Gambrel Roof Calculator

1. Enter the building length in feet (the dimension along the ridge).
2. Enter the building width in feet (the dimension perpendicular to the ridge).
3. Enter the lower slope angle in degrees. The classic barn angle is 60 degrees.
4. Enter the upper slope angle in degrees. A typical shallow upper slope is 25 degrees.
5. Enter the upper run in feet: the horizontal distance from the break point to the ridge. The lower run is calculated as half the width minus the upper run.
6. Enter the eave and gable overhangs in inches. The eave overhang extends along the lower slope; the gable overhang extends past each end wall.
7. Select a waste factor: 10% for a simple gambrel, 15% with dormers, or 20% for complex framing.

Here is what each result line means:

For shingle quantities from the roof area, use the roofing calculator.

A gambrel roof calculator estimates the geometry and framing dimensions for a gambrel (barn-style) roof. A gambrel roof has two different slopes on each side: a steep lower slope and a shallower upper slope that meet at a break point (also called the knee or curb). This dual-slope design creates a distinctive barn profile and maximizes the usable attic or loft space compared to a standard gable roof.

The calculator produces the total roof area, lower and upper rafter lengths, ridge length, total rise and break point height. Unlike a standard gable or hip roof, the gambrel requires two angle inputs instead of a single pitch, because each section has its own slope.

The calculator splits each side of the roof into a lower and upper section, then sums the areas.

Step 1: Lower and Upper Runs

Half the building width is divided between the lower run and the upper run. The upper run is entered directly; the lower run is the remainder: lower run = (width / 2) – upper run.

Step 2: Rafter Lengths

Each rafter length is the horizontal run divided by the cosine of the slope angle. The steeper lower slope produces a longer rafter per foot of run.

Step 3: Total Rise

Each section contributes its own rise: the lower section rises steeply, and the upper section rises more gradually to the ridge.

Step 4: Total Roof Area

Each side of the gambrel is a rectangle with width equal to the ridge length and height equal to the sum of the lower and upper rafter lengths. The eave overhang adds to the lower rafter length along the slope (overhang / cos(lower angle)).

A 40 ft long by 24 ft wide building with a 60-degree lower slope, 25-degree upper slope and 6 ft upper run, no overhangs.

Step 1. Half-width = 12 ft. Lower run = 12 – 6 = 6 ft.

Step 2. Lower rafter = 6 / cos(60) = 6 / 0.500 = 12.00 ft. Upper rafter = 6 / cos(25) = 6 / 0.906 = 6.62 ft.

Step 3. Lower rise = 6 x tan(60) = 10.39 ft. Upper rise = 6 x tan(25) = 2.80 ft. Total rise = 13.19 ft.

Step 4. Ridge = 40 ft. Total area = 2 x 40 x (12.00 + 6.62) = 1,489.6 sq ft.

With 10% waste, the ordering area is 1,489.6 x 1.10 = 1,639 sq ft.

Lower Slope Angle

A steeper lower slope increases the lower rafter length and break point height. At 60 degrees the lower rafter is twice the lower run. At 70 degrees it is nearly three times the run. Steeper lower slopes create more usable attic space but require longer lumber.

Upper Slope Angle

The upper slope is typically much shallower (20 to 30 degrees). A flatter upper slope sheds water more slowly but reduces overall roof height. The classic half-circle gambrel has the lower angle minus the upper angle equal to 45 degrees.

Upper Run Proportion

The split between upper and lower run determines the break point position. A larger upper run pushes the break point inward and down, creating a wider upper slope. For a detailed eave analysis, see the overhang calculator.

Overhangs

The eave overhang extends along the lower slope, so the overhang distance along the slope is longer than the horizontal overhang (divided by cos(lower angle)). The gable overhang adds to the ridge length on both ends.

Waste Factor

A simple gambrel wastes about 10% of material, similar to a gable roof. Adding dormers or shed dormers increases waste to 15% or more because of the additional valley and hip cuts.

For standard gable roof estimates, use the gable roof calculator.

Barn and Agricultural Buildings

The gambrel design is the standard barn roof shape. Calculate the rafter lengths and total area before ordering lumber and roofing materials for a new barn or barn-style building.

