[LazyPup]

The smallest diameter trap that is allowed on a Drain, Waste and Vent system is an 1 1/4" diameter trap, which may only be used on a bathroom lavatory sink.

By physical experimentation it was determined that an 1 1/4 trap could pass 7.5gal/min at standard gravity pressure, therefore 7.5gal/min was assigned as one Drainage Fixture Unit. Once the DFU was defined they they created a table of DFU values for each type of fixture that may be found in the structure.

The codes have tables that list each type of fixture, the minimum trap diameter required and the number of DFU's assigned to that fixture.
(From the International Residential Code)
Bathroom Lavatory 1DFU 1 1/4" trap
Bathtub/shower 2DFU 1 1/2" Trap
Shower Stall 2DFU 2" trap
Water Closet > 1.6gal/flush 4DFU N/A
Water Closet < 1.6gal/flush 3DFU N/A
Kitchen sink 2DFU 1 1/2"trap
Kitchen sink w/dishwasher 2DFU 1 1/2"trap
Kitchen sink w/disposal 2DFU 1 1/2"trap
Clothes washer standpipe 2DFU 2" trap
Laundry tub 2DFU 1 1/2" trap
laundry Group (washer & tub) 3DFU 1 1/2"
(Note that the table does not list a trap size for the water closet. Water closets-or toilets if you prefer, have a built in trap, therefore the line does not get a trap, however, the drain line must be equal to or greater than the trap diameter, therefore if the water closet has a 3" discharge port from its built in trap, the watercloset flange and drain must then be a 3" minimum.)

The table continues on and lists many other fixtures such as water coolers, Bidets, bar sinks, etc etc.

To layout the drain we first start with the "floor Plan" that shows the location of each fixture in the structure. Considering one fixture at a time we continue to add the DFU values until we know the total combined DFU load for the whole structure.

Consider a small house with two bathrooms, kitchen and laundry.

Bathroom #1
Water closet < 1.6gal/fl 3DFU
Tub/shower 2DFU
Lavatory 1 DFU
total 6DFU

Bathroom # 2 (master bath)
Water closet < 1.6gal/fl 3DFU
Tub 2DFU
Shower stall 2DFU
1st Lavatory bowl 1DFU
2nd Lavatory bowl 1DFU
total 9DFU

Kitchen
Kitchen sink/disposal 2DFU

Laundry
Washing machine standpipe 2DFU
Laundry tub 2DFU
4DFU (Laundry group May be derated to 3DFU)

Therefore the Total DFU load for the whole house is 20DFU's.

We then consult a table that lists the maximum DFU load permissible by pipe diamter.

1-1/4" pipe 1 DFU (Horizontal) 1 DFU (Vertical)
1-1/2" pipe 3DFU (H) 4 DFU (V)
2" pipe 6 DFU (H) 10DFU (V)
2-1/2" pipe 12DFU (H) 20DFU (V)
3" 20DFU (H) 48DFU (V)
4" pipe 160DFU (H) 240DFU (V)

First we must consider that the minimum line diameter that may serve a water closet is a 3" diameter line. From the table we can see that a 3" diameter line will carry a total combined load of 20DFU, therefore in this instance the proper size for the main drain, and house sewer line would be 3"

Depending upon the physical layout of the floor plan, it may be necessary to install a separate vent for each bathroom as well as the kitchen and laundry. At least one vent directly attached to the main drain (3") must be designated as the house "main vent" and must run from the main drain up and out through the roof, undiminished in size, meaning it must be 3" (Equal to the main drain)

The additioinal vents (Auxillary vents) may then be reduced to 1/2 the diameter of the line they serve once they are 6" above the flood level rim of the highest fixture they serve, but that line may be no less than 1- 1/4".

In a climate that is subject to freezing temps (northern climate) the auxillary vent must then be enlarged back to 3" diameter at least 1' inside the attic space before passing through the roof.

It is very important that we use these table to determine the proper line size.

When properly sized a should only be 1/2 full at full load. That allows the lower half of the pipe for waste flow while allowing the upper half to permit vent air to move.

If a line is too small, the load then fills the pipe and restricts the movement of vent air resulting in the water seal being sucked out of the traps.

If a line is too large the level of liquid in the line is not sufficiently deep enough to suspend and carry any solid wastes. Therefore, the solids will sit idle in the line while the liquids flow around the solids, then end result is a line that continually clogs up.