When necessary spiral stair codes limit dimensions allowed in a spiral staircase installation. Spiral stair code requirements fit a spiral’s unique geometrical requirements. That is the codes allow the spiral staircase to be steep enough to provide adequate headroom under the landing. At the same time these rules specify that the treads have enough width and depth to make a safe walking path.
To more fully understand the terms used below, it may be helpful to view the post called “Explanations of spiral stair parts and planning terms”.
Spiral stair codes require the following:
- Stair Rise – The stair rise relates to how steep the stairs will be. It’s the vertical distance from one step up to the next. All stairs have a maximum rise so that they are not uncomfortably steep. Codes allow rises of 7-3/4″ or so* for straight stairs. Spiral stair codes allow a maximum rise of 9-1/2″. The codes allow extra height for spiral stairs so that as you go down the spiral it is steep enough to allow headroom when you pass under the landing or the treads above. *Different codes allow slightly higher or lower rises for straight stairs.
- Headroom – Head clearance is the vertical distance from a spiral surface defined by the tread nosings* up to the bottom of any obstruction directly above – the top landing or another tread. Codes limit head clearance to no less than 78 inches or 6-1/2 feet. *For a more detailed explanation see the headroom discussions in the post called “Explanations of spiral stair parts and planning terms”.
Spiral stair codes dictate special tread dimensions
- Tread Depth – If treads had too little depth, they might be more narrow than your feet are long. It would be hard to descend such a spiral staircase. You would want to go down backwards as if you were climbing down a ladder and it could be dangerous. Treads are triangular — they have less depth near the center column and more out at the handrail. Spiral stair codes dictate that the tread has to be at least 7-1/2″ deep 12″ out from the narrowest point. The strictest definition of this rule uses the face of the center column as the narrowest point of the tread.
- Tread Width– Spiral staircases often make efficient use of smaller spaces. But there is a limit on how small the treads can be. The codes require tread width to be at least 26″. That means the spiral stair must provide 26″ from the spiral railing to the face of the center column.
- Baluster Spacing – Balusters run from the tread up to the handrail. A code approved spiral stair has balusters close enough together so small children cannot fit their heads between them. The code states that balusters must be spaced close enough so that a 4″ diameter ball can’t pass between them. Since each tread will have a set number of balusters each (2, 3, 4…) the space between them is usually much smaller.
- Opening Between Treads – This refers to the amount of open space between one tread and the next. It must be no more than 4 inches for the same reasons that dictate baluster spacing. Your spiral stair manufacturer should provide a means for your spiral to meet this rule.