Alternating Tread Devices: A Space-Saving Solution

Alternating tread devices offer a unique and compact alternative to traditional staircases, especially when space is limited. They feature a zig-zag design where the steps are offset, allowing for a steeper and more vertical climb.

Understanding Alternating Tread Devices

Imagine a ladder that’s been transformed into a staircase. That’s essentially what an alternating tread device is. You step on alternating treads as you climb, so you don’t have both feet on the same level at the same time. This design allows for a smaller footprint while still providing access to different levels.

Important Considerations:

  • Not for Main Exits: Alternating tread devices are generally not permitted as the primary means of egress (exit) from a building or space. This is because their design can be less intuitive and potentially more challenging for some people to navigate, especially in emergencies.
  • Alternative Access: They are allowed when a regular staircase or ramp serves the same space on each level, providing an alternative access option.
  • Limited Use: Alternating tread devices can also be used where a means of egress is not required, such as for access to a loft, attic, or storage area.
  • Exceptions: In some cases, alternating tread devices may be allowed as part of a means of egress for small spaces like lofts or mezzanines, as long as they don’t provide the only access to essential areas like kitchens or bathrooms.

Tread and Handrail Requirements

Alternating tread devices have specific requirements for tread dimensions and handrail configuration to ensure safety and usability.

Tread Dimensions:

  • Minimum Tread Depth: Each tread should have a minimum depth of 5 inches.
  • Minimum Projected Tread Depth: The tread should project out horizontally by at least 8 ½ inches.
  • Minimum Tread Width: The width of each tread should be at least 7 inches.
  • Maximum Riser Height: The risers should be no more than 9 ½ inches high.
  • Angle of Ascent: The angle of the staircase, measured from horizontal, should be between 50 and 70 degrees.
  • Tread Elevation: The first and last treads of the device should be level with the floor or landing surface.

Handrail Requirements:

  • Handrails on Both Sides: Alternating tread devices must have handrails on both sides to provide support and stability.
  • Handrail Height: The handrails should be uniform in height and between 30 and 34 inches high.
  • Other Requirements: Handrails must also meet the requirements for continuity and grip size discussed in the previous section on handrails.

By understanding these specific requirements for alternating tread devices, you can ensure your space-saving staircase is both safe and code-compliant.