Getting ready to bust up some floor and I just want one final reconfirmation as my layout plan has slightly changed:

What size branch pipe will I need horizontally under my basement floor to service all these fixtures:

Kitchen sink + dishwasher + disposal
Clothes washer
Bar sink
2” basement floor drain
Toilet, lav, and shower group

The kitchen DFU count is confusing, but assuming worst case 2 DFU each that’s:
6 DFU for kitchen
3 DFU for washer
1 DFU bar sink
6 DFU bathroom group
6? DFU floor drain

Which is 22 DFU. What I’m finding conflicting information about is whether I need to go 3” or 4” PVC for a horizontal run under the basement floor.

I’m looking at one table which says for a 3” horizontal branch, the limit is 20 DFU, so I’m leaning towards using 4” til the first subsequent split in the branch...

Appreciate your guidance!!



Edited 1 times.

These are the values for each residential fixture:

Toilet=3
lavatory=1
Clothes washer=2
Kitchen Sink=2 includes Disposal and Dish Washer
Shower=2 for 2" Trap
Bathtub=2 includes shower over the tub
Bar sink=1



Edited 1 times.

Srloren thank you.

How much DFU for the floor drain?

I think the stricter chart is for an individual fixture branch and not for the building drain branch. In CA, a horizontal building drain branch is good for 35 DFU's. Other charts show at least 42 DFUs.

back when toilets used 3.5 gallon per flush, we were allowed two toilets on a branch and three total in the whole house.
only time 4 inch was needed was with 4 or more toilets.
so, 3 inch is plenty big for what you have.
P/S, my chart says 42 DFU for 3 inch.
where did you get 6DFU for a kitchen? my chart says 2

Packy I said 6 “worst case” for kitchen, as I read online 2 DFU each for sink, dishwasher, garbage disposal. But it’s been clarified above that in a residential install the entire kitchen grouping is only 2 DFU. I guess if they are individually trapped, maybe they count as more.
Also, after doing more reading, it seems the DFU carrying capacity of the horizontal 3” pipe depends on its slope. So perhaps 20 DFU at 1/4” per foot, up to 50 DFU at 1/2” per foot of slope.

So it seems as long as my new branch is 3” at 1/4” per foot I will be fine with one bathroom, a kitchen, a washer, and a bar sink on the one line. It just seems like a lot to me but the math looks okay.

Sum to your question I’m planning on using a jackhammer because that’s what I know how to use. I’m not too worried about cracks as I’ll eventually install tile or something on the floor. Also I heard the saw is extremely dusty? Anyway, appreciate all the tips, I hadn’t thought about all that stuff and you’re right, better for me to figure it out now instead of learning the hard way later!

From the 2018 ICC-

That table is extremely confusing for a DIYer!!

Oh well here we go!!!