Circular stair, the Iron Shop, Broomall PA, installed in NY 1978 (C) Daniel FriedmanCircular & Curved Stair & Railing Construction

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Circular or curved stairs:

This document assists in building or inspecting indoor or outdoor circular or curved stairs, railings, landings.

Circular stairs have been used for centuries to ascend or descend towers or to provide access between floors where there is not enough horizontal space for a conventional straight-run or single-turn stairway. But circular and curved stairs present some special trip and fall hazards that should be carefully addressed. The circular stairs shown at page top were installed by the author in 1978.

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Circular & Curved Stairway Tread Inspection, Building Code, & Trip Hazards

Circular stairway stone treads (C) Daniel Friedman

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.

Article Contents

Circular & Spiral Stair Codes vs Winder Stairs

What are the differences among circular stairs, spiral stairs & winder stairs?

Definition of circular or spiral stairs

Circular stairway, San Miguel de Allende, Mexico (C) Daniel Friedman 2013

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.

Definition & Properties of Spiral Stairs & Curved Stairs

Spiral stairway (C) Daniel Friedman Celaya Mexico 2012A [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.

[Click to enlarge any photo]

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).


Below: tread dimensions for winder stairs. [Click to enlarge any image]

Winder stair tread dimensions from 2006 IRC (C) adapted from Juneau AK code

Above; minimum tread dimensions for a Winder stairway, adapted from the 2006 IRC as published by Juneau AK IRC R311.5. Details are at WINDER or ANGLED STAIRS

What's the Difference Between Interior & Exterior Radius-Secured Curved Stairs?

Curved stairway (C) Daniel Friedman Curved stairway on oil tank exterior (C) Daniel Friedman

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 above 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 above 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 Kit Choices and Tips

Circular stair schematic (C) Daniel Friedman Circular stair layout details (C) Daniel Friedman

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.

Advice on Choosing the Size or Diameter of Circular Stair Kits

Circular stairway, San Miguel de Allende, Mexico (C) D Friedman

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.

The circular stair shown at left, located in San Miguel de Allende, Guanajuato, Mexico, had wide open vertical balusters. We welded on an additional vertical baluster in each opening of the stair guardrail as well as at the top patio guardrailing. The result not only reduced the child fall hazard it also stiffened the whole stair structure.

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.

Reader Question: what is the minimum allowable width for a circular stair?

2006 IRC Spiral Stair Code Illustration (C) adapted from Juneau AK publication3 September 2015 deb said:

Looking for current New York State code on the MINIMUM WIDTH of exterior spiral stairs. I have a very old entry to a 250 sq foot apartment (with another egress for furniture). It is definitely not to current code -- too narrow & steep. It is inside a porch enclosed by equally old lattice (not the nasty kind!!) The width is 5'5".

My architect says min. is 6', but I thought it was 5'2". Can someone clarify for me? I would like to do this before the snow flies outside Rochester, NY. And this 150-yr-old house deserves an entry that is not a cob-job!!

This question was originally posted at BUILDING CODES for STAIRS

Reply: IRC Stair Building Code Citation:

Deb: from my reading, the BOCA Code, UBC Code, IRC Code, and IFC codes all specify: [We use the terms circular stairway and spiral stairway synonymously and assume there is a center post - a feature not present on some custom circular or spiral stairways].

If your architect has found a different citation requiring 6-foot diameter circular stairs as a minimum, please ask her for it and share that with me - we need to know about it.

Or perhaps the architect's custom design uses a larger center column for the circular stair and thus needs a larger overall stair width.

Circular stair with large diameter center post (C) Daniel Friedman

To meet the minimum stair tread width the circular stair shown above, at the Nirvana Restaurant in Dolores Hidalgo, in Guanajuato, would have to increase in its total diameter thanks to that incredibly fat center wooden column.

[Click to enlarge any image]

My OPINION is that you should put in the largest round stair that you can fit in the space. I find that walking on triangular stair treads is inherently dangerous and the larger stair width means larger stair treads as well as a far easier time carrying something up the stairwell.

The narrow diameter stone stairwell shown below is in Goodrich Castle, Ross on Wye in Herefordshire, England in the U.K. I had a hell of a time walking up and down these stairs that are so narrow that there is no room for a handrailing. You can see that my feet don't fit on the stair treads.

