3 inch sewer main???

[misterg]

We are doing a major remodel (basically a rebuild) on our house. We currently have an envelope of just the foundation walls; the basement slab is gone.

When the project is done, we will have 4 finished bathrooms and 1 basement bathroom rough-in, so 5 bathrooms someday. Additionally, there will be a standard kitchen (one sink plus dishwasher) and laundry (wash machine and utility sink). If we finish the basement, we will have about 3800 finished square feet and 5 bedrooms.

Today the plumbing contractor came out to place the portion of the sewer system that is beneath the slab, and he used a 3 inch main line which he tied into the existing 4" sewer lateral outside (goes to the septic tank). Every house I've ever owned, from 1500 sq feet with 3 bathrooms to 2500 square feet with 4 bathrooms, has had a 4 inch sewer main.

The plumber said that it is allowed by code (We're on the 2006 IRC and 2006 IPC). I did a quick check and as best as I can tell he is right, based on the number of fixtures that we have.

So, a few questions:
1) Is this really allowed by code?

2) Does this increase the potential for problems down the road, even if allowed by code?

If I'd known in advance, I would have spec'd a 4" sewer main even if not required. At this point, it would probably cost me quite a bit to change, but it's not too late until the slab is poured. Is this simply a case where current research has shown that the old way (4" for a typical house) was overdesigned?

Thanks,

Jeff

[nhmaster]

Yes and no.

[FloridaOrange]

I would've wanted a 4". However, looking at Florida Code which closely follows 2009 IPC, I figured you would be at around 35-36 DFU's (as a building drain). a 3" pipe can carry 36 DFU's at 1/8" per foot. I learned on the 94 Florida code which said that any more than 2 water closets called for a 4" line and I still design that way.

In all reality, not all the fixtures in your house will be used at one time so you should'nt have anything to worry about.

[master plumber mark]

It will work fine.... if their is 4 inch stubbed into the slab
that he has tied into with the 3 inch , basically that makes it all the better...

you probably wont ever know the difference and the
3 inch line will actually stay more scoured out over a 4 inch
one....

as long as their is a place to clean out the system,
I would not sweat about it

[hj]

Here, regardless of the fixture load, once you have more than 3 toilets, the rest have connect to a 4" line. And the third toilet has to be at least 15' from the other two when you use a three inch line.

[misterg]

Thanks for the replies...

3 inches it will be... too late to change now. We will have new low-flow (1.6 GPF) toilets.

Jeff