Dutch Colonial and Gambrel Additions

Dutch Colonial homes use the gambrel profile. Use the calculator to estimate the roof area for re-roofing or adding a gambrel dormer to an existing structure.

Attic and Loft Planning

The break point height tells you how much vertical wall space is available in the attic. A break point at 8 ft or higher creates a full-height room with standard walls on the lower portion.

Comparing Roof Shapes

Enter the same building dimensions in this calculator and the gable or hip roof calculator to compare total area, rise and material needs before choosing a roof shape.

Insurance and Appraisal

Determine the gambrel roof area from the building footprint for insurance documentation or damage estimates.

1. Using a Single Pitch for Both Slopes

A gambrel has two different slopes, not one. Using a single gable pitch multiplier underestimates the total area because the steep lower section adds significantly more roof surface per foot of run.

2. Confusing Upper Run with Half-Width

The upper run is only a portion of the half-width. The lower run is the remainder: (width / 2) – upper run. Entering the full half-width as the upper run leaves zero lower run and crashes the calculation.

3. Forgetting the Eave Overhang Is Along the Lower Slope

A 12-inch horizontal overhang on a 60-degree lower slope extends 24 inches along the rafter (12 / cos(60) = 24 in). This is twice the horizontal distance and significantly increases the total area. For rafter tail details, see the roof overhang calculator.

4. Not Checking the Break Point Height

The break point must be high enough for the intended use. If the lower slope is too shallow or the lower run is too short, the break point may be too low for a usable attic room. Check the break point height result before finalizing the design.

5. Using Gable Waste Factors

A simple gambrel wastes about the same as a gable (10%), but a gambrel with dormers wastes 15% or more because of the additional intersecting cuts at the dormer valley and ridge.

This calculator gives exact trigonometric results for a rectangular building with symmetric gambrel slopes on both sides.

A gambrel roof has two different slopes on each side: a steep lower slope and a shallower upper slope that meet at a break point. The design creates a distinctive barn-shaped profile and maximizes usable attic or loft space compared to a standard gable roof.

Divide half the building width into a lower run and an upper run. Calculate each rafter length as the run divided by the cosine of the slope angle. The total area is 2 times the ridge length times the sum of the lower and upper rafter lengths. Add the eave overhang along the lower slope.

The classic barn gambrel uses a 60-degree lower slope and a 25-degree upper slope. Dutch Colonial homes often use 65 lower and 30 upper. The traditional half-circle method sets the difference between the two angles at 45 degrees.

A gambrel roof is typically 10 to 30 percent larger than a gable roof on the same footprint, depending on the angles. The steep lower section adds substantial area that a single gable slope would not cover. The exact difference depends on the gambrel angles and the gable pitch.

The break point (also called the knee or curb) is where the steep lower slope meets the shallower upper slope. Its height above the eave determines how much vertical wall space is available in the attic. A break point at 8 feet or higher creates a full-height room.

Yes. Dormers are common on gambrel roofs, especially shed dormers on the lower slope to add headroom and light. Adding dormers increases the roof complexity and waste factor from 10% to 15% or more because of the valley and ridge intersections.

A gambrel has two slopes on two sides with vertical gable walls at each end. A mansard has two slopes on all four sides with no gable walls, like a four-sided version of the gambrel. Mansard roofs are more complex to frame and estimate.

The half-circle method inscribes the gambrel profile in a half-circle of radius equal to half the building width. This sets the lower angle minus the upper angle to exactly 45 degrees and produces a visually balanced curve. It is a design guide, not a structural requirement.

Add 10% for a simple gambrel without dormers. Add 15% for a gambrel with dormers or shed dormers. Add 20% for complex gambrels with multiple dormers, valleys or intersecting roofs.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• Omni Calculator, Gambrel Roof Calculator (primary: full gambrel formula set with trigonometric derivation).
• CD Calculators, Gambrel Roof Calculator (typical gambrel angle presets and design guidelines).
• IKO, Estimating How Much Roofing Is Required (rake-line method for roof area estimation).