Drains

Drain and Sewer Materials

Aboveground: Codes vary on the type of materials allowed for aboveground drain lines. The most commonly accepted materials include ABS, PVC, cast iron, and copper.

For best results, always use cast iron for drain lines that run through a living space. To save money, switch back to ABS or PVC under the house, but only cast iron will provide silent drains in the living space.

Belowground: Most building sewers and drain lines below grade are made of ABS and PVC. While both types of pipe must be cleaned before joining, ABS pipe is much more forgiving to work with in substandard conditions.

Sizing Drains

The smallest drainpipe allowed for underground use is 2 in., and the smallest building sewer allowed is 3 in. in diameter.

Minimum drain sizes for individual fixtures are shown in Figure A. Best practice calls for upsizing these minimums.

Sizing Branch Lines and House Mains

For branch lines, drain stacks, and building sewers, the sizing is based on drainage fixture units (dfu), which are different from fixture-unit values for supply piping (Figure B). One dfu equals one cubic foot of water per minute. Each fixture is rated for a specified dfu, which determines the trap size and the size of the drain following from it (Figure C).

Drain sizes must never be reduced in diameter, though they often are enlarged as branch lines accommodate more and more fixtures.

Combined drains from several fixtures are sized based on the total dfu minus a factor based on the low probability that all the fixtures will discharge at the same time (Figure C).

Groundwork

Underground drainpipes must be adequately supported. The bottom of the trench should be backfilled with 4 in. of 1/4-in. fines to continuously and uniformly support the drain line.

Trenching

Sewer lines should be below the frostline.

Sewer lines must be at least 12 in. below water lines that run in the same trench (some codes permit 10 in.). Sewer lines and water lines running at the same depth must be laid in separate trenches spaced at least 5 ft. apart (see Figure: Water Service Installation in Groundwork). However, if the sewer is made of building drain materials (such as ABS, PVC, cast, or copper), then separation is not required.

Excavations near footings. Drain lines running parallel to a foundation footing must not fall within a 45-degree angle outside the footing (Figure D).

Drain lines running perpendicular to the footing should pass a minimum of 2 in. below the footing. Do not bring the drain line through the footing.

A drain line may pass through the foundation, but it must be sleeved with a pipe at least two sizes larger than it.

Backfill. Backfill with 1/4-in. fines (some codes allow 3/4-in. material) at least 6 in. above the drain lines. Above drain lines, maintain a minimum of 12 inches "tamped earth" and keep soil free of rocks, frozen chunks, concrete, etc.

Figure D: Drain Lines Near Footing

Drain Slope and Orientation

Horizontal drains (under 3 in. in diameter) should have a 2% slope, equivalent to 1/4-in. drop per foot of length. This allows the contents to develop a velocity (approximately 2 ft. per second) that is fast enough that things won’t bog down and slow enough that the water flows ahead of solid and semi-solid wastes.

Drains 3 in. or more in diameter may slope 1% (or 1/8 in. in 12 in.).

Vertical drains should be plumb to allow the water and waste to fall as a unit (called a slug). On a horizontal run that drops to vertical, the turn can be abrupt. But vertical drops that turn to horizontal and turns on horizontal runs should have room to make a wide sweep and a cleanout, if practical.

Cleanouts

• Cleanouts are generally required every 40 ft. wherever drain lines change direction more than 45 degrees.
• Cleanouts are typically the same size as the drain lines they serve.
• Each cleanout must be installed with the flow of the drain so that a snake inserted into it will also go with the flow.
• Horizontal drains must be equipped with cleanouts at least every 100 ft.
• All cleanouts must be accessible. They can’t be buried in the wall and must have clear space in front of them.  For any drain 6 in. and smaller this clear space must be at least 18 inches (Figure E).
  

Figure E: Cleanout Clearance

Vent Cleanout

In an existing building, a vent stack may serve as a cleanout — if it meets local code and the following requirements:

• The vent stack is completely vertical, with no offsets.
• The vent stack is the same diameter as the drain line it serves.
• The drain line below the vent does not have more than one 90-degree turn.
• A 4-in. vent may not reduce to less than 3 in.; a 3-in. vent may not reduce to less than 2 in.

Using a vent to clean out a drain is not supported by code as a required cleanout in new construction.

Drainpipe Fittings

Most plumbing codes are specific about which type of fitting can be used on drain lines to change direction. The guidelines shown in Figure F, are typical of what most codes allow. Note the limitations on quarter bends, short sweeps, and sanitary tees.

