---|---|---|---|---|
| Enter the building size and slopes to compare | |||||
How to use the gambrel roof calculator
Four quick steps turn your building size and two slopes into the roof height, both rafter lengths, the knee cut, roof area, roofing squares and loft headroom — with a slope check, a live cross-section, by-pitch and by-width tables, a chart and a downloadable report.
Enter the building
Give the building width (span) and length in feet, plus an optional wall height for the overall height.
Set the lower slope
Pick the steep lower pitch and the lower run from the wall to the break — or let it set equal rafter lengths.
Set the upper slope
Choose the shallow upper pitch to the ridge — it must be lower than the lower slope.
Read results & save a PDF
Get rafters, roof area, squares, shingles, sheathing and loft width, then download a report.
How a gambrel roof is calculated
A gambrel is just two slopes per side. Each slope uses the same right-triangle math as any pitched roof; you add the two rises for the total height and the two rafters for the full slope. Here is the exact math the calculator runs.
Worked example — 24 × 36 ft barn, 24/12 lower slope, 4 ft lower run, 6/12 upper slope, 10% waste
Upper run: 24 ÷ 2 − 4 = 8 ft · Rises: 4 × 24/12 = 8 ft + 8 × 6/12 = 4 ft = 12 ft tall
Rafters: lower 4 × √(1+2²) = 8'11" · upper 8 × √(1+0.5²) = 8'11" (equal — the classic barn)
Area: 2 × 20'1" × 37 ft ≈ 1,489 sq ft · +10% ≈ 16.4 squares ≈ 50 bundles
≈ 12 ft roof height with matching 8’11” rafters — about 16.4 squares (50 bundles), and 17 ft of the width keeps a full 7 ft of loft headroom.
Gambrel roof reference charts
Handy lookups for how the upper pitch and the building width change the height, rafters, roofing squares and loft space.
| Upper pitch | Roof height | Upper rafter | Squares | Loft at 7 ft | Shingles |
|---|---|---|---|---|---|
| 3/12 | 10′ 0″ | 8′ 3″ | 15.8 | 17′ 0″ | Double underlay |
| 4/12 | 10′ 8″ | 8′ 5″ | 16.0 | 17′ 0″ | OK |
| 5/12 | 11′ 4″ | 8′ 8″ | 16.2 | 17′ 0″ | OK |
| 6/12 | 12′ 0″ | 8′ 11″ | 16.4 | 17′ 0″ | OK |
| 7/12 | 12′ 8″ | 9′ 3″ | 16.6 | 17′ 0″ | OK |
| 8/12 | 13′ 4″ | 9′ 7″ | 16.9 | 17′ 0″ | OK |
A steeper upper slope mainly adds height, not floor space — the loft width stays 17 ft because it’s set by the steep lower slope. Below 4/12, asphalt shingles need a double layer of underlayment.
| Width | Roof height | Lower rafter | Upper rafter | Squares | Bundles |
|---|---|---|---|---|---|
| 20 ft | 11′ 0″ | 11′ 2″ | 6′ 8″ | 14.6 | 44 |
| 24 ft | 12′ 0″ | 11′ 2″ | 8′ 11″ | 16.4 | 50 |
| 28 ft | 13′ 0″ | 11′ 2″ | 11′ 2″ | 18.2 | 55 |
| 32 ft | 14′ 0″ | 11′ 2″ | 13′ 5″ | 20.0 | 61 |
| 40 ft | 16′ 0″ | 11′ 2″ | 17′ 11″ | 23.7 | 71 |
Holding the 4 ft lower run fixed, the lower rafter (incl. 12″ overhang) stays 11’2″ — only the upper slope grows with width. All rows use a 36 ft length and 10% waste.
| Lower pitch | Angle | Look |
|---|---|---|
| 12/12 | 45° | Gentle gambrel |
| 18/12 | 56° | Traditional barn |
| 21/12 | 60° | Classic barn |
| 24/12 | 63° | Steep, max headroom |
The steeper the lower slope, the more the walls climb vertically and the wider the usable loft. 60° (21/12) is the traditional barn angle.
