Plumbing fixture calculations in a multi-story building: by building or by floor?

[Tim Mailloux]

As usual I am probably over thinking this, but for multi-story buildings I have started wondering if plumbing fixture calculations should be performed as one calculation based on the entire occupant load of the entire building, of multiple calculations based on the occupant load of each floor.

It is very likely in a multi-story building you could have a floor or two with a much higher occupant density that the other floors in the building, and unless you do fixture calculations floor by floor you may not provide enough fixtures on those denser floors. The plumbing code also allows you to go one floor up and one floor down to get to a required plumbing fixture. Again, unless you calculate floor by floor how can you prove that the floors below or above have excess fixture capacity to accommodate persons from another floor?

The down side to calculating floor by floor is that the math will end up with more required plumbing fixtures than calculating by the building. This is due to the rounding. When one building calculation is performed the sub-totals (a number with a decimal) are round up once to get the total. When done floor by floor, each floors sub-total gets rounded up adding a lot of additional fixtures in a multi-story building.

 

[Mr. Inspector]

By building. Restrooms are not required on every floor. Only one service sink required for whole building, not one in each floor.

If different tenants are in the same building you need to calculate per tenant area unless restrooms, drinking fountains and service sink are in a shared area.

 

[mtlogcabin]

2012 IBC
[A] 107.2.3 Means of egress.
The construction documents shall show in sufficient detail the location, construction, size and character of all portions of the means of egress including the path of the exit discharge to the public way in compliance with the provisions of this code. In other than occupancies in Groups R-2, R-3, and I-1, the construction documents shall designate the number of occupants to be accommodated on every floor, and in all rooms and spaces.

The simplest method is add up the occupant load that is going through each exterior exit door and you will have the total for the building for your plumbing fixtures. In mixed occupancy type building example a restaurant on the main floor and offices on the upper floors you would have to calculate the OL based on the occupancy of each floor.

 

[steveray]

The correct answer is...kind of both....

[P] 2902.3.2 Location of toilet facilities in occupancies
other than malls. In occupancies other than covered and
open mall buildings, the required public and employee toilet
facilities shall be located not more than one story above
or below the space required to be provided with toilet
facilities, and the path of travel to such facilities shall not
exceed a distance of 500 feet (152 m).

Calculation is by building OL but calculated placement is by floor within the parameters set in 2902.3.2...

 

[JPohling]

What code are you using?
I would never use the occupant loads used for exiting from the building code to determine the minimum plumbing fixtures.
Use the occupant loads required by the plumbing code.
For a shell B occupancy building you calculate the minimum required fixtures based upon the total building area and then distribute them to comply with the requirements regarding access to them.

 

[Tim Mailloux]

What code are you using?
I would never use the occupant loads used for exiting from the building code to determine the minimum plumbing fixtures.
Use the occupant loads required by the plumbing code.
For a shell B occupancy building you calculate the minimum required fixtures based upon the total building area and then distribute them to comply with the requirements regarding access to them.

Im in CT, this state has amended the plumbing code to base fixture counts on building occupants as calculated per chapter 10 of the IBC. The section of the IPC model code that allows the local BO to set occupant load to be used for plumbing fixture calculations has been deleted from our code. The only way to use a smaller occupant load than what is calculated per chapter 10 is to submit a code modification request to the State Building Official. In my experience we have only had success is getting a mod for this for K-12 Schools.

 

[mtlogcabin]

I would never use the occupant loads used for exiting from the building code to determine the minimum plumbing fixtures.
Use the occupant loads required by the plumbing code.

How do you determine the OL using the plumbing code when they do not provide a sq ft per occupant factor to use
The IPC requires you to use the IBC to determine OL

2018 IPC Table 403.1 footnote a
a. The fixtures shown are based on one fixture being the minimum required for the number of persons indicated or any fraction of the number of persons indicated. The number of occupants shall be determined by the International Building Code.

 

[JPohling]

Simple, I work in California where the CPC provides that information.

 

[steveray]

The one thing in the Cali code I see as being better than ours....

 

[mtlogcabin]

Simple, I work in California where the CPC provides that information.

So the CPC is based off of the UPC. Did the CPC amend (2015) Section 422.1 of the UPC telling you to use the adopted building code to determine OL and if so are the figures the CPC use different than the IBC for determining OL ?
Just curious how you do it in CA. We amend the numbers in the UPC table and even eliminate the laundry sink and drinking fountain requirements

 

[JPohling]

Here you go. not all of it but you get the picture. 1 per 200 SF for a B

 

[mtlogcabin]

Thanks for the info
I am really surprised by the numbers you use which would reduce the plumbing code fixture amounts. I would have thought CA would have gone the other way and required more fixtures. I do like the numbers you use for B occupancies and worship assembly areas

 

[JPohling]

These are also code minimums, so in some instances the clients understand that they would be better served to provide more if they choose too.
When we work outside CA typically doing B occupancies less than 50 occupants and we have to provide two single room restrooms we are always wondering.
We provide hi/lo fountains typically in CA, many times substitutions are allowed for bottle fillers or filtered water systems at break rooms, etc.

 

[steveray]

Tim I have seen several mods for fixture count, mainly commercial/ industrial plus the schools.....Maybe not so much on the office building stuff...

 

[Harrison Staab]

Piggy backing with my own puzzle:

A multi-family residential building with lots of common space on the ground floor and a roofdeck (5 stories up). Total fixture calc for the building requires 3 water closets each. Obviously needs to provide a pair of restrooms for the roofdeck occupants on the roofdeck level (or on the story below).

The first floor has 86% of the occupant load that makes up the overall fixture calc, so proportionately that's 2.59 required water closets. Or, if I just calculate the first floor distinctly, 2.37 water closets are required.

I had assumed I needed to provide 3 w/c (ea) serving the first floor and 1 (ea) w/c serving the roofdeck, resulting in 4 w/c (ea), above the 3 w/c required by the overall building fixture calculation. A colleague suggested that the intent of the code allows for proportional distribution, but does not mean this requires a total number of fixtures greater than required based on the total occupant load of the building. He suggested providing 2 w/c (ea) on the first floor and 1 w/c (ea) on the roofdeck is compliant.

So, hoping this all makes sense, do I need to provide 3 w/c (ea) on the first floor so that these spaces meet the location requirements of 2902.3.2 for the rounded up fixture requirement for that space? Or am I compliant with 2 w/c (ea) on the first floor distributing the total minimum fixture requirement for the building?