Figure F: Fittings for Change of Drainpipe Direction

Type of Fitting	Horizontal to Vertical	Vertical to Horizontal	Horizontal to Horizontal
A. 1/16 bend (22 1/2 degrees)	OK	OK	OK
B. 1/8 bend (45 degrees)	OK	OK	OK
C. 1/4 bend (90 degrees)	OK	OK 1	OK 2
D. Short sweep	OK	OK 3	OK 4
E. Long sweep	OK	OK	OK
F. Sanitary tee 5	OK	NO	NO
G. Wye	OK	OK	OK
H. Combination wye and 1/8 bend 6	OK	OK	OK

Parallel offsets. Allow plenty of room for an offset between parallel pipe runs. Figure G shows a method of calculating offset distances and the length of pipe required between common offset fittings.

Figure G: Calculating Parallel Offsets

Degree of Offset	When A=1, B=	When B=1, A=	When A=1, C=
60	0.573	1.732	1.155
45	1.000	1.000	1.414
30	1.732	0.577	2.000
22 1/2	2.414	0.414.	2.613

Drainpipe Installation

Supporting Pipe

Spacing for pipe supports depends on the material used (Figure H).

When hanging pipe, best practice calls for using a solid material, such as notched 1-by or approved pipe hangers from a plumbing supply house (Figure I). Solid hangers resist upward thrust and vibration, thereby providing more secure support, while also helping to quiet drain lines. Wrap the pipe with insulation at each pipe support to further dampen vibration.

Figure I: Plastic Drain Support

Notching and Boring Studs and Joists

For notching and boring framing to accommodate plumbing lines, it’s critical that plumbers follow the guidelines shown for lumber joists (Figure J), engineered wood I-joists (Figure K), regular studs (Figure L), and engineered studs for tall walls (Figure M).

When using plastic piping (PVC or ABS), install nail guards where lines run near the surface of studs or joists.

Figure J: Notching and Boring Lumber Joists

Joist Size	Max. Hole	Max. Notch Depth	Max. End Notch
2x4	None	None	None
2x6	1 1/2	7/8	1 3/8
2x8	2 3/8	1 1/4	1 7/8
2x10	3	1 1/2	2 3/8
2x12	3 3/4	1 7/8	2 7/8

Figure K: Notching and Boring Engineered Wood I-Joists

Min. Distance (ft.-in.) from Inside Face of Support to Near Edge of Hole
I-Joist Depth	TJI/Pro	Hole Diameter
9 1/2"	150	1'-0"	1'-6"	3'-0"	5'-0"	6'-6"
	250	1'-0"	2'-6"	4'-0"	5'-6"	7'-6"
11 7/8"	150	1'-0"	1'-0"	1'-0"	2'-0"	3'-0"
	250	1'-0"	1'-0"	2'-0"	3'-0"	4'-6"
	350	1'-0"	2'-0"	3'-0"	4'-6"	5'-6"
	550	1'-0"	1'-6"	3'-0"	4'-6"	6'-0"
14"	250	1'-0"	1'-0"	1'-0"	1'-0"	1'-6"
	350	1'-0"	1'-0"	1'-0"	1'-6"	3'-0"
	550	1'-0"	1'-0"	1'-0"	2'-6"	4'-0"
16"	250	1'-0"	1'-0"	1'-0"	1'-0"	1'-0"
	350	1'-0"	1'-0"	1'-0"	1'-0"	1'-0"
	550	1'-0"	1'-0"	1'-0"	1'-0"	2'-0"

Figure L: Notching and Boring Regular Studs

Figure M: Notching and Boring Engineered Studs

Traps

A trap — the bend in the drain below every plumbing fixture — creates a water seal that blocks sewer gases from escaping into the house (Figure N). A vent prevents the trap water from siphoning away. Vents maintain equal air pressure on both sides of the trap seal. This keeps negative pressure from building up so that a slug of water running down the drain doesn’t pull the trap water out with it (Figure O).

Figure N: Typical Trap Seal

Trap Rules

The following guidelines apply to all fixture traps:

• No fixture trap outlet may be larger in diameter than the drain to which it is connected.
• No fixture can be double trapped.
• A slip joint is allowed at the trap inlet, outlet, and within the trap itself. On the outlet side, the slip joint may not be allowed above the top of the water seal. In any case, any trap made with slip joints must be accessible through a minimum 12x12-in. opening.
• Each fixture trap must have a water seal that is at least 2 in. deep and not more than 4 in. deep.
• Most codes require a horizontal leg on the discharge side of the trap that’s at least two pipe diameters long. This leg must be sloped 1/4 in. per foot from the top of the water seal toward the vertical discharge.
• Code limits the horizontal distance before the trap arm tees into a vent, as shown in Figure P and Figure R.
• Most codes limit the length of the vertical drop from the fixture inlet to the trap. For sinks, lavs, showers, and bathtubs, the maximum drop is typically 18 to 24 in. (Figure P); check with local code.

