Exit Stair Width

Egress Stair Width, exit stair width

The required width of an egress stair is governed by BC Section 1005 and BC Section 1009.1. The latter tends to be the more strict and therefore governing requirement. However, you still need to demonstrate that your stair complies with both BC Sections.

Building Code Section 1005 requirements

You must first calculate the occupant load of the space being served by the stair. Then the minimum width of the egress stair is calculated using the factor from Table 1005.1.

OCCUPANT LOAD X 0.3 = MINIMUM WIDTH OF EXIT STAIR
egress width table

TABLE 1005.1 Egress Width

Example: For a space that has an occupancy load of 100 people, 100x 0.3 = 30. Therefore, a minimum stair width of 30 inches is required.

Building Code Section 1009.1 requirements

Egress Stair Width, exit stair width

Egress Stair Width

Generally, this section requires a minimum stair width of 44 inches. So if you have an occupant load of less than 147 (44/0.3=147), BC Section 1009.1 will govern over BC Section 1005.1 when it comes to the width of the stair.

Exceptions to BC Section 1009.1:

A minimum stair width of 36 inches is also permitted according to BC Section 1009.1 in the following (most common) cases

  • the total cumulative occupant load of all stories is 50 or less
  • in R2 occupancies with no more than 30 occupants per floor and with a building height of 125 feet or less.
  • for more exceptions see BC Section 1009.1

An Exit Stair Width Mistake to Avoid

 

Egress Stair Bottleneck, Exit Stair Bottleneck

Egress Stair Bottleneck

Here are two excerpts from the NYC Building Code that you need to keep in mind for your overall design.

“The maximum capacity required from any story of a building shall be maintained to the termination of the means of egress.” – BC Section 1005.1

“Once a given level of exit protection is achieved, such level of protection shall not be reduced until arrival at the exit discharge.” – BC Section 1017.1

In other words, design your stair width for the floor with the highest occupant load or comply with requirements of BC Section 1009.1 if this is more strict and then don’t reduce the width of your stair until you reach the exit door.

So…

Here is an example of what NOT to do.

You provide more width then necessary in an upper floor and then reduce the width at a lower floor. Even if the reduced width still complies with requirements, this would not be okay. You can probably imagine why this would be a dangerous scenario, creating a bottle neck effect if an evacuation was required. Yikes!

 


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