What are the most common bathroom plumbing mistakes?

The toilet is the fixture people most often set themselves, and the same few errors come up.

Setting the flange at the wrong height. The flange is the ring that anchors the toilet to the drain. Oatey's guidance is that "The flange should sit on top of or even with the finished floor." Oatey says that placement "ensures a proper wax ring seal and prevents leaks." If the flange is too low, Oatey says "Leak paths will form because the flange is not at the correct height to accept the horn at the bottom of the toilet." This shows up in remodels, when new tile raises the floor and nobody raises the flange with it. A spacer or extension kit fixes that.

Stacking wax rings. It is a popular shortcut, and Oatey says it backfires: "Stacking wax rings will not result in a better or more secure seal." Oatey's reasoning is that the two rings do not fully mold into one solid piece, so gaps can form. The better fix is a single extra-thick ring or a flange spacer.

Reusing the wax ring or the old bolts. Oatey says "Once compressed, a wax ring cannot be reused - even if you remove it minutes after installing." Old bolts may be corroded, so Oatey recommends you "always use a new wax ring and new bolts."

Not anchoring the flange to the floor. The flange has to be fastened to the floor, not just held by the toilet bolts. Oatey says without that anchoring "Movement can break the wax seal, allowing sewer gas or leaks." Rust-proof screws are the right choice, not drywall screws, which "will deteriorate over time if they come in contact with water."

Ignoring a rocking toilet. A toilet that moves under you is a broken seal waiting to happen. Our page on fixing a rocking or sloped toilet covers the fix.

Drains are hidden, so mistakes here can hide for years.

Building an S-trap. Oatey's P-trap guide says an S-trap drops through the floor and can siphon dry, which lets sewer gas into the home, and that it is often not code-compliant. A P-trap that leaves the fixture and goes into the wall is the standard. If your bathroom smells like a sewer and the trap shape is odd, see why a bathroom drain smells like sewer.

Over-tightening slip nuts. More force is not a better seal. Oatey warns "Excessive force can crack plastic fittings or strip threads, causing leaks." Hand-tight plus a small snug with pliers is the rule. Slip joints seal with washers, so tape is not the fix.

Skipping the leak test after a trap change. Oatey's P-trap guide says to "run water for 30–60 seconds" and inspect each connection. Do it, and look again later.

Forgetting venting. A trap needs air behind it to drain quietly and keep its water seal. When a new sink or shower is added without a proper vent path, you get gurgling and slow drains. Our page on what a plumbing vent does shows how it works. Adding or moving a vent is a good moment to call a plumber.

Ignoring slope. Drain pipes have to fall toward the sewer, and a flat run collects waste. Our page on drain pipe slope covers the code numbers.

Mixing pipe materials without the right fittings. Oatey says to match the drain fitting to your pipe: "PVC to PVC: Use a PVC flange with PVC cement." Oatey says mixing materials "often requires transition fittings or rubber couplings, which add work and risk."

Water damage from a shower is the costly kind, because it happens behind tile. Oatey's article on common shower installation errors lists the repeat offenders.

• Weak subfloor. Oatey says "Any movement in the shower system because of an unstable substrate will most likely cause failure."
• Water-resistant board instead of waterproof. Oatey explains that a water-durable product "can withstand water penetration to some degree, but not entirely, whereas a waterproof material or product is impervious to water."
• No pre-slope, or blocked weep holes. Oatey says "Many installers make the mistake of not pre-sloping before installing the waterproofing layer; or leaving the weep holes in the drain base unprotected." Over time, it says, this "causes deterioration of the thin-set below the tile or mortar bed."
• Poking holes in the liner. Oatey says "Penetrating the waterproof membrane or liner is another common error that can lead to shower failure."
• Skipping the flood test. Oatey calls it "the best way to determine if you have a leak before installing your shower system," and says "Check with local code jurisdiction for appropriate testing requirements."

Each of these is covered step by step on our page about shower pan liner code. And if the job is a tub-to-shower conversion, the drain moves, and that is plumber territory.

Ignoring small leaks. A drip is easy to live with, and it is not free. The EPA says "The average household's leaks can account for more than 9,300 gallons of water wasted every year." It lists "worn toilet flappers, dripping faucets, and other leaking valves" as the common types. It also gives a home test for toilets: "Identify toilet leaks by placing a few drops of food coloring in the toilet tank. If any color shows up in the bowl after 10 minutes, you have a leak." Flush right afterward so the tank does not stain.

A leak under a bathroom sink or behind a shower valve can also rot cabinet floors and subfloor quietly. Those are worth a look under the vanity every few months.

Skipping permits. Some work is exempt and some is not. Phoenix's Work Exempt from Permit sheet says a permit is not required for replacing water closets, valves or fixtures with new ones complying with the code's conservation requirements, "except that a permit shall be required for the relocation of any valves, pipes or fixtures." So swapping a toilet in the same spot is one thing, and moving it is another. Our page on what plumbing work needs a permit in Phoenix lays out the rest.

Follow this short checklist.

1. 1Read the instructions that come with the fixture and the drain parts. Manufacturers publish rough-in and height details for a reason.
2. 2Get the rough-in right before the finishes go on. Flange height, trap shape, vent path and shower slope are all easier to fix before tile.
3. 3Test before you close the wall. Pressure test the supply lines, run water through the drains, and flood test the shower.
4. 4Use new small parts. New wax rings, bolts, washers and supply lines cost little next to the damage they prevent.
5. 5Match the pipe and fittings. Do not mix materials without the right couplings.
6. 6Ask about permits before moving anything. Then follow the inspection steps.
7. 7Fix drips early. A toilet dye test takes ten minutes.

If you are doing a full bathroom remodel, our page on when to involve a plumber in a remodel has the timeline. Call a licensed plumber whenever the job involves moving a fixture, changing the drain or vent, or cutting into a slab or wall you cannot easily reach again.

HQ Plumbing & Air is licensed and insured, and we give upfront pricing and free estimates. If you find a leak you cannot stop, our team is available 24/7.