Home›Calculators›Construction›Gambrel Roof Calculator

Gambrel Roof Calculator

Construction

Calculate total roof height and upper and lower rafter lengths for a gambrel (barn-style) roof from building width and each slope's pitch instantly.

Reviewed by the thecalcu.com team · Last updated July 27, 2026

880
0.20.5

Total Roof Height (ft)

32.7
Lower Rafter Length (ft)
21.416
Upper Rafter Length (ft)
14.062

This calculator computes your Total Roof Height (ft), Lower Rafter Length (ft), Upper Rafter Length (ft) from the values you enter.

Inputs
Building WidthLower Slope Pitch (rise per 12 in run)Upper Slope Pitch (rise per 12 in run)Lower Slope Run Fraction
Outputs
Total Roof Height (ft)Lower Rafter Length (ft)Upper Rafter Length (ft)

What is a Gambrel Roof?

A Gambrel Roof Calculator determines the total roof height and the rafter lengths for both the steep lower slope and shallower upper slope of a barn-style gambrel roof, based on your building's width and each slope's pitch. Unlike a simple gable roof with one continuous pitch, a gambrel roof's two-slope geometry requires calculating each segment separately before summing them for total height.

This distinctive roof shape is common on barns, storage sheds, and some house styles specifically because it maximizes usable interior loft space for a given building footprint. Pair this calculator with the Birdsmouth Cut Calculator for the rafter-to-wall notch dimensions, or the Metal Roof Cost Calculator if you're budgeting roofing material.

Why Use a Gambrel Roof Calculator?

Manually calculating a two-slope roof's dimensions requires breaking the geometry into two separate right-triangle calculations and summing the results, a multi-step process that's easy to make errors in without a dedicated tool. This calculator handles both slope segments together, giving you total height and both rafter lengths in one calculation.

It's also useful for comparing how the run fraction (how much width the steep lower slope covers versus the shallow upper slope) affects your final roof height and rafter lengths, helping you fine-tune the barn's proportions before framing.

Who Should Use This Calculator?

Barn builders and pole barn contractors use this calculator to determine roof framing dimensions before cutting rafter stock. DIY shed builders planning a gambrel-roof storage shed use it to maximize interior loft headroom for their design. Timber framers working on traditional barn restoration or new-build projects use similar two-slope geometry calculations. Architects designing barn-style residential additions use it during early design to establish roof proportions. Agricultural building suppliers reference gambrel roof geometry when providing framing packages for barn kits.

What Insights Does the Gambrel Roof Calculator Give You?

The Total Roof Height is the primary result, the combined vertical rise of both roof slopes from the wall top plate to the ridge, useful for confirming your structure fits within height restrictions or matches your design intent. The Lower Rafter Length and Upper Rafter Length results give you the minimum stock length needed for each rafter segment, before accounting for any birdsmouth notch or overhang. Comparing results across different run fractions shows how shifting more roof width to the steep lower slope increases interior loft headroom at the cost of a taller overall structure.

How to use this Gambrel Roof calculator

  1. Enter your Building Width in feet.
  2. Select the Lower Slope Pitch (rise per 12 in run) for the steep section near the eaves.
  3. Select the Upper Slope Pitch for the shallower section near the ridge.
  4. Adjust the Lower Slope Run Fraction to control how much of the building's half-width the steep lower slope covers.
  5. Review the Total Roof Height, Lower Rafter Length, and Upper Rafter Length results to plan your framing material and confirm the design fits your height requirements.
Show formula & methodology ↓Show less ↑

Formula & Methodology

The calculator splits the building's half-width between two slopes and computes each segment's rise and rafter length:

Half Width = Building Width ÷ 2

Lower Run = Half Width × Lower Run Fraction; Upper Run = Half Width − Lower Run

Lower Rise = Lower Run × (Lower Slope Pitch ÷ 12); Upper Rise = Upper Run × (Upper Slope Pitch ÷ 12)

Total Roof Height = Lower Rise + Upper Rise

Lower Rafter Length = √(Lower Run² + Lower Rise²); Upper Rafter Length = √(Upper Run² + Upper Rise²)

Worked example: For a 24 ft wide building with a 60/12 lower pitch, 18/12 upper pitch, and 0.35 run fraction:

Half Width = 24 ÷ 2 = 12 ft

Lower Run = 12 × 0.35 = 4.2 ft; Upper Run = 12 − 4.2 = 7.8 ft

Lower Rise = 4.2 × (60 ÷ 12) = 4.2 × 5 = 21 ft; Upper Rise = 7.8 × (18 ÷ 12) = 7.8 × 1.5 = 11.7 ft

Total Roof Height = 21 + 11.7 = 32.7 ft

Lower Rafter Length = √(4.2² + 21²) ≈ √(17.64 + 441) ≈ 21.4 ft

Upper Rafter Length = √(7.8² + 11.7²) ≈ √(60.84 + 136.89) ≈ 14.1 ft

Frequently Asked Questions

What is a gambrel roof?
A gambrel roof, also known as a barn-style or Dutch barn roof, has two slopes on each side, a steep lower slope near the eaves and a shallower upper slope near the ridge, which maximizes interior loft or attic space compared to a simple gable roof. It's the classic shape seen on traditional American barns and many storage sheds.
How do I determine the run fraction for a gambrel roof?
The run fraction determines how much of the building's half-width is covered by the steeper lower slope versus the shallower upper slope. A larger fraction (like 0.4-0.5) shifts more of the roof width to the steep lower slope, maximizing interior headroom near the walls, while a smaller fraction shifts more width to the flatter upper slope.
Why does a gambrel roof have two different pitches?
The steep lower slope sheds water and snow efficiently while creating vertical wall space that maximizes usable interior loft area, and the shallower upper slope reduces overall roof height and material compared to continuing the steep pitch all the way to the ridge. This two-pitch design is what gives barns their distinctive silhouette and generous hayloft space.
What pitch is typical for the lower slope of a gambrel roof?
Lower slopes on gambrel roofs are often quite steep, commonly in the range of 45/12 to 72/12 (well beyond typical house roof pitches), since the steepness is what creates the vertical interior wall space characteristic of barn lofts.
How does building width affect total roof height?
A wider building requires more horizontal run for both slopes to cover, which directly increases the rise (and therefore total roof height) for any given pitch, doubling the building width roughly doubles the total roof height for the same pitch and run fraction.
Can I use this calculator for a shed with a gambrel roof?
Yes, gambrel roofs are popular for storage sheds specifically because the barn shape maximizes interior storage headroom for a given footprint, this calculator works the same way regardless of whether you're framing a full barn or a smaller shed.
How do rafter lengths from this calculator help with material ordering?
The Lower Rafter Length and Upper Rafter Length results tell you the minimum stock length needed for each rafter type before accounting for any birdsmouth notch or overhang, see the [Birdsmouth Cut Calculator](/birdsmouth-cut-calculator/) for the wall connection notch dimensions.
Does this calculator account for roof overhang?
No, this calculator computes the core rafter lengths based on the roof's structural geometry; add extra length separately for any eave overhang beyond the wall line, similar to how the [Rafter Length Calculator](/rafter-length-calculator/) handles overhang for simple gable roofs.
Also known as
barn roof calculatorgambrel roof rafter calculatordutch barn roof calculatorgambrel roof pitch calculatorgambrel roof height calculator