Gambrel Roof Calculator
Estimate gambrel roof for your project with our free calculator. Get accurate material quantities, costs, and specifications.
Reviewed by Abdullah, Technical Content Specialist
Formula
Rafter Length = Run / cos(atan(pitch/12))
A gambrel roof is calculated as two separate roof sections on each side. For each section, determine the horizontal run based on the break point percentage of the half-span. The rise equals run times pitch divided by 12. The rafter length equals run divided by cosine of the pitch angle. Total roof area is 2 times the sum of lower and upper rafter lengths times building length.
Worked Examples
Example 1: Standard Barn Gambrel
Problem:Calculate a gambrel roof for a 24 ft wide, 40 ft long barn with 24/12 lower pitch and 6/12 upper pitch, 50% break point.
Solution:Half-width = 12 ft\nLower run = 12 x 0.50 = 6 ft, Upper run = 6 ft\nLower rise = 6 x (24/12) = 12 ft\nUpper rise = 6 x (6/12) = 3 ft\nTotal rise = 15 ft\nLower rafter = 6/cos(63.4) = 13.42 ft\nUpper rafter = 6/cos(26.6) = 6.71 ft
Result:Total roof area = 1,610 sq ft, 16.10 squares, 49 bundles
Example 2: Small Storage Shed
Problem:Calculate for a 12 ft wide, 16 ft long shed with 18/12 lower and 4/12 upper pitch, 50% break.
Solution:Half-width = 6 ft, Lower run = 3 ft, Upper run = 3 ft\nLower rise = 3 x 1.5 = 4.5 ft\nUpper rise = 3 x 0.333 = 1 ft\nLower rafter = 3/cos(56.3) = 5.41 ft\nUpper rafter = 3/cos(18.4) = 3.16 ft
Result:Total roof area = 274 sq ft, 2.74 squares, 9 bundles
Frequently Asked Questions
What is a gambrel roof and how does it differ from a gable roof?
A gambrel roof has two different slopes on each side instead of the single uniform slope of a gable roof. The lower slope is steeper (typically 60-70 degrees) and the upper slope is shallower (typically 20-30 degrees). They meet at a break point partway up the roof. Gambrel roofs are commonly seen on barns, Dutch Colonial homes, and storage sheds. The design maximizes usable attic or loft space compared to a gable roof of similar width and provides a distinctive architectural appearance.
What are typical pitch ratios for a gambrel roof?
A traditional gambrel roof uses a lower slope of 18/12 to 24/12 (56 to 63 degrees) and an upper slope of 4/12 to 8/12 (18 to 34 degrees). The classic proportions place the break point at about 50% of the half-span. Some designs use a lower pitch as steep as 30/12 (68 degrees), which creates nearly vertical walls at the lower section and maximizes interior space. The upper pitch should be steep enough to shed water and snow effectively, with 6/12 being a common minimum for good drainage.
How do I calculate the break point on a gambrel roof?
The break point is where the lower steep slope transitions to the upper shallow slope. It is typically located at 40-60% of the half-span distance measured horizontally from the eave. For a 24-foot wide building, the half-span is 12 feet, so a 50% break point would be at 6 feet from the eave. Moving the break point outward (higher percentage) creates more loft space but requires a steeper lower slope. The break point height equals the lower run multiplied by the lower pitch divided by 12.
Is a gambrel roof stronger than a gable roof?
Gambrel roofs are generally less resistant to wind loads than gable roofs because the steep lower slope catches more wind. However, they handle snow loads well because the steep lower section sheds snow quickly. The structural design requires proper engineering at the break point connection, which is the weakest area of the gambrel. Using collar ties, knee walls, or engineered gusset plates at the break point is essential. In high-wind regions, gambrel roofs need additional bracing and stronger connections per local building code requirements.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy