3" or 4" PVC for septic

srs · May 7, 2006

3\" or 4\" PVC for septic

I posted this on CBN with negative responses. Hopefully within a year /forums/images/graemlins/smile.gifI will have a Morton building built. I'm considering putting in running water plus a toilet so I won't have to run into the house if I'm all dirty working in the outbuilding. My question is should I use 3" or 4" PVC pipe from the Morton to the septic tank? Throughout the house I have 3" PVC but when it exits the house to go to the septic tank the plumber used 4". I will "tee" into the main 4" PVC prior to it reaching the septic tank. Any pros or cons using either one? Is it code that you have to have 4" under ground? By the way I reside in Maryland. Thanks in advance.

Farwell · May 7, 2006

Re: 3\" or 4\" PVC for septic

srs,
Depending on cost difference I would go with the 4 inch pvc. I do not know the code for your state but I like to error on the plus side.
Farwell

srs · May 7, 2006

Re: 3\" or 4\" PVC for septic

Farwell, I only have to go 60+ feet so the cost difference won't really be that much. I just thought it was kinda strange to have 3" inside and 4" outside. /forums/images/graemlins/crazy.gif Thank you.

EddieWalker · May 7, 2006

Re: 3\" or 4\" PVC for septic

One toilet and a sink will work fine on a three inch line. The cost isn't gonna make a difference to you, but for all your doing, there's no big advantage to 4 inch over three inch in my opinion.

What is important, especialy for a work shower that will see more dirt and grime is to have as many cleanouts as you can. I like one before the points you access it on the side of the building, then when you leave the building and where you change directions.

Cheap insurance if you ever need to use it.

Eddie

srs · May 7, 2006

Re: 3\" or 4\" PVC for septic

One toilet and a sink will work fine on a three inch line. Actually it will be a toilet, sink, and slop sink for the outbuilding, plus a hose bib. 

The cost isn't gonna make a difference to you, but for all your doing, there's no big advantage to 4 inch over three inch in my opinion. I really value your opinion Ed with all the projects that you have completed and still working on. You are doing one **** of a job. Once I get to eventually placing the pvc I'll check the slope to make sure I can use 4". I'm going to be real close with the 1/4" slope and I might have to build up the yard a little to hide the pipe. I'll git-r-done though. /forums/images/graemlins/smirk.gif If I can use 4" I might as well do it. And thanks on the advise on the clean outs. Certainly easier to do while the trench is open.

PILOON · May 7, 2006

Re: 3\" or 4\" PVC for septic

As far as I know, they always use 4" from the building to the tank and usually 3" inside with a cleanout where they meet.

Here is some logic:

Whatever blocks or plugs a 3" will pass in the 4"
Also most of the 3" will have a fair vertical or drop for good flow while the 4", being buried will generally have very slight slope to it and can accumilate sediments easier..
I would never use 3" in buried conditions, besides you will find that all fabricated tanks are made for 4" pipe and that in itself tells us something.

CurlyDave · May 8, 2006

Re: 3\" or 4\" PVC for septic

Sometimes there is no real rhyme or reason to plumbing codes.

I second Eddie's idea of lots of cleanouts, and have always designed my plumbing this way.

The only clogs I have ever had in burried lines were from tree roots. And only in the older bell & spigot systems. Properly installed plastic lines don't seem to have this problem.

Sometimes the higher water velocity in a 3" line will keep sediments in suspension and prevent them from settling out.

I once talked at length with a buliding inspector, who told me that I had to have a 2" or larger drain line in a stall shower, but that if I put the same shower head over a bathtub, the outlet from the bathtub was only 1-1/4" and he had to approve them since that was all the bathtub manufacturers made. Neither one of us could understand why the 2" line was necessary in the stall shower, but I put it in because he wouldn't approve it otherwise.

Some of the codes are the result of the way things were done a century ago.

iris49truck · May 8, 2006

Re: 3\" or 4\" PVC for septic

The 3" may be used inside the walls of the house because it will fit in a 2x4 stud wall. 4" will not fit.

Terry

theboman · May 8, 2006

Re: 3\" or 4\" PVC for septic

( The 3" may be used inside the walls of the house because it will fit in a 2x4 stud wall. 4" will not fit.

Terry )

Plumbed 2x4 walls should be double walls, so you can get all the plumbing inside the walls.

I have Clayton manfactured home. From the outlets in the bathrooms (2) it was all 3 inch pipe. There are cleanouts on each and they run to a T. From the T to the existing septic system that's 4".

I'm pretty sure on new spetic systems the inlets and outlets are 4".

A 3" line should carry a toilet/shower/sink without problem. A tip is keep everything a straight as possible too. Clogs happen at the turns, corners where things pileup against the walls of the pipe.

Farwell · May 8, 2006

Re: 3\" or 4\" PVC for septic

Dave,
A friend of mine has a place in San Francisco where according to her, code demands copper pipe for all of the drain systems in her home. This includes the vent stack as well. I am sure that 3"and 4" copper pipe exists but I have never seen it for sale. Imagine the cost at today's copper price.
I do all of my own plumbing as well. The home I had in California had a 4" cast iron vent stack for the toilet and the walls had 2"x6" studs to enclose the pipe. Where the drain left the house the cast iron pipe was changed to a pipe made from paper impregnated with a tar like product. It was 50 years old when I left and it was still working with the exception of root invasion on occasion.
Farwell