Mezzanine Size Calculation

[AaronA]

Hello,

I am designing a home with a mezzanine level. The home will have sprinklers. I am trying to max out the size of the mezzanine and am told that I can build up to 50% of the room below. My question is how are the stairs treated to and from the mezzanine. Are stairs to the mezzanine counted toward the room below? Are they counted towards the mezzanine? What about the stairs to the floor above the mezzanine? Can you please let me know if there is a source I can site.

Thank you!

 

[Yankee Chronicler]

The 2021 IRC definition of "Mezzanine" is:

MEZZANINE. An intermediate level or levels between
the floor and ceiling of any story.

The first question, then, is whether your mezzanine is, in fact, a mezzanine? Is it actually an intermediate level between the floor and ceiling of a story?

Then there's the issue of the allowable size. Mezzanines are addressed in section R325 of the IRC. The starting point is that the area is limited to 1/3 of the area of the room or space in which the mezzanine is located. This increases to 1/2 with sprinklers as an exception ... but only if all of the following criteria are met:



Without seeing the plans, I don't care to offer an opinion. My concerns:

• Is this mezzanine actually "in" a larger room or space?
• Is the mezzanine open to the room or space "in" which it is located?
• Are you relying on any of the exceptions to R325.5?

 

[AaronA]

I can confirm that it is a mezzanine. I have been in communication with the city planning and building departments. Again, just trying to see what the max size can be.

 

[Joe.B]

I'll take a stab at your specific question, but I don't have a reference to cite, so it's probably meaningless for you. And then I'll duck out of the way because someone here is sure to throw something at me.

I would count the area of the stairs as part of the floor area the stairs bring you up to. The stairs wouldn't be there if there was not a floor to go up to, that's my logic.

 

[Yankee Chronicler]

With sprinklers, the maximum size can be 50% of the area of the room in which the mezzanine is located, IF you meet all three conditions in the exception to R325.3.

Stairs to the story above make no difference to the allowable area for the mezzanine. Measure the size of the room in which the mezzanine is located, and divide by two.

 

[bill1952]

So the area of the stairs are included in the ground floor? And is the room or space in which it's located limited by same "open" restrictions?

 

[Yankee Chronicler]

The area of the room or space a mezzanine is located in is measured to the four walls (or three, five, or however many) that enclose the space.

Think of a mezzanine as being like a piece of furniture or equipment. You start with a room. That room has dimensions, from which you can calculate its area. Now you plug in a mezzanine. That mezzanine can be up to 1/3 (or 1/2 if you meet all the conditions for the exception) of the area of the room before you thought about a mezzanine.

What I often encounter in commercial plan reviews (and what I also used to find often when I was doing residential plan reviews in the past) is that many designers think they can treat any upper floor space as a mezzanine just by labeling it "mezzanine" on the plan -- even though it meets none of the criteria (or the definition) for a mezzanine.

So the area of the stairs are included in the ground floor?

Which stair? The OP asked about two stairs: the stair to the mezzanine, and the stair to the second story.