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Plumbing in the Home/Maximum Drainage Fixture Units


QUESTION: I want to add a bathroom upstairs and trying to figure out if I am within the maximum Drainage Fixture Units (DFU) allowed. I keep reading up on the subject and still confused so hoping you can shed some light.

I currently have 3 baths.  All 3 baths are on the 1st floor with a slab below. I am adding a forth bathroom upstairs. So from what I can find a bathroom group is 6 units so that is 24 units, plus washer 2 units, plus kitchen sink 2 units. It says Kitchen sink with dishwasher, but it is not clear to me if the dishwasher is part of the sink or separate. But with that I have 28 units. All will be run into a 3 waste stack. Now this is where it gets confusing.

I am looking at the Florida Plumbing codes.  I see one table that shows if I have a slope I can do 42 units total for the building and another that talks about 20 units off a single branch.
Is each bathroom unit considered its own branch??

The drain pipe is under concrete so I cannot be sure of this but a 4" pipe comes to the house is reduced to 3" (that I can see) which feeds the 1st bath and then continues to the other baths. Is this pipe considered the trunk?? Then it is Tee'd off to the 2nd and 3rd bath. Are these branches. I plan to tee off the 2nd bath pipe running horizontally and then vertically to the 2nd floor 4th bath and then take the stack through the roof.

I might be wrong but from what I read if I had a trunk that then ran vertically through the roof (the stack) I could have branches off that stack of 48 DFUS feeding into that trunk, but no single branch could have more than 20 DFUs

But with 3 baths on the 1st floor and then one on the second feeding into the vertical stack I am confused to how it counts.

I am looking for help in understand what exactly defines a branch and the Maximum DFU and if I am OK or have a problem.

Also a 2nd question. In order to run the 3 upstairs I will have to run it through the attic of the 1st floor and also in the floor joist above the garage. I am in Northern Florida but it can still freeze here. Is there an issue with doing that? I would have the insulation placed above the pipe in the attic and below the pipe in the garage.

ANSWER: Hi Robert,
What you are contemplating doing really shouldn't be a problem. I think you're still within the capacity of your main sewer drain. Also, it is unlikely that all fixtures will be discharging their full volume at the same time. The codes contemplate "worst-case scenario" for design purposes and safety margins.

As far as freeze protection goes, the only thing you really need to worry about being exposed to freezing conditions would be P-traps since those actually have standing water in them. The rest of the system only has water when fixtures are draining and should never have standing water at any time so freezing is not likely to be a problem.

I have seen frozen solid sewer lines but only when there was a blockage and there was water standing in the line. Not fun to try to clear either. Can you say "turd popsicle? LOL

Good luck with your project.

---------- FOLLOW-UP ----------

QUESTION: When I look at the code it says no more than 20 DFUs per branch interval and the definition of a branch interval is a vertical measure of 8' or more. So another way I look at this is the entire 1st floor is one branch and the 2nd floor another branch. If that is the case my 1st floor would be over its DFUs even before adding the 2nd floor bath. But the 2nd floor bath would be its own branch interval so it would not add to the 1st floor branch interval. The whole think is still very confusing.

Can you explain branch interval and is the entire 1st floor considered a branch interval.  If so would I not be over the 20 DFUs.  Of is each bathroom its own branch interval and the main waste line that feeds those branches not considered a branch?

The key terminology is "vertical" here. There is effectively no vertical component of any significance on the ground floor of your house. The drainage from the new upstairs bath would fall under this requirement.

The term "branch interval" means a vertical length of stack of not less than 8 feet, within which branches are connected. The total fixture units on all branches connected to a stack within any 8-foot length should not exceed the maximum permitted by the table in one branch interval.

The only issue is that you need to have a 4" main drain going out for 4 toilets. You also need an aggregate of 4" in venting.  Same amount of venting in sq inches as there is drainage in sq inches.
This does not need to be all in one vent, just the aggregate of all vents through the roof.

Formula for circle areas is Pi x R squared or Pi x Dia if you forgot this from school. LOL  
4" pipe is 3.14 x 2 x 2 = 12.56 sq in.

Yes, the codes can be very confusing.

Here is a good reference site.

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Dana Bostick


Pretty much any residential plumbing questions. For "item specific" details such as a specific model of fixture, I will need to research and there may not be any useful information available. Note: I live and work in Southern California. We do not, as a rule, use hot water or steam heating systems, oil fired boilers or private water wells so my knowledge in those areas is pretty limited. There are others here on AllExerts that can probably answer those questions better.


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