Why a Bathroom Floor Goes Soft

The Short Answer

Tile Does Not Go Soft. The Material Under It Does

When a bathroom floor feels spongy underfoot, the tile is not the soft part. Ceramic and porcelain are fired hard. What is soft is the layer under the tile, and that layer is usually the subfloor, the backer board, or the framing below it. Water got in, the material it soaked into lost its strength, and now the floor moves when you step on it.

The Two Questions That Decide It

So the useful question is not what is wrong with my floor. It is where did the water come from, and is it still coming. Those two answers decide whether this is a repair or a rebuild, and they decide what it costs.

Causes

What Causes a Bathroom Floor to Go Soft

There are five common sources. More often than not, more than one of them is going at the same time.

• A shower with no waterproofing behind the wall. In a house built before pan systems became standard practice, a shower was tile set directly on the wall. Water goes through the grout, hits the framing, and rots it. The floor gets soft from the splashing and the running water.
• A tub surround that has failed. The tile and grout on a tub surround are not waterproof. When the silicone at the tub joint goes, water gets behind the tile and runs down the wall and into the floor below it.
• A toilet that leaks at the base. A slow leak around the wax ring or the flange wets the subfloor right where nobody looks, and the floor softens under the toilet and out into the room.
• A supply line that has been dripping for a long time. This one is slow enough that people put a towel down and move on, and the subfloor goes soft over months.
• A slab that was tiled over without a membrane. On a poured basement or slab floor, tile with no waterproofing and no isolation membrane can pass water straight down into the slab and the subfloor below it.

The Seasonal Version Of A Soft Floor

There is also the seasonal version. A floor that is soft in the spring and firmer in the fall is usually telling you the ground under the house is wet and the framing is picking up moisture. That is a crawl space or grading problem rather than a bathroom problem, and it needs to be looked at as one.

Diagnosis

How It Gets Diagnosed

Nobody can diagnose a soft floor from the doorway. The order below is the order we work in, and each step tells you what the next one has to look at.

1. 1.Find the extent of it. Walk the floor and press down along the edges of the soft area, the tub side, and the toilet side. The soft area is usually bigger than the part you noticed.
2. 2.Look for the source of the water. Check the toilet base, the supply lines, the tub joint, and the shower. Look at the ceiling in the room below for a stain. A soft floor with no visible leak usually means the water came in a long time ago and dried up.
3. 3.Read the grout and the tile. Grout cracking in a straight line across the floor points at movement in the structure. Scattered cracked tiles point at water getting under the floor and lifting them.
4. 4.Take up a sample. One tile lifted at the edge of the soft area shows what the subfloor looks like, whether the backer board is wet, and whether the framing under it is rotted. That is the step that turns a guess into a finding.
5. 5.Check the framing for rot and for the level of the floor, and check for the deflection that lets a floor move under load.
6. 6.Check whether the leak that caused it has been fixed. If it has not, whatever gets built on top of it gets damaged the same way.

Repair or Rebuild

What Can Be Repaired and What Has to Be Rebuilt

The line between a repair and a rebuild is usually found by lifting one tile. That is why we do it before the estimate rather than after the demolition, because it is the difference between two very different scopes and nobody should have to find out mid-job.

Warning Signs

Warning Signs to Look For

• The floor moves or gives when you step on it, especially near the tub or the toilet.
• Grout lines that are cracking in a straight line across the floor.
• Individual tiles that sound hollow when you tap them, in a pattern rather than all over.
• A tile that has started to lift or rock at the edge.
• A cold or damp patch near the tub, the toilet, or the shower threshold.
• A water stain or a bulge in the paint on the ceiling in the room below.
• A bathroom door that has started to catch on the floor, or a door that no longer closes square.
• A floor that is soft in the wet season and firmer when the ground dries out.
• A musty smell in the room that comes and go.

One warning sign is worth pulling the whole job forward. If there is a stain on the ceiling in the room below a bathroom, water is coming through the assembly above it. That is the point where a shower pan, a tub, or a toilet needs to be looked at rather than the floor.

What to Do Next

What to Do Next

Do not pull the tile yourself to look. Lift one tile at the edge of the soft area, take a photo of what is under it, and put a piece of cardboard or a sheet of plywood over the hole so nothing walks into it. That photo is enough for a contractor to tell you whether you are looking at a subfloor problem or a framing problem, and it costs you a few dollars.

Fix The Leak First

Then get the source of the water fixed before anything else gets scheduled. If the leak is still active, a new floor built over it fails in the same way, and that is the most expensive version of this problem.

What The Estimate Covers

On the estimate we tell you what we found under the floor, what the repair involves, what would make it a rebuild, and what has to happen to the leak first. If there is framing work in it, we tell you that on the estimate and you can get the trades lined up.