Drain Pipe Sizea and Maximum Drainage Fixture Units.

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Rwbil

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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 someone can break it down for me in clear understandable terms.

I currently have 3 baths and I might be removing one of them, but for now let us assume I keep it. 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 2units. 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 see one table that shows if I have a ¼” slope I can do 42 units and another that talks about 20 units. I think it might be tied to the way the system is branched. On the table that says 20 DFU max., it states total discharge into one branch interval.

Is each bathroom unit considered its own branch??

The 3” pipe I am tying into is under concrete, so I have no way to tell how things were branched but I do know all 3 bathrooms feed into that 3” pipe as well as the kitchen sink, dishwasher and washing machine.

I will tie into that 3” line run a stack upstairs and through the roof for venting.

I am looking for help in understand 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.
 

BMWpowere36m3

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If I remember correctly a 3" waste line can support two bathrooms or toilets and you'll need a 4" waste line for the third toilet/bathroom. I'm sure someone with more knowledge will correct me if I'm wrong. I used this for calculating individual DFUs and this for maximum vertical/horizontal waste DFU capacity.

From the second link, you'll see it breaks downs maximum waste capacity into vertical stacks, horizontal branches and total maximum for building drain branches (i.e. where waste lines converge and leave thru the foundation or slab horizontally and head to either a sewer/septic/cesspool.

So for example a 3" waste line, maximum horizontal branch capacity is 20 (two toilets max) and vertical is 48. However the maximum building drain branch is 42.

Well, that's at least how I understand it. Confusing I know.
 

Tom Sawyer

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It all depends on your state and local code. Contact the local plumbing inspector.
 

hj

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Here, it makes no difference how many fixture units you have. You can only have a maximum of 3 toilets on a 3" line. As soon as you add a fourth toilet the pipe has to be increased to 4" at the connection of the fourth toilet.
 

Rwbil

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I am looking at the Florida Plumbing codes. I do not see anything about 2 closets limit off a 3" drain pipe. I will have to double check. I think I am confused when I look at those tables with terminology. So let me ask about that. All the piping is under concrete so I can not 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 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 horizontally feeding and then a vertical stack and branch I am confused to how it counts.

So when it says 20 total discharge into 1 branch interval, what exactly does that mean. 20 DFUs into the trunk or is each bathroom considered a branch.
 
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Rwbil

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Looking at the tables it looks like Florida just follows the IPC. Here is a drawing as best I can tell. I am clearly within the 42 DFUs for the building. The blue 3" line is the new planned line to bathroom group 4. I did not show it on the drawing but all the bathrooms and kitchen are vented separately through the roof.

But I am not clear on the 20 DFU branch maximum. Based on this drawing am I over that limit.

http://screencast.com/t/kbrRItwHzuu
 

Rwbil

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Maybe I am wrong but it just did not seem that simple to me. In doing my research I saw a diagram with a main drain line going up stairs. So it was it horizontal at the ground and then vertical and it had multiple branches off of it and it showed as long as none of these individual branches was not more than 20 DFUS it was OK, but if one branch was greater than 20 DFUs then you needed a 4" pipe for that branch and for the drain pipe it feed into. Or if you had 2 branches discharging into the main pipe at the same point and the total was greater than 20 DFus then you have an issue. Better yet here is a video I saw showing this.


I am discharging into the main drain line at different points, which is why I think the building 42 max number is the number I have to worry about, but obviously I am not confident in that, because 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.

Another reference I read said this was really about venting so if each branch is vented then there is not issue. The whole think is way more complicated than it should be.
 
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hj

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You are making this more difficult than necessary and also confusing the issue by talking about individual branches and their maximums. The ONLY thing that is important is "how many fixture units are flowing past any specific point in the main line?". It is a cumulative number, arrived at by adding EACH BRANCH to the previous total as you progress downstream. A "branch" is ANY pipe discharging into a 'main' pipe, regardless of whether this 'main pipe' is also a branch off another line. Think of a tree with a trunk and then branches off it and also branches off those branches.
 

Rwbil

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hj,

Maybe I am slow, but the terminology is still confusing. The reason I brought up branches is because the code does. Table 710.1-1 states 42 DFUs maximum but Table 7101.1-2 states 20 DFus per branch. So if that main horizontal pipe that feeds all the branches is not a branch I am fine, but if it is considered a branch then I am over the 20 DFUs maximum. So based on my drawing it your opinion am I over the maximum or not.
 
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