This is a definite sign that there is a rot issue under that area of the flooring.
Bathroom floor wood rot.
Subfloor damage in the bathroom usually comes from a leaking toilet but it can also be the result of a persistent sink drain leak or even a drip behind.
Pick a time to do it when the bathroom traffic will be lighter like during the summer months when people are outside more if possible and get started.
If the remaining solid wood is wet ventilate the area and allow it time to thoroughly dry before installing replacement materials.
Sure it will take you some work but the end result is well worth the effort.
If only the area directly below the flange is rotted you can install a two piece steel closet flange support.
It goes under the flange and transfers the load of the flange and toilet and you out onto more solid surrounding wood.
The homeowner had recently completed some plumbing upgrades which took care of the water source for the rot but the floor damage was already done.
A step by step video of how to repair the floor under your toilet replace it put new flooring and set the toilet etc.
Exposure to moisture invites the onset of mold and mildew which has a damaging effect on wood.
The only sure way to find out is to remove a section of the wallboard and check the wood behind.
Use a circular saw set to 1 depth to cut the subflooring back beyond the damaged areas until you reach solid structural framing.
In the worst case scenario the damage is so severe that the floor gives way sending the bath tub toilet or shower stall crashing down to the floor below.
You will need to pull up the flooring and repair or the spot will.
Look at the floor surface in the room.
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When you discover a section.
If you find mold growing the wood floor plates behind the wall are at risk of wood rot.
Use a pry bar to remove any protruding nails and vacuum up the demolition debris.
Rotted bathroom floor if you find that your bathroom floor has rotted through you will need to replace it.
When moisture gets under the tub or shower basin or between the shower liner and the wall it encourages mold and mildew to form which eventually leads to wood rot in walls and the subfloor.
But if your floor is severely rotted say more than an inch beyond the flange you re stuck replacing the flooring around the toilet.
Dry rot involves less water but the destructive fungi that cause dry rot still need some moisture to do their wood fiber eating thing.
If there is an area that has not had a spill which caused a spot then it is likely that the wood in the subfloor is rotting.