After seeing the popularity of my post on Residential Stair Design, I thought it made sense to explain the guidelines from the 2006 International Building Code for commercial buildings and common stairs in buildings with more than two dwelling units. It is much more complex and there are some exceptions if you need them so please contact me if you need help. These are the general rules.
Stairways have to be at least 44″ if you have an occupant load over 50 (and may be more if your occupant load is large). If under 50 occupants you can go to 36″ width. There are other exceptions for spiral stairs, aisle stairs and incline stairs but these types will have to be covered in a later post.
You need at least 80″ of head height measured off the tread nosings and it needs to be maintained for one additional riser depth at the bottom of the stairs.
The riser height must be between 4″ and 7″ and the treads must be at least 11″ deep.
If you have winders they need to be at least 11″ deep when you’re 12″ in from the narrow edge and the minimum depth must be 10″. You can’t use winders in required stairways unless they are within a dwelling unit.
The landings must be at least as deep as the stairs are wide (up to 4′-0″) and you can’t go up more than 12′-0″ vertically without a landing. Where a door opens into the landing it can’t project more than 7″ into the required depth of the landing.
Handrails must be mounted between 34″ and 38″ above the tread nosings and landings. Where they are circular they need to be 1.25″ to 2″ in diameter. If the handrail isn’t circular it needs to have a perimeter between 4″ and 6.25″ with a maximum cross section of 2.25″. The handrail needs to be mounted a minimum of 1.5″ off the wall.
Handrails in IBC buildings need to return to a wall, a guard or the walking surface or continue to another handrail. The IBC requires that your handrail extend 12″ past the top riser and one tread depth past the bottom riser. It important to note that the ADA will require you to go one tread depth plus 12″ past the bottom riser so it generally overrides the IBC.
Your handrails need to be on both sides of the stair and can’t project more than 4.5″ into the stair width on each side. On a very wide stair you need to space intermediate handrails no more than 60″ apart.
Most commercial buildings will also require the ramp design guidelines.