Feb 20

How To Determine the Minimum Stair Width on the Stair Design Vignette

On the stair design vignette, understanding how to determine the minimum stair width is essential in completing your final design.

Like most ARE vignettes, an important factor in scoring your vignette is the EFFICIENCY of your solution.

Most vignettes can be solved in multiple ways so the key to determining a successful solution is largely based on the efficiency of the solution. So basically, you need to make sure that your design is as efficient as possible while still meeting all of the code requirements.

In the stair design vignette, it would be easy for any candidate to draw stairs wider than they need to be but you have to remember that this is also extremely wasteful.

The challenge in this vignette will be to determine the MINIMUM stair width required by Code and design your solution to that. This is the only way to ensure that you’re designing the most EFFICIENT solution possible.

Stair Rules

Before we get into the factors for determining your minimum stair width, it’s important to understand some general rules about the stair design vignette.

For starters, you CAN have multiple stair widths in your design. If you do though, you have to make sure that your stair width DOES NOT decrease in width in the direction of travel (per the Program and Code).

This means that if you’re allowed by Program and Code, you could theoretically have a smaller stair width at the top of your stairs and then have a larger width at the bottom. Again, this is assuming that the Program allows for it when determining the stair width factors so make sure you only do this if allowed!

Multiple Factors

In order to determine the minimum width of your stair, there are 3 different factors that you have to look at. In the end, you will need to figure out which factor yields the LARGEST minimum width and use that in your design.

Fortunately, all of the information you need is given to you in the Program and the Code so it’s your job to find the correct information and properly interpret it.

Some of the information is directly given to you in the Code while other factors require you to do some easy calculations. Either way, all of the information you need to determine the stair width is already given to you. I would certainly recommend taking time BEFORE you start drawing to determine this.

Let’s take a look at the 3 different factors you need to look at when figuring out the minimum stair width:

1) Code Required Minimum

The first factor is the most straightforward and it’s typically given to you directly in the Code under the “Stairways” section.

For example, the NCARB Practice Vignette states “The minimum width shall be computer in accordance with the Capacity of Exit Components, above, but shall not be less than 44 inches

This 44″ INCLUDES the handrails so it’s important to understand that the dimension is meant to be from stair edge to stair edge. (We’ll in the next step that the number given in the Program is between handrails so it’s important to understand the difference).

2) Area of Refuge Requirement

The requirement regarding the Area of Refuge is also given in the Program, but there are some differences that you need to be aware of.

For starters, you must note and understand exactly where your Area of Refuge needs to be provided. In the case of the NCARB Practice Vignette, the Code states “An Accessible area of refuge serving the second floor shall be provided within the stair enclosure” . There are some cases where the Area of Refuge occurs at a different floor so pay attention.

In that same area of the Code, you need to determine the minimum width at the Area of Refuge. For example, the NCARB Practice Vignette states “When areas of refuge are required, stairway width shall have a minimum clear width of 48 inches between handrails”.

They key to that statement is “BETWEEN HANDRAILS” because this is different than the first requirement we looked at which included the handrails.

In this case, your minimum stair width needs to be 48″ + 4″ on each side for handrails (so 8″) = 56″ MINIMUM WIDTH.

So far, the 56″ width in Factor #2 would govern over the 44″ width in Factor #1 because it’s more restrictive.

3) Capacity of Exiting Components

The final factor that you need to look at when trying to determine the minimum stair width is based on the capacity of exiting components.

In the Code, you are provided with the “Total Occupant Load” and the “Number of Exits” AT EACH FLOOR.

To determine the minimum stair width, you need to divide the Total Occupant Load by the number of exits at each floor level. Then take that number and multiply it by a width factor given to you in the Code.

For example, the NCARB Practice Vignette states that a width factor of 0.3 must be applied to each floor. So in the case of the ground floor, you would do the following calculation to determine the width of the stair:

(360/3)(0.3) = 36″

Remember, you have to calculate the minimum at each floor level, so in this case you’re doing 3 separate calculations. The higher of the 3 calculations will then be compared against the widths that you figured out in the first 2 factors discussed above.

It’s important to remember that depending on the level in the building, the stair width requirement may change so please pay attention. If this is the case, you need to make sure that your width does NOT DECREASE in the direction of travel.

Which One To Use

Now that you’ve figured out the 3 different factors used to determine the minimum stair width, you have to choose the LARGEST of the 3 numbers.

To review:

• Factor 1 (Code Requirement) – 44″ stair width
• Factor 2 (Area of Refuge)56″ stair width
• Factor 3 (Exit Capacity) – 36″ stair width

So, your MINIMUM STAIR WIDTH would be 56″ wide.

Since the area of refuge determined your stair width and is located on the second floor, you don’t need to worry about your stair increasing in width on the way down. It can stay at 56″ the entire run.

Now say that in Factor #3 you determined a wider stair width at the intermediate landing, then you would have to go with 56″ at the top but then increase your minimum width when you get to that level. This is a rare occurrence but could happen so it’s important that you understand how this works.

Final Thoughts

I can’t stress enough the importance of determining your minimum stair width BEFORE you start designing your solution.

You don’t want to get too far into your design before realizing that you’re using the wrong width so I recommend taking your time up front to figure it out. Keep in mind that the factors you see on your actual exam will vary from the examples in the NCARB Practice Vignette so make sure you check everything carefully.

Remember, you need to look at all 3 determining factors and choose the LARGEST one.

If you’d like a closer look at some tips and tricks of the Stair Design Vignette, please see the Building Design & Construction Vignette Guide, part of the ARE Advisor Vignette Series.

This guide will give you a step-by-step breakdown of the BDCS Vignettes (Ramp/Accessibility, Stair Design and Roof Plan) and what you can expect. The guide uses the examples provided by NCARB so you can follow along using the NCARB practice software.

Since a fatal error on a vignette will cause you to fail an entire ARE exam division, I certainly recommend taking your time to understand exactly what you need to do to pass the vignette section. In my opinion, the vignette sections of each exam are the most important parts since one mistake can cost you the entire section.