r/Plumbing has a weekly thread called “What Did You Walk Into Today?” and I swear half the posts are DIY toilet installations gone wrong. The plumbers in there are saints — they’ve seen things that would make you question humanity. After spending way too long reading through these, I noticed the same mistakes over and over. If you’re about to install a toilet yourself, read this first.
Mistake 1: Not checking the flange before buying the toilet. This is the big one. Probably 40% of the disaster posts start with “I bought a new toilet at Home Depot, took the old one off, and the flange is…” then a photo of some rusted, cracked, half-buried-in-concrete nightmare. Now you have no working toilet and a flange that needs replacing. The time to check the flange is before you buy the toilet. Not after.
Mistake 2: Not using shims. Like, at all. I’d say a third of the leak-call posts on r/Plumbing end with the plumber saying “I shimmed the toilet.” That’s it. The toilet was rocking slightly — maybe the floor is uneven, maybe the flange sits a hair too high — and that tiny movement compressed the wax ring unevenly until it failed. Shim it. Even if the floor looks flat. Especially if the floor is tile.
Mistake 3: Overtightening the bolts. This one actually makes plumbers angry. There’s this guy — I forget his username but he’s a regular on r/Plumbing — who has his flair set to “STOP CRACKING YOUR TOILETS.” Because people crank the flange bolts down with a socket wrench like they’re torquing lug nuts on a truck. Porcelain doesn’t flex. It cracks. Then you have a cracked toilet base AND a leak. Tighten the bolts until the toilet stops rocking, then stop. Use a hand nut driver, not a power tool. This isn’t an engine rebuild.
Mistake 4: Stacking wax rings. Sometimes the flange sits below floor level and the solution people land on is “stack two wax rings.” Do not do this. Wax rings are not designed to stack. The weight of the toilet plus a person will squish them unevenly and you’ll get a leak. Use a flange spacer kit or an extra-thick reinforced wax ring instead. They cost maybe 3 dollars more.
Mistake 5: Not testing before caulking. This is the one that causes the most damage. Someone installs a toilet, caulks it 360 degrees immediately, and calls it done. Meanwhile there’s a slow leak at the flange that no one can see because it’s sealed in. Fill the tank, flush 10 times, check for water around the base with a paper towel. Then wait a day. Flush 10 more times. THEN caulk the front and sides. Leave the back open.
The Reddit plumbers have a saying: “Caulk hides leaks. Don’t use it until you’re sure there aren’t any.”