A Dirty Dozen: Twelve Common Plumbing Mistakes

When plumbing fails, it's not a pretty sight. You're either in for a noxious mess, noisy pipes or an expensive redo. So think of the list below as a heads-up for what not to do—and an appreciation for all the good plumbers out there. The next time you gasp at a plumber's bill, keep in mind all the specialized tools and hard-won knowledge he or she brings to a job.

If you're doing any plumbing, this article will help keep you from going astray. The tools that make for a top-notch job are described in our Plumber's Toolkit.

1. Sweating (soldering) dirty copper fittings. The shine of a brand new fitting is misleading: it isn't really clean enough to solder. Use a stiff wire fitting brush on the inside of fittings (sandpaper won't work because you can't push it in far enough with your finger). On the outside use either a brush, 120-grit sand cloth or open mesh abrasive cloth. Be sure to clean the lip of the fitting, too. Check out our article on how to solder copper pipe.

2. Putting in drains too small to vent well. Most pros rely on charts to calculate potential flow through a drain; do-it-yourselfers guess. But both groups often forget that there's also air in that drain. To allow proper venting, the top half of the drainpipe must stay open when water flows through it. To be on the safe side, size the drain one full pipe-size larger than the absolute minimum required by your local building code (up to a 4-inch diameter). You'll get better venting and fewer clogged pipes.

3. Misplacing the toilet. There is a mistake that most installers make once, but the pros are too embarrassed to make it twice. What is it? Installing the toilet flange (the round inlet, attached to the floor, that the toilet base bolts to) too close to the wall or too far away from it. A proper installation has the tank lid just touching or within 1 inch of the wall behind it. Mounting the flange too close to the wall means the toilet won't fit. Having it too far away looks ugly, takes up valuable space, and announces to the world: "Boy, did somebody screw up!"

Always install the center of the flange 12 inches from where the finished wall surface is projected to be. Never just measure from the edge of the rough wall framing. This screw-up is so common that toilet manufacturers make special bowls to compensate: a 10-inch rough-in bowl if the flange is too close, a 14-inch if it's too far away. But you can also replace the flange with an offset toilet flange, which allows up to two inches of play.

4. Running a toilet drain above a living area. A common error, frequently discovered too late. When the toilet is flushed, noisy water flows through the drain lines. Not a nice thing to have happening right over the dining room table. The solution: run the drain lines somewhere else. If that's not practical, wrap fiberglass sound insulation around a plastic drain. But the best solution is cast iron pipe: because it's denser than plastic, it's quieter, too.

5. Weak strapping for drainpipes. Pipes that run horizontally are often suspended from joists with straps. These pipes bear lots of weight when they're full of water, and if they're not strapped properly, they'll sag. If that happens, especially with cast iron pipe, there's a good chance that a fitting will weaken and leak. Always use heavy-duty thick metal strapping, also called plumber's tape, to hold drain lines in place. The thinner gauge metal straps can rust through, and plastic strapping tends to stretch and snap.

6. Not quite gluing plastic drainpipes. An error that's easy to avoid happens too often with large-diameter plastic pipe: not getting enough glue into the joints. This invariably happens when the applicator (called a dauber) is too small. The solvent cement that welds the plastic parts together has to be applied to large-diameter pipe with a large brush, or dauber because you have to spread it quickly, before it starts to cure. Small cans of cement come with small daubers that can't cover large areas fast enough. The solution: use a large can of cement that comes with a large dauber.

7. Installing the wrong main shutoff valves. There are two important valves in your house, called mains. One is at the water heater and another is where the water enters the house. Both of these must be full-flow valves. Examples are the gate valve and the ball valve. A full-flow valve can be fully open or fully closed, and because of the way its handle works, it's easy to see whether it's one or the other. With other kinds of valves, it isn't as easy to tell. If your main isn't fully open, your water pressure may be affected. If your water flow seems limited, check the main valve first. Just opening the valve wider may fix the problem.

8. Mounting the toilet flange too soon. The toilet flange must always be mounted on top of the finished floor—not level with it and not under it—or you may end up with an improper seal. If the finished floor isn't installed yet, temporarily shim up the flange so that the bottom of it will sit snugly on top of the flooring once it is installed. The only exception is linoleum or vinyl flooring. Being thin, such flooring will not raise the base of the toilet high enough to make a difference.

9. Trouble down the kitchen drain. The kitchen drain is often too small and poorly supported, and the result is clogging just a few years after it's installed. A lot of kitchens have super hot water going down the drain, as well, which can soften plastic pipe, causing it to sag between supports—and that also encourages clogs. So ... although code requires only a 1 1/2-inch kitchen drain and supports spaced every 4 feet—upsize! Make your kitchen drain 2 inches in diameter and support it every 2 feet.

10. Strainers that leak? Homeowners squeezing a plumbing bid too hard usually cause this one: their plumber installs plastic sink strainers. Big mistake. These strainers screw into the sink basin and connect it to the drainpipe. Plastic strainers don't seal as well as metal ones, and after a few months they can loosen up and leak. A slow drip into the cabinet under the sink will make a mess and eventually cause rot. Always use metal strainers—preferably high-quality ones.

Traditionally, installers used plumber's putty to seal the strainer to the sink bowl. But plumber's putty can harden and crack. Clear silicone caulk gives superior results.

11. Faulty foundation for a fiberglass shower base. Installers often fail to provide solid support under shower pans, so fiberglass bases flex each time someone stands on them. This flexing base, in turn, pushes down on the rigid drainpipe below. It's a formula for leaks around the shower drain. To avoid this trap, set pans in mortar or foam, according to the manufacturer's directions. An alternative is installing a flexible PVC drain fitting immediately below the base, but it's a distant second as a solution.

12. No caulk in the wall. Faucets in the tub and shower invariably leak around the spout and handle covers because their holes are not properly caulked inside the wall. Always use silicone caulk behind and outside the handle covers and spout.

Rex Cauldwell is a third-generation electrician and a master plumber. He has written widely on both subjects, most notably in Fine Homebuilding magazine and the Journal of Light Construction. His book Wiring a House was published by Taunton Press in 1996.