[LazyPup]

Since this line will be coming into the house direct from the house sewer and septic tank it must be treated as a completely separate "Main Drain".

The first step is to determine the diameter of the main drain line. In order to do so we must first determine the total load that this drain must handle.

In the code book we have a list of all the fixtures that would commonly be found in the home. The list then assigns a DFU (Drainage Fixture Unit) value and it defines the required size of trap for the fixture.

In this example we have a watercloset, lavatory bowl, tub/shower & a kitchen sink. From the International Residential Code table 3201.7 we get the following values:

Water closet (less than 1.6gal/flush) 3DFU 3" line
Lavatory bowl 1DFU 1-1/4" trap
Tub/shower 2DFU 1-1/2" trap
Kitchen sink 2DFU 1-1/2" trap.
Total DFU 8DFU

Understanding that a drain line may never be reduced in size in the direction of flow we must then use a minimum 3" line to meet minimum for a watercloset.

A 3" main drain or sewer with a 1/4" per foot pitch is rated for up to 42DFU so the main will be a 3" line.

Under the International Residential Code we are required to have one "Main Vent" which must run undiminished in size from the main drain up through the roof. Auxillary vents may then be reduced to 1/2 the diameter of the line they serve and may termintate through the roof, in the attic space, under limited conditions through the wall or you may use an ulimited number of AAV(air admittance valves).

Under the Uniform Plumbing Code all vents may be reduced to 1/2 the diameter of the line they serve but the combined aggregate total of cross sectional area must be equal to or greater than the cross sectional area of the main drain. The UPC requires all vents to go through the roof and it only permits a maximum of one AAV per structure with the expressed permission of the local AHJ. (In this example we will only have two vents so one of them must be 3" to get the required cross sectional area)

Begin at the closet flange and run a 3" main line straight out of the buiding to the sewer connection. For slab constrution you will need a "Main cleanout" at the junction of the building Main Drain & Building Sewer approximate 3' outside the foundation wall.

Code prohibits installing a Sanitary Tee in a horizontal line so on the 3" line at the wall you will install a 3" double sanitary Wye with a street 1/8th bend in each side opening to give you a 90 deg angle. On the left side you will install a second 3" sanitary Wye with the side opening point up and run a 3" vent vertical up through the roof. On the end of the second wye install a 3"x1-1/2" reducer bushing and run an 1-1/2" line horizontal to the lavatory position and turn it up to the lavatory. Under the IRC if the combined length of sections A & B do not exceed 6' there is no additional venting required for the lavatory.

From the double Wye position GOING RIGHT install a 3" x 2" reducer bushing then a 2" dia. 1/8th bend to turn the pipe at right angles to the 3" main. Continue the 2" to the tub location. Install an 2"x 1-1/2" x 1-1/2" wye (Or a 2" wye with 2" x 1-1/2" reducers on the side & upstram ends) From the side opening install a 1-1/2"--1/8 bend and a short section of 1-1/2" to the tub trap.

From the end of the Wye continue the 1-1/2" left until you reach a point just on the right hand side of the window by the kitchen sink. Install a Wye & 1/8th bend and run an 1-1/2" line vertical for a vent. Then continue the 1-1/2" toward the right until you reach the sink location and turn up to the sink.

As a side note: Your floor plan shows a water heater and furnace in the utility space between the bathroom and kitchen. The door to that utility space is opening into a bedroom. The Plumbing code prohibits installing a Gas Water heater in a Bathroom, Bedroom or a utility room which opens into a Bathroom or Bedroom. An electric water heater would be ok. You should also check your local mechanical code to see if there are similar restrictions on a gas furnace.