The codes provide us a table that lists all the different types of fixtures that can be attached to the drainage system and each fixture is assigned a DFU (Drainage Fixture Unit) value.

We begin listing all the fixtures and their corresponding DFU value to find the Total DFU load for the structure. Once we have the total DFU load we then consult another table that lists the maximum number of DFU's that can be conveyed by each size of pipe. By example let us consider a few fixtures:

Lavatory................... 1DFU
Tub/shower................. 3DFU
Watercloset Watercloset >1.6gpf........ 4DFU
Shower stall............... 2DFU
Kitchen sink............... 2DFU
Kitchen sink w/disposal.... 2DFU
Kitchen sink w/Dishwasher.. 2DFU
Clothes washer standpipe... 2DFU
Laundry Tub.................2DFU
Laundry Group.............. 3DFU
Floor drain................ 0DFU

As you can see if you have a standard bathroom group, lavatory, tub/shower & watercloset we can use the Bathroom Group value but if you have extras such as two lavatories, watercloset, tub and separate shower or perhaps a bidet we must then use the value of each individual fixture to compute the bathroom group.

We would then use the total DFU value for the whole structure to determine the size of the "House Sewer line" (line from the structure to the septic tank or municipal main.

WE must then go back to each bathroom, Kitchen and laundry ad compute the total DFU value for that room to determine the proper size of the branch lines. As these branch lines connect together we must compute the combined load of the lines connecting to determine the size of the line downstream from that intersection.

The codes then give us a table listing the Maximum DFU load that can be conveyed by each size of pipe. Example;

..........................Horizontal........Vertical
1-1/4".................... 1DFU ............. 1DFU
1-1/2".................... 3 .................4
2"........................ 6 ............... 10
2.5" .................... 12 ................20
3" ...................... 20 ............... 48
4" ...................... 160 ............. 240

The code has some exceptions to this rule by example, while a watercloset is rated at 3 or 4DFU which could be conveyed by a 2" line however we must also consider that a watercloset is expected to convey solid fecal matter therefore the code requires a watercloset to be set on a 3" or 4" line.

Contrary to popular belief an oversize line will clog much faster than an undersized line.

When a drain line is properly sized in this manner the line will only be 1/2 full under full load which provides a sufficient liquid level to convey any solids that may be present in the waste. In order to insure proper velocity of flow this keeps the upper half of the line open for the free transfer of vent air.

If a line is oversized it results in the level of liquid being much lower. In many cases the level of liquid is then not sufficient to properly suspend the solids and the solids will rub and scuff on the bottom of the pipe dramatically reducing the velocity of flow. This also allows the liquids to then seep past the solids and continue down the line leaving deposits of the solids in the line. Once the liquid has passed the solids will then begin to dry in place creating a small dam for the next cycle until the solids build up and totally clog the line.

The plumbing in this house wasn't installed by a professional.
It was my Dad. And since he payed for it he probably saved
money everywhere he could. Starting with what I am certain
is an undersize supply line from the well house.
I will have to dig it up and measure it to be sure.

Let's see, fixtures in this house.
Kitchen sink = one faucet only and one sprayer.
Dish washer.
Ice maker for fridge.

Washing machine and hot water heater

bathroom sink = one faucet.
One toilet
One shower/tub combo

Two outside faucets

I assume the drainage system is layed out in a fairly logical manner.
But I would have to actually crawl under there to check it.
I could see that a 3 inch pipe drops from the toilet and
goes into the main line which was also 3 inch.
The shower/tub and sink each have their own separate
connections to the main line. 2 inch pipe I think.

Again, this system was not installed by a pro.

Thanks for the info.

Pooh Bear

Your total DFU load is 11DFU therefore a 3" pipe would be the correct size for your main drain and house sewer.

That's interesting.
I have never before seen 3 inch line used for the main line.
Guess I'll have to look for another reason the toilet frequently
stops up. I noticed several other posts about the same problem.

Oh, and thank you very much for the information.
An actual scientific way I can calculate this stuff.
That I can deal with. Great answers.

Thanks.

Pooh Bear

Hi,
Presently I'm in a vacation home which is on the market for sale. I route my washing machine waste to a small dry well but would like to reroute it to the septic tank. I'm very handy but far from a pro. Could you please give me some insight as to what this entails. Many thanks.
Don