A flooded bathroom floor is the loss that looks smallest and is most often underestimated. Bathrooms and kitchens sit on assemblies full of joints, penetrations and absorbent material, and the water that spreads across the tile for ten minutes is a fraction of what has already gone through the floor. The useful inspection is almost never in the room that flooded; it is in the ceiling of the room below and in the cavity between them.
Where bathroom flooding water actually goes
Straight down, through grout lines, through the gap at the perimeter, around the toilet flange, around pipe penetrations and into the subfloor. Tile and vinyl are not waterproof assemblies; they are water resistant surfaces over a floor that absorbs. In a kitchen the same is true around the dishwasher, the sink base and the refrigerator line, which is why dishwasher flooding so often shows up as a stain on a ceiling downstairs rather than as a puddle in the kitchen.
After a bathroom flood, read the room below, not the room above
Within hours of a bathroom flood the ceiling underneath will tell you the extent: a stain, a sag, a wet light fitting, or a hollow sound when tapped. Check the joist bays with a meter through the ceiling if you can, and check the subfloor edges where they meet the wall. A bathroom that is dry on top and a ceiling that is wet below means the assembly between them is holding water, and that assembly does not dry on its own with a fan pointed at the tile.
What has to come up
Vinyl and sheet flooring over a wet subfloor almost always comes up, because it traps moisture underneath. Tile over a sound substrate frequently stays if the cavity can be dried from below, which is one of the few arguments for opening a ceiling rather than a floor. Particle board and chipboard subfloor that swelled is finished and is replaced. Cabinetry with a particle board carcass that wicked water at the base is usually a write off even when the doors look perfect. Underlay, pad and any insulation in the cavity come out.
Why bathroom water damage becomes a mould job
Because the cavity stays wet and nobody looks. The room dries, the tile looks fine, the ceiling stain gets painted, and the joist bay and the subfloor keep a moisture content high enough to grow mould for weeks. The signature a month later is a musty smell strongest at floor level, a stain that returns through the new paint, and growth at the base of the wall in the room below. The prevention is one meter reading through the cavity on the day it happens.
Questions people ask about flooded bathroom floor
My bathroom floor flooded but it looks dry now. Is it fine?
Check the room below and the cavity between. The surface dries first and the subfloor and joist bay last, and that is where the mould problem starts if the assembly is left wet.
Does tile have to come up after a flood?
Often not, if the substrate is sound and the cavity can be dried from below. Vinyl and sheet flooring usually do come up because they trap moisture against the subfloor.
After a kitchen flood, why is the cabinet ruined when the doors look fine?
Particle board carcasses wick water at the base and swell irreversibly. The visible doors are frequently solid or faced and survive, while the structure behind them has failed.
Do I need to open the ceiling below?
Sometimes, and it is often the cheaper option. Opening the ceiling lets the cavity dry and the insulation come out without lifting a tiled floor that is otherwise sound.