A wet room on a first floor timber deck is the job people underestimate. The fall has to be formed in the deck itself before anything else happens — you cannot lay a flat floor and add a slope afterwards, whatever anyone tells you. That means cutting the deck, sometimes trimming between joists, and building the fall in. In a Victorian terrace where the joists are what they are, this is usually the part that takes the time, and it is entirely invisible when it is done.
Then the tanking, which is the actual product. The membrane goes over the whole floor, up the walls to shower height and beyond, bonded at every corner and every junction. On a ground floor a failure is a nuisance. On a first floor above a kitchen it is a ceiling. We do the tanking ourselves rather than subbing it out, because it is the one part of the job where nobody sees whether it was done properly until it is far too late.
Space is the other Altrincham constraint. These bathrooms are often the smallest room in the house, and the soil stack is where the Victorians put it, which is rarely where you want it. Moving a WC means the falls on the waste have to work, and there is a limit to how far you can chase a soil connection before gravity stops cooperating. It is usually solvable — it just needs deciding at design rather than discovering at second fix.
The upside: soft water. No scale on the screen, no white bloom on the taps, no softener needed. In a small bathroom where the shower screen is a large part of what you look at, that is worth more than it sounds.
