IBC Stair Width Calculation

Table of Contents

Calculating Single-Story IBC Stairway Width

ARE YOU MEASURING STAIR EGRESS WIDTHS CORRECTLY?

Stair egress measurements are commonly misunderstood and can be complicated because it is determined by the building’s occupant load. Should stairs be measured between handrails since this is where occupants can walk, or is it the entire width of the treads? This article is a simplified version of the IBC codes and addresses common questions associated with stair egress width.

Egress Width Factors

How do I determine my “calculated stair egress width”?

IBC Section 1005.3.1 requires that “the capacity, in inches, of means of egress stairways shall be calculated by multiplying the occupant load served by such stairway by a means of egress capacity factor of 0.3 inch per occupant”.

Exceptions are if there is a compliant sprinkler system installed and equipped with an emergency voice/alarm system for all occupancies other than Group H (High Hazard) and I-2 (Health Care), multiply the occupant loads by 0.2 and 0.15 inch per occupant, respectively, not 0.3.

For stairs serving one floor, multiply the floor’s occupant load by 0.3. For stairs serving multiple floors, multiply the occupant load of the floor that serves the most people by 0.3. These calculations give you the required COMBINED width of all exit stairs serving that floor. However, the minimum clear width between handrails is 44″. If the stairway serves a floor with an occupant load of less than 50, the minimum clear width is reduced to 36″.

Multi-story stairs

Single-story stairs

Here’s where it gets tricky.

There is a minimum number of exits that is based on the occupant load per story:

Occupant Load Per Story	Minimum Number of Exits or Access to Exits from Story
1-500	2
501-1,000	3
More than 1,000	4

The combined width calculation performed above must be distributed between the minimum number of required exits while still meeting the minimum width requirement.

AND if one exit stairway is “lost” (i.e. occupants cannot access it due to a fire) the remaining stairway width(s) cannot be reduced to less than half the required combined width. This applies typically when there are only two stairways because if you lose one, there is only one stairway left. If you originally have three stairways and lose one, you still have two stairways left.

Summary: calculate the total width per floor you need by multiplying the occupant load by 0.3″ and in some cases 0.2″. Take the largest width that you calculate, and use it for the entire stairway system. Keep in mind, the minimum clear width is typically 44″, but it is sometimes reduced to 36″ for low-capacity floors. Then, figure out how many exits are required by referencing the chart above, and divide the required width between the required number of stairways. That’s it! Reference the IBC codes below to check your work.

Calculating Single-Story IBC Stairway Width

Scenario: We need exit stairs from the second story of a building. There is a sprinkler system, but the system was not installed per code requirements. The occupant loads are as follows:

Floor 1: 175 occupants
Floor 2: 300 occupants

The required total stairway width per floor is as follows:

Floor 1: Not applicable because this floor is ground level.
Floor 2: 300 occupants x 0.3” = 90”

We will need two stairways because we have an occupant load between 1 and 500 per story. The 90″ width can be divided between the two stairways. BUT in case a stairway is destroyed or inaccessible during an evacuation, the remaining stairway needs to be at least half of the required minimum (144″). We cannot have a stairway that is 44” wide and the other one is 46”. Why? If we lose the 46” stairway to a fire, we only have the 44” stairway which is less than half the required width.

Summary: We calculate the minimum required combined width by multiplying the occupant load of the floor by 0.3″, which is 90″. We need two stairways, so we can divide the 90″ between the two stairs. However, since we only have two stairways, each stairway must be at least half of the 90″.

Calculating Multi-Story Stairway Width

Below is a scenario in which we calculate the required width of multiple stairways that serve multiple floors.

Scenario: We need exit stairs for a building that has 3 floors that have varying occupant loads. There is a sprinkler system, but the system was not installed per code requirements. The occupant loads are; Floor 1: 40 occupants, Floor 2: 480 occupants and Floor 3: 110 occupants

The required total stairway width per floor is as follows:

Floor	Occupant Load	Required total stairway width
1st	40	40 occupants x 0.3” = ~~12”~~ 36″ *Minimum width for less than 50 occupants is 36”
2nd	480	480 occupants x 0.3” = 144”
3rd	110	110 occupants x 0.3” = ~~33”~~ 44″ *Minimum width for a floor serving more than 50 occupants is 44”

What do we do when each floor requires a different width? We use the largest minimum width because we don’t want to create a bottleneck effect where a wide stairway leads into a narrower stairway.

Therefore, we want to use the 144” measurement for the minimum combined stair width.

We will need two stairways because we have an occupant load between 1 and 500 per story. The 144″ width can be divided between the two stairways. BUT in case a stairway is destroyed or inaccessible during an evacuation, the remaining stairway needs to be at least half of the required minimum (144″). We cannot have a stairway that is 60” wide and the other one is 84”. Why? If we lose the 84” stairway to a fire, we only have the 60” stairway which is less than half the required width.

Summary: We calculate the minimum required combined width by multiplying the occupant load of each floor by 0.3″ and using the largest number, which is 144″. We need two stairways, so we can divide the 144″ between the two stairs. However, since we only have two stairways, each stairway must be at least half of the 144″.

Referenced IBC Code:

1005.3.1 Stairways The capacity, in inches, of means of egress stairways shall be calculated by multiplying the occupant load served by such stairways by a means of egress capacity factor of 0.3 inch per occupant. Where stairways serve more than one story, only the occupant load of each story considered individually shall be used in calculating the required capacity of the stairways serving that story.

Exceptions: For other than Group H and I-2 occupancies, the capacity, in inches, of means of egress stairways shall be calculated by multiplying the occupant load served by such stairways by a means of egress capacity factor of 0.2 inch per occupant in buildings equipped throughout with an automatic sprinkler system installed in accordance with Section 903.3.1.1 or 903.3.1.2 and an emergency voice/alarm communication system in accordance with Section 907.5.2.2.

1005.4 Continuity The maximum capacity required from any story of a building shall be maintained to the termination of the means of egress.

1005.5 Distribution of Minimum Width and Required Capacity Where more than one exit, or access to more than one exit, is required, the means of egress shall be configured such that the loss of any one exit, or access to one exit, shall not reduce the available capacity or width to less than 50 percent of the required capacity or width.

1011.2 Width & Capacity The required capacity of stairways shall be determined as specified in Section 1005.1, but the minimum width shall be not less than 44 inches. See Section 1009.3 for accessible means of egress stairways.

Exceptions: Stairways serving an occupant load of less than 50 shall have a width of not less than 36 inches.

1022.1 Exits Once a given level of exit protection is achieved, such level of protection shall not be reduced until arrival at the exit discharge.

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