allowable number of fixtures on one stack?

kirkhallMay 30, 2012

I know very little about plumbing, and have learned a lot more through posts on this site. I have learned that there are limits to the number of fixtures/types of fixtures on any drain line/stack (I think).

I have an addition planned. In it, I would like to replace one bathroom with a toilet, sink, and shower/tub with a new hall bath with double sink vanity, shower/tub, and toilet, as well as add a master bath with double vanity, toilet and shower (only).

Is it possible to put all of this on the one stack? My house is a 2 story, on the main level there is a double sink vanity, toilet, tub/shower bathroom. The main house stack runs from near that toilet to near the upstairs toilet directly above.

I know I have black pipe for my stack (some sort of pvc type pipe, not cast iron). How do I find its size?

My house was built in 1982.

And, if I have to add a stack, do I only need to add one, or more than one?

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There are no limits to the actual number of or types of fixtures that may be connected to a stack.

Instead of limits to the number or types of fixtures the code has a table listing all the types of fixtures that may be connected to a DWV (drain, waste & vent system) and the table defines the minimum drain & trap size for the fixture and it defines a specific DFU (drainage fixture unit) load value for the fixture.

The code then has another table that lists all the listed pipe sizes and the maximum allowable load on each specific size of pipe for both vertical and horizontal application, expressed in dfu's.

By example, a lavatory bowl is listed as requiring a minimum 1-1/4" drain & trap and it is assigned a load value of 1 dfu.

A WC (water closet -commonly called a toilet) that has a flush of 1.6gallon/flush or less is listed as requiring a 3" drain and it has a load value of 3dfu's.

When designing the DWV system we begin by making a list of all the fixtures in the house and computing the total dfu load.

From that we can check the pipe size chart to determine the size of the house main drain and house sewer. We then lay out the branch lines from the main drain to each respective fixture or fixture group such as bathrooms, kitchens, laundry etc and we compute the dfu load for the branch. Here again, we check the pipe size table to determine the size of branch pipe required. The exception being that if any fixture in the group is required a line larger than the dfu load table requires, we must use the line size required by that fixture. An example,,

A tub/shwer combo is 2dfu's, the lavatory 1dfu and the WC is 3dfu's for a total of 6dfu's. The pipe size table says a 2" pipe may carry up to 6dfu's, however because of the size of solids in the discharge from a WC code requires a WC to have a 3" drain and we cannot reduce the size of a line in the direction of flow so we would be required to make a branch serving a WC with 3" pipe.

Next we have to consider the venting. A length of pipe running from a fixture trap to a vent is defined as a "Fixture arm" while any length of horizontal pipe from a vent downstream to the stack or main drain is defined as a "Vented Branch".

When attaching fixtures to a vertical stack we may not attach any fixture to the stack by means of a fixture arm if it is below a WC being served by the stack. by example, your stack is serving a WC on the second floor so you may not connect a fixture arm directly to the stack on the first floor. The option would then be to install a separate stack for each floor or you could install a vent between the fixture traps on the first floor and the point where the line ties into the stack. In this configuration, the section of pipe from the trap to the vent is a fixture arm, but the section from the vent to the stack is a vented branch, which may be attached to the stack.

    Bookmark   May 31, 2012 at 1:23AM
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Lazypup, I sure do appreciate your plumbing knowledge, and ability to mostly spell it out for a non-plumber.

That said, I did get a little lost right at the end. But, I think I now know why there are some "minor" pipes that run up from the first floor (I was pretty sure they were vent pipes, and you've confirmed--and now I really know why) to the stack.

Thankfully, I will not be doing this plumbing. I will leave that to the experts. I am just trying to figure out the best (read, least costly) configuration for the addition of all my fixtures.

If I have, upstairs, a total of 10 dfus on this one stack (and drain), with 5 dfus on this same stack downstairs, am I going to be okay? Or, should I plan on an additional drop from upstairs to downstairs?

The laundry will be moved floors, but it has its own drain line/vent which I figure could just be extended straight up and used (the vent goes straight up from the current washer drain and is separate from the main house plumbing stack. I was thinking that they could just tie the new washer drain into the current laundry vent line... (I guess I don't actually know if that is okay either; but if it was, that saves on plumbing there.)

The kitchen sink is the only other source of interior water/drainage and it has an AAV, which is allowed in Washington, though I don't actually know where it drains to (oh, and there is a DW attached to that).

I will post my layouts to GW soon for general comment, and will post them here as well to get a better sense of what I will need.

    Bookmark   May 31, 2012 at 10:46AM
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Here is my new attempt at a layout. I am getting feedback on the small homes forum now, and later will get some from the remodeling forum.

But, for plumbing, what are my big challenges here?

Stack is marked with "S" and will have to move laterally (within the joist space) a foot or 2.

I might need a second main drain line from this second floor, in which case it will go approx where the "S" is on the far left.

Joists run "N-S" Am I going to run into big trouble getting my drainlines to the stacks? Can my framers fix that by making a plumbing "channel" within the joists (by rehanging on a support cross joist?) From GWfloorplans

    Bookmark   May 31, 2012 at 7:54PM
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    Bookmark   June 15, 2012 at 5:45PM
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you raised too many questions that cannot be answered by your drawing. Post a link to the other thread you have going.

    Bookmark   June 17, 2012 at 9:36PM
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I have several other threads I have posted in the past.

Small homes forum (has links to the build forum too from early on):

Do you mean this one from bathrooms forum?

Here is a link that might be useful: small homes forum

    Bookmark   June 17, 2012 at 10:13PM
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