Circular & Curved Stairs & Railings: Guide to Building Code, Construction, & Safety Inspection
     

• Circular or Curved Stair Inspections, Defects, & Codes - Circular stair, railing, guardrail, landing, tread, and step specifications
  • Where & How to Measure Stair Tread Depth on Circular or Curved Stairways
  • Circular Stair Kit Choices and Tips
  • Advice on Choosing the Size or Diameter of Circular Stair Kits
  • Rules for Constructing Circular or Curved Stairways
  • What's the Difference Between Interior & Exterior Radius-Secured Curved Stairs?
  • Codes for Stair Balusters as Circular Stair Safety Enclosures / Barriers
• Questions & Answers about building or inspecting circular stairs, stair treads and railings
• References

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Readers of this material about circular stairs and stair kits should also review the building code and trip hazards discussed at Winder stairs - stairways that include a turn without a landing platform. See these detailed articles on specifications for proper dimensions for stairs, railings, platforms in our list of Related Topics near the top of this page.

The traditional circular stairway shown in our photo at left was observed in Barcelona, Spain.

This circular stairway accesses four floors in a building and uses stone treads that gave good traction except when they were wet during cleaning. The hand-rail was grasp-able and enclosed. Our opinion was that this was a good stair design.

As we elaborate at HANDRAILS & HANDRAILINGS, while a good handrail is important at every stairway, on curved or circular stairs where treads are not uniform in tread depth, railings are more likely to be grasped by stair users - and are perhaps even more critical.

Definition of circular or spiral stairs

1009.7 Circular stairways. Circular stairways shall have a minimum tread depth and a maximum riser height in accordance with Section 1009.3 and the smaller radius shall not be less than twice the width of the stairway. The minimum tread depth measured 12 inches (305 mm) from the narrower end of the tread shall not be less than 11 inches (279 mm). The minimum tread depth at the narrow end shall not be less than 10 inches (254 mm).

Our photo (left) illustrates a circular stairway installed outdoors in San Miguel de Allende, Guanajuato, Mexico. We had a welder double the number balusters to the as-built stair to correct an unsafe (too wide) spacing between those elements.

A benefit of that safety improvement was increased overall rigidity in the entire stair assembly.

A [spiral stair is a] stairway having a closed circular form in its plan view with uniform section-shaped treads attached to and radiating from a minimum-diameter supporting column.- IBC-1002 - [14a]

Our spiral stair photograph (left) illustrates this design in a building in Celaya, Guanajuato, Mexico.

1009.9 Spiral stairways. Spiral stairways are permitted to be used as a component in the means of egress only within dwelling units or from a space not more than 250 square feet (23 m²) in area and serving not more than five occupants, or from galleries, catwalks and gridirons in accordance with Section 1014.6.

A spiral stairway shall have a 7.5-inch (191 mm) minimum clear tread depth at a point 12 inches (305 mm) from the narrow edge. The risers shall be sufficient to provide a headroom of 78 inches (1981 mm) minimum, but riser height shall not be more than 9.5 inches (241 mm). The minimum stairway width shall be 26 inches (660 mm).

Not much, but as you can see in our two photographs, the support system for curved stairways must be considered.

Curved stairways may be self-supporting or the stair may be supported by securing a curved stair stringer on the inner or outer radius of the stairs directly to a building wall structure.

At far left the curved stairway is actually a self-supporting structure. A curved stair that is not self-supporting may be secured by connections between its outer radius and a curved building wall.

At right the stairs curving around the exterior of a heating oil storage tank are secured on the stair's inner radius by welding to the tank sides.

Circular stair kits such as the 1978 kit shown in our page top photo and in our sketch at left are a quick way to add a stairway between floors. that stair was produced by the Iron Shop, a Pennsylvania company in the U.S.

Companies offering stair kits offer an advantage to inexpert stair builders: the stair manufacturer knows a lot about stair codes, dimensions, trip hazards, and tips and tricks for installation.

Which way to face the circular stair: The sketches at left show the basic construction of a typical circular stair kit. The right-hand sketch explains a "right hand" circular stair layout. Notice that for most circular stairwells, the user enters and exits the stairway facing in the same direction.

This "direction facing" detail for circular stairs is important when determining where to place the stairwell, since you don't want your user to reach the top of the stairway and find herself facing a wall.

Install the widest possible circular stairway: When we installed the circular stair shown at page top in the 1970's we had intended to save floor space by ordering the smallest-diameter available circular stair that we could buy. The stair company's expert advised installing the largest circular stairway that would fit in our building. Our own calculations had failed to consider just how small the individual stair tread walking space would be.

After dividing the width of the stair opening in half (since treads have to be placed on either side of the center support post) and after subtracting the diameter of the supporting stairway post, our four-foot diameter circular stair kit was able to provide a triangular stair tread which was about 21" across in its long dimension (post to outer tread edge). The 21" stair tread gave us about ten inches of comfortable walking space near the outer third of the tread, and a tread depth between 12" and 7".

If we had opted for the smallest diameter stair, the comfortable walking area on the triangular treads would have been substantially reduced to just a few inches, increasing the trip hazard of the stairs.

[Our all-steel circular stair photo (left) shows an outdoor metal stairway located in San Miguel de Allende, Mexico after we installed an additional baluster between each of the more widely spaced components, making this stair more child-safe.]

Watch out when choosing the size of circular stairs or when building a structure or area that is accessed only by a circular stair. Ask how you will be able to move large furniture or other objects between floors.

Having inspected a few A-frame cabins whose upper floor was accessible only by a single circular stairway, we had learned that it is difficult to impossible to carry the box-spring for a bed up a circular stairway. (We once had to saw a box spring in half, fold it doubled, and then repair it when we got it upstairs.)

When we built a second floor addition on our laboratory building and knowing that the indoor stairwell would be a 4-foot diameter circular stair, we included a second means of entry to the upper floor: a large outdoor stairway and landing.

On our 1970's circular stair kit shown at page top and at left, the standard stair balusters were placed 11 3/4" apart - a child hazard by modern standards. None of our kids fell through the stairs, but we were worried about visitors. Our dog Katie did fall out of these balusters and to the floor below. Retrofit balusters or child guards would be a good safety improvement for this stairwell.

For details about balusters (vertical spindles in railing construction) see details at Railings for a discussion of safety barriers along stairs, and Guards (railings on landings and open hallways, porches, screened porches, balconies that are more than 30" above floors or grade).

As our photo shows, a toddler (in this case very carefully held and supervised) can easily fall through open landing balusters or open balusters on a stairway.

We often see guards and railings enclosed using horizontal members or mesh or link fencing materials. Because a toddler can easily climb these materials, they are not safe for guard or railing enclosures and should not be used. Examples of unsafe horizontal rail and guard enclosures can be seen at Balusters.

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