Q: I installed a new toilet using a wax ring with no rubber gasket. I set the new bowl on the bolts, making sure it was lined up right. But the bowl still wobbles — front to back, not side to side. When I fill the tank with water the water angles toward the wall. When I sit on the toilet, the water moves forward.

My house was built in the early 1960s and has a cast-iron flange coming out of the floor with the bolts secured to the flange, so I could not replace them. The flange is in good shape, but it sits about a half-inch above the floor. I think the base of the bowl is hitting the top of the flange. Do you recommend shimming the bowl?

A: We think you’re right, the base of the new commode is resting on top of the closet flange, which becomes a fulcrum or balance point. The weight of the water in the filled tank moves the toilet to the rear. Because you are heavier than the water-filled tank, when you sit on the toilet, the balance moves to the front.

The bad news is that the situation is a disaster waiting to happen. If the toilet doesn’t leak now, it will in short order. The good news is that the problem is easily solved.

We do recommend shimming the toilet, but that is only one step in several to solve the problem.

The standard household commode discharges into a 3- or 4-inch waste line. The line is secured to the floor by a closet flange. We don’t know the origin of this name, but we suspect it comes from “water closet,” a name for a small bathroom that has fallen out of use in the United States but is still prevalent in Great Britain.

A wax ring is placed on the bottom of the toilet where it discharges into the waste line. The wax ring has two purposes. It creates a watertight seal at the joint where the toilet discharges into the waste line. To a lesser extent, it acts as a gasket to take up slack in what might be an uneven floor.

The porcelain commode is then secured to the closet flange by two bolts with nuts called closet bolts. When the toilet is placed over the bolts and tightened down, they provide a solid connection to the flange and the floor. The two flange bolts secure the toilet, but they do not prevent it from wobbling if it does not already sit perfectly flat on the floor.

When installing a new toilet, it’s much easier to install the bowl first all by itself, then assemble the tank onto it, in place. If one decides to disassemble a used toilet, the rubber gasket where the tank meets the bowl should be replaced. In your case, since the toilet is new, the existing gasket should be OK.

Next, clean and prepare the flange. Remove any large pieces of old wax from the bottom of the commode. Place the toilet over the bolts (this is a test fit, no wax at this time) in the position you like — there is a certain amount of play possible and it lets you align the commode with the wall. Hand-tighten the nuts on the bolts to secure the commode.

Now press strongly down on the edges around the bowl of the toilet and see if you can get it to wobble. If it wobbles, and yours will, install shims to stabilize the bowl. Long cedar shims sold in bundles as door shims serve best.

Install the shims with the tapered end pointing toward the center of the toilet, then break the shim to approximately the correct length so that not too much pokes out from under the toilet. Use as many shims as necessary to level the toilet. Give the toilet a final wobble test — press hard around the rim.

Then, without disturbing any of the shims, lift the toilet straight up off the bolts and set it right next to you. Place the wax ring onto the flange. We recommend the wax ring with plastic insert as this helps to keep the ring centered on the flange. Pick up the toilet and lower it straight down onto the bolts without twisting. Be careful not to move any of the shims.

Now install the plastic cap washer, the steel washer and the nut onto each closet bolt and tighten evenly. Don’t be a gorilla. On many marginal plumbing installations, it’s possible to pull the toilet flange up out of the floor.

Now test the toilet again for wobble and adjust the shims slightly if needed. Tighten the bolts one-quarter turn more, connect the water supply and test it with five or six flushes to be sure water doesn’t show up on the floor, or worse, downstairs. Use the point of a new utility-knife blade to cut the shims right at the base of the toilet. Finally, caulk around the base of the commode but leave about 4 inches at the back clear so you’ll see any leaks. Caulk serves a couple of purposes. Most important, it keeps the shims in place. It also hides the shims and adds a little extra “glue” to keep the toilet in place. Because caulk shrinks when it dries, you may need to do it again the next day.

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