As you search the web to find the perfect cupola for your garage, shed or barn, you’ll see that many retailers use the same formula to help you find the correct size. It’s easy to remember: for each foot of length of the main ridge of your building, a cupola should be at least one inch wide at its base. The base dimension is usually how cupolas are sized and sold. So, using the formula, a garage with a 24’ ridge should have a cupola that’s 24” wide, or larger, at its base. Easy to remember, right? The problem is that the formula is often wrong. It often produces a base dimension for a cupola that’s visually too small for the building that it’s mounted on.

Building guide books from the 1950s included that formula. The 1950s were when most towns in the US adopted building and zoning codes. The spirit of the times meant that everything had to be regimented. Codes didn’t just include safety requirements. The size, property line set-backs and, in particular, the heights of buildings were regulated. It’s logical that a rule for cupola sizes would be a product of the 50s.

The most popular new home style of the 1950s was the Ranch House. Low, wide, sprawling, suburban Ranch Houses were idealized as the perfect American homes on most TV sit-coms of the day. 

Those build-low zoning codes and the popularity of the Ranch House style meant that most new garages in the 50s were substantially lower in overall height than their predecessors. Attached garages on two-floor, “modified” Ranch homes had to have low pitch roofs to allow space for windows on the adjacent house walls. They were lower still.

Since Ranch Houses had some affinity, at least in name, to some rural structures “out-west” somewhere, many were built with cupolas on their attached garages. Cupolas were, and still are, a symbol of country life, so they were perfect for suburban developments where city families were buying and building new homes in what they thought of as the country. Small cupolas looked fine on the low roofs that were the rule back then.

The one inch to one foot of ridge formula works for sheds and small, low pitch garage roofs. But, we’re building differently now. Homes and their garages are often bigger and have steeper roof pitches. Away from suburbia, country garages and barns are even bigger. Traditional barns and carriage houses had lofts and steep roofs. Today, many people are building garages that look like barns and carriage houses because they are appropriate accessory buildings for older homes and for new homes in traditional styles. However, on larger buildings with steep roof pitches, a cupola is more distant from eye level and looks smaller. A proportionately larger cupola would look better.

Ventilation is essential to keeping animals healthy and hay and grain free from mildew. Old farmers used huge cupolas to keep their barns cool and dry.  Cupola sizes were determined by the volume of the space that had to be ventilated. Most old New England farm barns have cupolas that are at least half-again larger than the one inch to one foot rule would suggest. And, on some of the largest barns, old cupolas break the rule by a factor of two. 

To emulate the appearance of cupolas on old barns and carriage houses, it helps to use a formula that factors in the overall size of and height of a building instead of just its length.  Here’s a formula that works fairly well:

Height of Ridge (in feet) + Length of Ridge (in feet) X .7 = Base of Cupola (in inches)

Take a look at a few examples of how the formula works.

For a 12’ wide x 14’ long, Cape Cod style shed with no roof overhang, 7’ side walls and 8/12 roof pitch:

11’ Ridge Height + 14’ Ridge Length X .7 = 17.5   Use a cupola with an 18” or larger base.

For a 36’ wide x 36’ long horse barn with 12” roof overhangs, 12’ side walls and 12/12 roof pitch:

30’ Ridge Height + 38’ Ridge Length X .7 = 47.6   Use a cupola with a 48” or larger base.

And, how about a cupola for the attached garage of that classic 1956 Ranch house that you’re restoring?

For a 20’ wide x 20’ long garage with no roof overhangs, 8’ side walls and a 6/12 roof pitch:

13’ Ridge Height + 20’ Ridge Length X .7 = 23.1   Use a cupola with a 24” or larger base.

You’ll notice that all cupola sizes are rounded up to the next size. Cupolas are usually sold in sizes with 2”, 4” or 6” increments. Find the cupola style that you like, use the formula above to find the approximate size for your building and then order the next size larger. You won’t go wrong ordering a larger cupola than you need.

Yes, the formula isn’t as elegant or as easy to remember as the 1’ of Ridge = 1” of Cupola rule. But, it works. Cupolas are expensive. Don’t be disappointed with yours after it’s installed on your roof. Use the formula above and it will look great.

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Architect Don Berg' s book American Country Building Design is available from Amazon.com and other book sellers.