Spiral staircase dimensions are set by building codes and must be followed for the staircase to meet the code. Each locality may have its own rules for spiral staircases, but many will follow the Universal Building Code or another widely accepted standard. Under these standards, the general dimensions required for a spiral staircase include the following:
- Stair Rise – The stair rise refers to how steep the stairs are. It’s measured by the vertical distance from one step up to the next. All stairs have a maximum rise so that they are not uncomfortably steep. For traditional stairs, this is a conservative 7.75 inches, but for spiral stairs it’s 9.5 inches. The reason that spiral stairs are allowed the extra rise is so that there is ample headroom when you pass under the landing or the treads above. This is why spiral staircases sometimes feel “steeper” than regular staircases. This is the spiral staircase dimension that is most likely to be forgotten by a building inspector, since it’s different from the stairs they see most often.
- Head Clearance – Head clearance is the vertical distance from the top of any tread to the bottom of the tread or landing directly above it. Head clearance can never be smaller than 78 inches or 6.5 feet. That means a person 6’5″ tall could stand on any tread and their hair would just brush the tread above them.
- Tread Width – If treads are too narrow, people feel off balance and the stairs would be dangerous. But treads are triangular, meaning that they’re narrower at one end and wider at the other. The general rule is that the narrow end has to be at least 7.5 inches wide one foot out from the narrowest point. In other words, the first twelve inches of tread can be narrower as long as the rest of the tread meets the requirements.
- Width of Passageway – Spiral staircases are often used to make efficient use of smaller spaces, but there is a limit on how small they can be. The “passageway width” has to be at least 26 inches wide. That means it is 26 inches from the railing on the outside to the central column on the inside.
- Baluster Spacing – Balusters are the vertical rails that run from the tread to the handrail. They must be placed close together so it’s not easy to fall between them. Generally, code requires that they are spaced so that a 4-inch object can’t pass between them. Spacing of up to 4 3/8 inches may be permitted.
- Opening Between Treads – This refers to the amount of open space between one tread and the next. It must be at least 4 inches to allow the foot to make use of open space beyond the tread itself (although some codes require more space than this).
If your spiral staircase must meet different requirements for a specific local code, or if you aren’t sure, talk to us at Precision Pine. We will be gladly check with your local building inspection office for you and make the stairs to the correct dimensions. Call us today at 877-885-8902!