How to Install a Bathroom Drain-waste-vent System

Running pipe for a drain-waste-vent system can be tricky. You will have to plan your runs and buy the necessary fittings (always get a few extra) ahead of time. All different kinds of fittings are available to receive different sizes of pipe at various angles. For instance, one fitting may need three opening ports, all accommodating a different size pipe. Keep in mind that codes may regulate how fittings can be used (i.e. sanitary tees may not be laid horizontally; long sweep connection must be used here). So, you will need to know the diameter of your pipes, their angles, their codes, and their direction to determine each fitting.

While planning may take some time and study, the cutting and assembly of plastic pipe is very simple.

Step 1 - Cut and Clean the Pipe

Using a back saw or a fine-toothed saw, cut the pipe the required length, remembering that this attaches to a fitting a prescribed distance for each diameter of pipe. Then, use a small knife or rough sandpaper and remove the "burr" off the freshly cut pipe. Some cleaning solvent and a rag will clean the ends of the pipe and the inside of fitting where it will join. ABS does not need this solvent or primer; it can just be wiped with a rag.

Step 2 - Dry Fit and Mark

Always dry fit the run by cutting and assembling everything without glue to assure it all fits at the correct angles and dimensions. When you use glue, it will dry almost immediately and you can never get the pieces apart without cutting out the fitting and starting over. After everything is assembled temporarily, make marks across fittings and pipes so that once it’s all taken apart to be glued, you will know exactly how to realign everything to get it back together in the right fashion. Be sure the marks or lines are long enough on the pipe so that they will not be covered by the glue you will spread on the end.

Step 3 - Glue and Re-Fit

Spread a generous amount of the required pipe glue around the end and on the inside of the fitting with a dauber. Then, insert the pipe into the fitting until it bottoms out and give it a little twist to be sure that the glue is spread evenly in the joint.

Step 4 - Drill Out the Framing

One of the most difficult parts about running drain-waste-vent pipes is drilling the large (two to three-inch) holes necessary to run the pipe. You need to purchase, borrow, or rent special heavy-duty plumbers' bits that are notched to fit 1 1/2-inch, two-inch and 2 1/2-inch pipe. A cheaper hole saw is just too difficult to use on a large project. You will also need a heavy-duty, 1/2-inch or 3/4-inch drill, preferably a right angle drill or one with a right angle attachment. Be careful, as drilling with these large bits often causes the bit to bind and the drill to spin.

Most Common Mistakes

1. Ignoring Code Restrictions

Always review any codes regarding this kind of pipe work and follow them to the letter.

2. Incorrect Slope

DWV piping should always be installed with a 1/4-inch slope per foot of pipe, or one inch per four feet of pipe.

3. Not Venting Fixtures

Always properly vent or trap all fixtures to prevent clogging. You may also run into issues if your venting is too far from the fixture's trap.

4. Too Many Fixtures

Know how many fixtures your drain-waste-vent pipe can handle and do not exceed that number.

5. Too Small Pipes

Before starting to install your piping, be sure you know what size you need for your system. Using pipes that are too small will likely lead to issues down the line. Some people also reduce pipe size as the pipes run downstream, causing additional problems.

6. Improper Cleanouts

Take care to provide enough cleanouts for your system and make sure they’re at the prescribed places.

7. Misaligned Tubing

Not properly aligning tubing into fittings or stop valves can lead to leaks, such as forcing the nut onto the compression ring at an angle when the tubing is at an angle.

8. Using Fittings Improperly

Don’t use any of your fittings in a wrong position or it could lead to both clogs and leaks.

9. Installing Rough Plumbing in the Wrong Location

Always know which types of piping need to be placed where. This is one of the most important parts of your initial run plan.

10. Cutting the Wrong Lengths

Account for the ridge in the fittings when measuring your pipes to cut them. If you don’t, your pipe lengths will end up being too long and additional cuts will need to be made repeatedly.

11. Not Using Compression Tubing in Fittings

Forcing the trap and waste arm fittings out of alignment will put too much stress on the nuts and washers in the tubing. Make certain the compression tubing is put in the fittings so that it is evenly tightened.