How to Fix a Rotting Floor Under a Toilet

Turn off the main water supply valve at the back of the toilet. Loosen the bolts securing the tank to the toilet. Drain the water by flushing the toilet, and then remove the tank. Loosen the floor bolts securing the bowl to the floor, and then lift the bowl from the floor.

Inspect the flange for damage. Leave it in place if it’s not damaged. If it is cracked, remove the damaged flange and replace it after repairing the subfloor.

Pull up the flooring. Most flooring tiles and laminate boards are pulled up with a hammer and pry bar. Vinyl flooring may be cut and removed in sections with a utility knife. When you see the moisture damage’s extent, cut out the rotted section of underlayment and subfloor past the rotted wood into the good subflooring up to the closest joists on each side using a handsaw or a reciprocating saw. Do not to cut into the joists beneath.

Inspect the joists. If they are moist, leave the floor open until they dry, and then cut two 2-by-4s so that each extends at least 1 foot past the wood that was exposed to the moisture on each joist.

Nail the 2-by-4s along each of the joists, ensuring the top is flush to the old joist.

Measure the distance between the joists and cut two 2-by-2-inch pieces of lumber that fit this distance. Nail one 2-by-2 to each of the sides adjacent to the joists. These provide extra support for the new subfloor.

Measure the opened area of floor with a measuring tape, and then cut a piece of plywood and underlayment the same thickness as the old material.

Measure the outside diameter of the drain and the distance from the edge of the opening to the drain on all sides. Use these measurements for marking the drain’s location on the plywood and underlayment.

Cut the opening for the drain hole out of the plywood and the underlayment using a hole saw. If you have not removed the flange, cut the plywood and underlayment in half, so that the cut intersects the hole.

Fit the plywood into place so that each side fits beneath the flange’s lip. Fasten it to the floor joists by drilling drywall screws spaced about 10 to 12 inches apart through the plywood into the joist.

Install your underlayment so that the hole you’ve cut fits around the drain. Fasten this to the plywood using nails spaced 6 to 8 inches apart. The closer spacing of these fasteners prevents the underlayment from buckling from temperature or moisture changes.

Filling cracks, nail dents or holes with wood putty or floor leveler ensures the surface is smooth. Install your new flooring.

Scrape the old wax ring from the toilet bowl’s bottom with a putty knife and set a new wax ring over the bowl’s bottom. Press it into place firmly.

Reinstall the toilet and turn the main water supply on checking for leaks. Seal the gap at the toilet’s base between the bowl and the flooring with a bead of latex silicone caulk.