PVC & ABS Drain Assembly

Most drains in today's houses are plastic. White PVC or black ABS goes together quickly, but it does not forgive poor planning. A fitting that points the wrong way, a run that flattens out halfway, or a hanger placed too far apart can all look fine until an inspector or a clog finds them. Joining the pipe is only one part of the job. The rest is choosing the right fitting, setting the right fall, and supporting the run so it stays that way.

What You Will Be Able to Do

• Identify DWV pipe and read the print on it
• Pick the right fitting for each direction change
• Lay out a drain with correct slope
• Support PVC and ABS so runs do not sag
• Walk a rough-in the way an inspector will

Know Your Pipe

Both are usually sold as Schedule 40 for DWV. Read the print on the pipe. It tells you the material, size, and standard, and it will say "DWV" if it is rated for drains. Foam-core pipe is lighter and is allowed in many places for gravity drains only. Never use it where pressure is possible. Some jurisdictions do not allow ABS, so check first.

Common sizes for homes:

Fittings by Direction of Flow

DWV fittings have a smooth sweep that guides waste. They are not interchangeable with pressure fittings, which have a shoulder inside that catches solids.

The rule is simple. Follow the water and ask if the fitting helps it go where it needs to go. A sanitary tee installed upside down sends water upstream. On a vent, flow is air, so fittings are chosen differently.

A double sanitary tee (sanitary cross) on a stack can throw water from one fixture into the drain of the fixture across from it. Many codes do not allow it for fixtures like back-to-back toilets. Use a double fixture fitting or a double combo that is approved for that use.

Plan the Run

Plan before you cut. Good layouts have a few things in common.

1. Start from the sewer or stack and work up. The fall of every run depends on the height where it lands.
2. Check the distance from each trap to its vent. Code limits it, and it is much easier to catch on paper.
3. Look for conflicts. Joists, ducts, beams, and other pipes get in the way, so find them first.
4. Mark your cleanouts. Put them in places where a person can later work.
5. Dry fit, then mark. Assemble without cement, check slopes, then draw a line across each joint so you can line it up again.

Setting Slope

The fall of a drain is measured in inches per foot.

• Pipe 2 inches and smaller needs at least 1/4 inch per foot.
• Under the International Plumbing Code (IPC), pipe from 3 to 6 inches may run at 1/8 inch per foot. Other codes and local rules may set the break point at 4 inches. Many plumbers run 3-inch home drains at 1/4 inch per foot when there is room.

Check your adopted code. When you have the room, the steeper of the allowed slopes is the safer choice in small pipe, but do not go to extremes. In a nearly level pipe, solids settle. In a very steep horizontal pipe, water can run away from the solids. A vertical drop is fine, since a stack has no slope requirement.

Worked example. A 3-inch drain has to travel 24 feet. At 1/8 inch per foot, it falls 24 x 1/8, or 3 inches. At 1/4 inch per foot, it falls 6 inches. If the joists only give you 4 inches of room, you can only use the lower slope if your code allows it.

To check fall, use a torpedo level with slope marks, a longer level with a block under one end, or a laser. On a long run, measure the height at both ends and do the math. Do not guess. A low spot in the middle of a run holds water and collects solids, even if both ends are the correct height.

Support

Plastic gets soft as hot water runs through it, and a full pipe is heavy. Without support, it sags.

• Horizontal PVC and ABS: every 4 feet (48 inches) in most codes, and at changes in direction or branch connections.
• Vertical runs: at each floor, and more often if your code requires. Support at the base of tall stacks.
• Hold fittings close to a support. A heavy fitting between hangers pulls the line down.
• Use hangers wide enough not to dig in. Do not use bare wire. Metal straps need to be rated for the job and must not cut the pipe.
• Hangers must let the pipe move a little. Plastic grows as it warms, and long runs can creak or strain if strapped tight.

Plastic drains also need protection. Use sleeves through concrete and nail plates where a pipe runs close to the edge of framing. Fire-rated walls and floors need a listed firestop system around plastic pipe. Ask before you cut into one.

Cementing the Assembly

Join PVC or ABS using a full solvent-weld process, and use the right cement for each. PVC gets purple primer, which inspectors look for, then PVC cement. ABS uses ABS cement with no primer. Hold each joint for about 30 seconds.

Work in an order that does not trap you. Build small assemblies on a bench or the floor where you can reach every joint, then hang them. Line up your dry-fit marks before pushing a joint home, because you cannot rotate a fitting once the cement grabs.

Plumbing Through a Building

• Keep the sweep clean. Wipe out cement that squeezes inside small fittings.
• Cap every open stub at the end of the day. Open drain stubs collect drywall mud, sawdust, and trash.
• Protect the closet flange. Cover it so nothing falls in.
• Leave test access. Plan for test tees and plugs before the system is closed.

Testing

DWV is tested before it is covered. Allow the cement's cure time first. A water test fills the system to a head of at least 10 feet for 15 minutes. Because of the weight of water, support everything well and have a way to drain it. Some codes allow an air test on non-plastic systems. Makers of plastic pipe warn against it, so test plastic with water unless the manufacturer and inspector clearly say otherwise.

Walk the Rough-In

Before you call for inspection, go over the job with a flashlight.

• Is primer showing on every PVC joint?
• Do all tees and wyes point with the flow?
• Is every horizontal run falling the correct way, with no low spots?
• Is every hanger in place and every cleanout reachable?
• Are all vents rising before they turn horizontal?
• Are all openings capped and nail plates installed?
• Is the test up and holding?

Common Mistakes

Check Your Understanding

1. A 2-inch laundry drain runs 16 feet. How much must it fall?

At 1/4 inch per foot, 16 x 1/4 gives 4 inches.

2. You need to connect a horizontal 2-inch branch into a horizontal 3-inch drain. What fitting do you use?

A combo, which is a wye with a 1/8 bend. A sanitary tee on its back is not allowed.

3. An inspector points at a run of 3-inch PVC hung every 6 feet. What is wrong?

Most codes require horizontal PVC to be supported every 4 feet. Add hangers.

4. A horizontal line has correct height at both ends, but it dips in the middle. Does it pass?

No. The dip is a low spot where water and solids sit. Support it and correct the slope.

Key Takeaways

• Follow the flow. Fittings must help water go where you want it to go.
• Keep slope steady. Small pipe gets at least 1/4 inch per foot.
• Support horizontal plastic drains about every 4 feet.
• Plan, dry fit, and mark before you cement anything.
• Cap stubs and keep cleanouts reachable.
• Test with water and walk the job before calling the inspector.