Two bathrooms exist side by side in this neighbourhood. In an original cottage or bungalow it is the Chicago standard: five by seven, tile in a mortar bed, a cast-iron tub in an alcove and galvanised supply behind the wall.
In a house from the rebuild boom it is a generous primary bath with a double vanity and a glassed shower — and, very often, a shower pan formed without enough fall and a fan that recirculates into a ceiling void.
Only one of those is a rescue job, and it is usually the newer one. A hundred-year-old bathroom is honest about its age. A fifteen-year-old one that looks fine can be moving water through the substrate, and that is the failure worth finding early.