Why soaked subfloor stays weak
The Department of Energy's Building America Solution Center says hidden leaks can cause subfloor swelling, warping and rot, and that plywood and OSB that get saturated and then dry may not return to their original size. Water can also break down the glue between the plies of plywood, so the layers separate and the panel loses its structural strength. So a floor that firms up after a dry spell isn't necessarily fixed.
Where the water is probably coming from
Water on the floor at the base doesn't prove the seal under the toilet failed. Work from the top down and find the highest wet point:
- Supply line and shutoff. A drip at the coupling nut where the supply line meets the tank. Fluidmaster's instructions say to tighten that nut only enough to stop the leak, since overtightening causes its own problems.
- Tank bolts and tank-to-bowl gasket. Water running down from under the tank onto the bowl. Fluidmaster's troubleshooting says a leak there usually means loose bolts or a tank-to-bowl gasket that isn't centered and seated.
- The seal between the toilet and the drain. Water that shows up at the base, often right after a flush, with no drips above it. A toilet that rocks is a strong hint, because movement breaks that seal.
- Condensation. Beads of water all over the outside of the tank in a humid room, not one wet connection. EPA recommends keeping indoor humidity below 60% and ideally between 30% and 50%. It can wet a floor over time, but don't stop there if the floor is already soft.
Dry everything, then check again after the toilet sits unused and again after a flush. If you can't find the source, How to use the water meter to check for a hidden leak shows how to confirm water is escaping somewhere. A tank that sweats a lot often points to a bathroom fan that isn't moving enough air; see Why a bathroom fan runs without clearing condensation.
What to check yourself, and what not to
Stand on solid floor and look. Note how big the soft area is, whether the flooring is lifted, stained or has open grout, and whether the room smells musty. EPA says a musty smell is a reason to suspect hidden mold. Take photos so you can tell whether the area grows.
If there's an unfinished basement below, look at the underside of the floor near the toilet's drain pipe for water stains, dark wood or plywood layers pulling apart. Don't jump on the soft spot, rock the toilet or pry up flooring. If the floor sags visibly, keep people off it and stop using that toilet until someone has looked.
Who to call
Start with a plumber to find and stop the leak. Ask them to pull the toilet and show you the flange and the subfloor around the drain, and to tell you whether a new seal alone will hold on that floor. If the wood is soft, dark or separating, or the joists below look damaged, you need a carpenter or general contractor for the floor, and a structural engineer if anyone has doubts about the joists.
Questions that keep the quote honest:
- What's the leak source, and how did you confirm it?
- How far does the damaged subfloor go, and how did you check past the visible spot?
- Are any joists damaged, and will you sister or replace them?
- How will you confirm the remaining wood is dry before new subfloor and flooring go down?
EPA advises drying wet materials within 24 to 48 hours to keep mold from growing in most cases, and its water-damage guidance says to make sure the underflooring is dry before it's covered. If the water came from the drain side and you suspect sewage, EPA's cleanup guidance doesn't apply; that work belongs to a restoration company that handles contaminated water. EPA also suggests hiring a professional when mold covers more than about 10 square feet.
If the toilet itself is old or cracked, it may make sense to replace it while it's off the floor. Should I repair or replace a toilet that keeps having problems? walks through that call.
Philadelphia permit rules
The City of Philadelphia says no plumbing permit is needed to stop a leak or to remove and reinstall a toilet when no pipes or fixtures are replaced or moved. Installing a new toilet, or replacing or moving the flange and drain piping, is plumbing work that needs a permit, which a licensed plumber can pull for you.
New floor coverings such as tile or vinyl don't need a building permit, but the City lists major repairs that aren't part of regular maintenance as permit work, so ask your contractor whether joist or structural floor repairs need one. In Bucks and Montgomery County, check with your township, because rules vary by town.