Toilet Traps

• Toilets have integral traps, and may not discharge into a trapped drain, as this would create a double trap.
• The vertical drop below a toilet may not exceed 24 in. (again, check with local code to confirm this maximum).

Tub and Shower Traps

Most fixtures should be made accessible. The common exceptions are bathtub and shower traps, which may be concealed. However, these fixture traps must be made of approved materials and may not be equipped with a cleanout. (Cleanouts must always be accessible; see Cleanouts).

While best practice calls for keeping the trap as close as possible to the fixture, most code inspectors will allow a maximum of 30 in. horizontally from sink outlet to trap inlet to get around framing obstacles (Figure P). However, be sure to check with your local official. Also, there are offset tub drain-overflow assemblies with side or rear outlets that may help overcome framing obstacles.

Figure P: Tub Trap

Traps for Double Sinks

Sinks and laundry trays with two or three compartments may be connected to a single trap with a continuous waste. In this case, the following guidelines must be met:

• The sink’s drain inlets must be within 30 in. o.c.
  
• Two or three single-compartment sinks may be used on a single trap, as well, provided the drain inlets are not spaced more than 30 in. o.c. (Figure Q).
• If three single sinks are installed on a single trap, the trap must be on the center sink.

Drum Traps

Drum traps are no longer allowed by code.

Figure R: Max. Distance Between Fixture Traps and Vents

Size of Trap (in.)	Size of Fixture Drain (in.)	Slope (in. per ft.)	Distance from Trap (ft.)
1 1/4	1 1/4	1/4	3 1/2
1 1/4	1 1/2	1/4	5
1 1/2	1 1/2	1/4	5
1 1/2	2	1/4	6
2	2	1/4	6
3	3	1/8	10
4	4	1/8	12

Basement and Crawlspace Drainage

Basements should have a floor drain to remove standing water. Dig a shallow hole at the low point, then run solid PVC drainpipe through the foundation wall to daylight or a municipal sewer. (For more information on foundation drainage, see Perimeter Foundation Drains.)

Backwater valves. A basement drain connected to the municipal sewer should have a backwater valve in the drain line. Under concrete, install backwater valves with sleeves and access panels, so that they can be accessed for maintenance.

Sump Pumps

If the exterior grade won’t permit gravity drainage of a basement drain, use a sump pump and discharge the water well away from the building (Figure S).

Retrofit sump pump. To retrofit a sump pump to replace a utility drain, remove the existing drain but leave the trap. Dig a hole for a plastic bucket or sump kit. Connect the sump pump’s 2-in. discharge connection to the existing drainpipe under your slab, using watertight threaded or glued fittings. The sump pump must include a check valve in its discharge fitting. This enables it to pump out under pressure any liquid that flows into the sump, even when the utility drains are full.

If a basement or crawlspace is generally wet but water isn’t pooling in one spot, dig shallow trenches across the floor and use perforated PVC drainpipe to collect the water. Then pipe it to daylight or to a sump pump.

Ejector Pumps

A basement toilet that has a discharge outlet below the house main drain will require a sewage ejector pump specifically designed for lifting effluents to the drain line (Figure T). A typical installation consists of a drum-shaped plastic or metal reservoir, or sump, which receives discharge from a toilet, tub/shower, and lavatory. Inside the sump basin, which holds about 30 gallons of effluent, is a specially designed pump triggered by a float mechanism. The unit churns the waste and periodically pumps it to the main drainpipe as needed. This churning is loud, so avoid positioning the ejector pump near a bedroom or study.

Figure T: Ejector Pump

Drainage and vent. The sump pit will require two pipes at the top: a high-pressure discharge pipe, usually 2 in. in diameter, and a dry vent pipe, usually 2 to 3 in. in diameter (Figure U). To connect the vent, try to find a dry vent in the house plumbing and tap into that, or run a new vent to the roof.

Sealing. After the ejector pump is installed, seal all openings around the sump pit with hydraulic sealant or caulking to prevent any radon or groundwater from entering the basement. If radon is an issue, be extra careful to properly seal these and other penetrations through the basement floor. (For more, see Radon Abatement.)

An option worth considering is an alarm system, which monitors the basin level. An integral float furnished with the pump provides automatic on/off action in response to usage. If the pump does not cycle, an audible alarm sounds to warn against usage until the system is checked.

Figure U: Venting a Basement Toilet