Users can grab onto a vertically-hung knotted rope as they swing out into space while attempting to arrest a fall down these steps - I had to let go and totter on these steps while taking the photo. Imagine having to run up and down these stairs carrying bows and arrows, lances, or buckets of hot tar!

Narrow stone stairwell with knotted rope handrailing at Goodrich Castle, Ross on Wye, Hereford England UK (C) Daniel Friedman

Showing tread width measurement on a circular stair (C) Daniel Friedman

There are other circular stair tread and stair design guides:

And of course there are added requirements for the circular or spiral stair handrailing, guardrailing, and baluster spacing.

Thanks to Salter Industries for some of these details though they're also in the codes and at a plethora of other stair websites.

Rules for Constructing Curved Stairways

Angled or Curved stair treads are a particular trip hazard, especially because of the lack of uniformity and because the tread width at the inside of the curve can be too small for safe walking.

Curved stairway (C) Daniel Friedman Stairway Lighting requirements (C) Carson Dunlop Associates

Above we provide a photo and a sketch of a curved stairway with unsafe railing enclosures and of curved stair tread designs. The sketch of curved stair treads is courtesy of Carson Dunlop and shows the minimum tread width and radius for curving stair treads such as the stairs in our photograph.

Curved stair tread dimensions adapted from 2006 IRC as published by Juneau AK (C)


Where & How to Measure Stair Tread Depth on Circular or Curved Stairways

Our photos illustrates a sample of measurements of the tread on a conventional iron-framed circular stair (below left) and the width of the stair baluster opening (below right).

At about 12-inches from the center post the stair tread provides 9 1/2" of tread width. At 5-inches from the center post, the treads are just 6-inches wide and not safely walk-able. You can observe from the stair tread wear pattern that users step in a space between 8-inches and 16-inches away from the center post.

Circular stairway stone treads (C) Daniel Friedman Circular stairway stone treads (C) Daniel Friedman

If you (click to) enlarge the photos you can get an idea of the dimensions of this circular stairway and the spacing between its balusters (an un-safe 12-inches).

Codes for Stair Balusters as Circular Stair Safety Enclosures / Barriers & Handrails

Chloe Mei checking an open landing and stair baluster (C) Daniel Friedman

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.

Circular Stair Guardrail Baluster Spacing Code

Circular Stair Guardrailing at Stair Top or Landing Code for Height & Balusters

The guardrailing around the top of the spiral stairwell, referred to as a "balcony / well enclosure" shall also

For details about balusters (vertical spindles in railing construction) see details at RAILING CODES & SPECIFICATIONS for a discussion of safety barriers along stairs,

and GUARDRAILS on BALCONIES, DECKS, LANDINGS (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.

Circular Stair Handrailing Codes

Spiral stair no center post New York City (C) Daniel Friedman

The spiral stair shown above, in a retail store in New York City, has no center post and relies on the wall and perhaps a hidden inner spiraling stair carrier for its support.

Notice that there is both a guardrailing and a lower handrailing along the inside of the stairwell which is open, and there is another handrailing where you see the walker's legs - along the outer edge of this stairwell. Notice also that users tend naturally to walk along the outer portion of the stairway where the treads are widest.

Small turning stair with triangular treads and special center hand-hold (C) Daniel Friedman

The small-width turning stairwell shown above uses triangular treads in the turn-area that is so steep that the stair builder included a vertical hand-hold (at the right side of my photo). I like this idea because the hand-hold also literally blocks clumsy people like me from stepping on the narrowest portion of the stair treads where falls are more likely to occur.

Example of graspable and non-graspable hand railings along a curved stair, Poughkeepsie Galleria (C) Daniel Friedman

Above my daughter Michelle is showing where her sister should have been holding on when descending these curved stairs at the Poughkeepsie NY Galleria mall. Mara, tripping on these steps, tried to grasp the too-fat upper guardrail top and instead, fell. Later the builder added the lower and more graspable handrailing that Michelle is demonstrating.

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.

See the detailed model building code specifications for proper dimensions for circular, curved, and spiral stairs, railings, platforms at CIRCULAR CURVED & SPIRAL STAIR CODES - 2006 IRC & others


Continue reading at STAIRS, RAILINGS, LANDINGS, RAMPS - INSPECTIONS, CODES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see WINDER or ANGLED STAIRS - stairways that include a turn without a landing platform.

Suggested citation for this web page

CIRCULAR & CURVED STAIRS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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