Leaking Toilet Base: Subfloor Damage You Can't See

A toilet leaking at the base causes subfloor damage because most of the water never reaches the surface. When the wax seal under the toilet fails, a little water escapes with every flush and soaks into the wood around the drain opening, under the flooring where you cannot see it. By the time water shows at the edge of the toilet, the floor underneath has often been wet for months.

Why a toilet leaks at the base

A toilet sits on a flange, which is a ring fastened to the floor at the top of the drain pipe. A wax ring or a rubber seal is squeezed between the two, and two bolts hold the toilet down. The seal only works while the toilet stays perfectly still.

• Failed wax ring. Wax does not spring back. If the toilet shifts even slightly, the seal opens and stays open.
• Loose bolts or a rocking toilet. Movement breaks the seal, and the leak then makes the floor softer, which lets the toilet rock more.
• Broken or low flange. A cracked flange cannot hold the bolts. A flange that sits below the finished floor, common after new tile is laid over old flooring, leaves the wax too thin to seal.
• Cracked bowl or base. A hairline crack in the porcelain seeps slowly.

Not all water at the base comes from the seal. Check these first, since they are simpler:

• The supply line and shutoff valve at the wall.
• The bolts and gasket between the tank and the bowl.
• Condensation. In humid summer weather a cold tank sweats and drips down the outside of the bowl to the floor.

To sort it out, dry everything and lay strips of toilet paper around the base, under the tank and under the supply connection. Wait an hour, then flush several times. Paper that is wet before flushing points to the supply, the tank or sweating. Paper that is wet at the base only after flushing points to the seal.

Signs of subfloor damage from a toilet leaking at the base

• The toilet rocks or shifts when you sit down.
• The floor feels soft or spongy when you press with a foot beside the toilet.
• Sheet vinyl has turned grey, yellow or dark around the base, or has lifted and bubbled.
• Tile grout near the toilet is cracked, dark or loose, or tiles have lifted.
• A sewer smell that returns after cleaning.
• A stain on the ceiling of the room below, close to under the toilet.

The ceiling below is the most reliable clue in a two-storey house.

Why this water is treated differently

Water that escapes at a toilet seal has been through the bowl. It is not clean water, and restoration practice treats water contaminated with sewage as the most serious category. That changes what can be saved. Hard surfaces can be cleaned and disinfected. Porous materials that soaked it up, such as carpet, underpad, particleboard underlay and saturated drywall, are generally removed instead of dried in place. Wear gloves when cleaning up, wash well afterward, and keep children and pets away from the area.

What the repair involves

The plumber's part is to shut off the water, drain and lift the toilet, scrape off the old wax, inspect the flange, repair or raise it if needed, and reset the toilet on a new seal with the bolts snugged evenly. Ask them to stop before resetting if the floor under the toilet is wet or soft, because a new seal on a rotten floor will fail again.

With the toilet off, the floor can be checked properly. A moisture meter shows how far the wet area extends under the finished flooring, which is usually wider than the stain. Sound plywood that is damp can be cleaned and dried until it reads under 16% moisture content. Subfloor that is soft, delaminated or black is cut out back to solid wood, and the joists below are checked and dried. Any mold on the framing is removed, in line with Health Canada's guidance that indoor mold should come out whatever the type. Water damage jobs typically run $1,950 to $11,000, depending on how far it spread.

We do the moisture mapping, removal of contaminated material, cleaning, drying and mold work. Replacing the subfloor section and the finished flooring goes to a contractor.

About caulking the base

Caulking around a toilet keeps mop water and spills from getting under it and helps steady it. It does not stop a leaking seal. It only hides one, and the trapped water then has nowhere to go but down. If you caulk, do it after the seal is confirmed sound, and some plumbers leave a small gap at the back so a future leak can show itself.

What to do now

Do the toilet paper test today. If the seal is leaking, stop using that toilet if you have another, or shut the supply valve and flush once to empty the tank between uses. Press around the base for soft spots, check the ceiling below, and take photos. Then have the toilet pulled and the floor checked before a new seal goes on.

Common questions