What the gambrel calculator gives you
Building size and two slopes give you the full framing geometry plus a materials shopping list.
Gambrel roofs at a glance
Who uses a gambrel roof calculator
From a backyard shed to a barn or a loft conversion, the same calculator sizes the slopes and the materials.
Building a gambrel shed or small barn and wants the rafters, roof height and shingle count before buying lumber.
- Set both slopes
- Use equal rafters
- Read the bundle count
Chasing usable upstairs space and needs to know how wide the loft stays with full standing headroom.
- Steepen the lower slope
- Check loft at 7 ft
- Watch overall height
Quoting a re-roof or new build and wants squares, bundles, sheathing and underlayment with a waste margin.
- Set the waste %
- Grab the squares
- Check the shingle slope
Tips for framing a gambrel roof
A gambrel works best when the break is placed well, the knee joint is solid and the shallow slope suits your roofing.
Gambrel roof calculator FAQ
Quick answers about gambrel pitch, the break point, shingles, usable loft space and building code.
A gambrel is the classic barn roof: each side has two slopes — a steep lower slope rising from the walls and a shallow upper slope running to the ridge. The extra steep lower slope opens up headroom, which is why gambrels suit barns, sheds and lofts.
On a 24 ft barn with a 24/12 lower slope and a 6/12 upper slope the roof stands about 12 ft tall.
Split each side into two slopes. For each slope, rise = run × pitch ÷ 12 and rafter length = run × √(1 + (pitch÷12)²). Add the two rises for total roof height and the two rafters for the slope.
On a 24 ft building with a 4 ft lower run at 24/12 and an 8 ft upper run at 6/12, the rises are 8 ft and 4 ft for a 12 ft roof.
A gambrel uses two pitches. The lower slope is steep, often around 60° (about 21/12 to 24/12), and the upper slope is shallow, commonly 4/12 to 8/12.
The classic barn look sets the slopes so the lower and upper rafters are the same length.
The break (knee) is where the steep lower slope meets the shallow upper slope. The lower run from the wall to the break must be less than half the building width.
For the traditional barn shape, place it so the lower and upper rafters come out equal — on a 24 ft building that’s a 4 ft lower run with a 24/12 lower and 6/12 upper slope.
Yes, on the slopes that are steep enough. Asphalt shingles need at least a 2/12 slope, and from 2/12 up to 4/12 they need two layers of underlayment.
The shallow upper slope is the one to check; if it’s below 2/12, use a membrane or standing-seam metal rated for low slope instead.
The steep lower slope pushes the walls almost straight up, so a gambrel gives far more headroom than a plain gable.
On a 24 ft wide barn with a 24/12 lower slope, about 17 ft of the width keeps a full 7 ft of standing headroom — enough for a real loft or second floor.
A square is 100 sq ft of roof. Measure both slopes on both sides including overhangs, add a waste allowance, then divide by 100.
A 24 × 36 ft barn with the standard slopes works out to about 16.4 squares at 10% waste — roughly 50 shingle bundles at 3 bundles per square.
Yes. Rafter sizes, the gussets at the knee joint, collar ties, snow and wind loads, and uplift connectors are all set by your local building code, and many gambrels are built from engineered trusses.
Treat this calculator as a layout and materials estimate and confirm the framing against your code or an engineer.
Layout & materials estimate only — confirm on site. This calculator works in US units (feet and inches) with pitch as rise per 12 in of run. Rise = run × pitch ÷ 12 and rafter = run × √(1 + (pitch÷12)²). Material counts use 3 bundles per square, 4×8 sheathing, 1,000 sq ft underlayment rolls and 10 ft drip edge plus your waste allowance; ridge cap, starter strip, vents and flashing are extra. Asphalt shingles need a 2/12 minimum and double underlayment below 4/12. Rafter sizes, knee gussets, collar ties, snow load and uplift connectors are set by your local building code — use engineered trusses or an engineer’s design for the framing.