Guide
Tanking and waterproofing bathrooms on timber floors
In a converted house or a mansion flat your bathroom floor is somebody else's ceiling, and timber moves.
Why timber changes the problem
A suspended timber floor deflects under load, and it moves seasonally as moisture content changes. Tile and grout do not. The junction between them is where failures start, which is why waterproofing in this stock is treated as a system with movement designed into it, rather than a coat of something applied before tiling.
In practice that means a rigid, decoupled substrate rather than tiling straight to floorboards, a waterproof membrane taken up the walls to a defined height, sealed corners and junctions, and a proper detail where the floor meets the wall. Nothing about that is exotic; it is simply not optional here.
Where the water actually goes
Most leaks that reach a neighbour are not dramatic. They are slow, and they come from the same handful of places: the joint between a shower tray and the wall, the perimeter of a bath, a poorly formed corner in a wet room, and the seal around a waste. Each is a detail rather than a product.
Half-landing bathrooms in converted terraces deserve particular attention because they sit directly over a shared hallway or another flat's room, often on the shortest joist span in the house, and because they were rarely designed as bathrooms in the first place.
Wet rooms: where they are honest
A level-access wet room needs fall across the floor to a drain, and on a timber floor that fall has to be built into the structure or the floor build-up. Where joist depth allows it to be formed properly, a wet room is a good answer. Where it does not, forming one means notching structure, and that is a different job from a bathroom refit.
The alternative is a low-profile tray, which achieves most of the look with none of the structural argument. We will say which the floor supports at survey rather than discovering it once the room is open.
Testing before the room closes
Waterproofing is tested before tiling, and the wet areas are tested again before decoration closes around them. It is a slower sequence than tiling straight over and hoping, and it is the only way to find a problem while it is still cheap.
Where a managing agent asks for evidence as part of a licence to alter, this is usually what they mean: the system specified, and confirmation it was tested. It is worth doing whether or not anyone asks.
Questions
In a shower area, yes, whatever was there before. Older bathrooms in this stock frequently had no membrane at all, which is why we so often find damp timber under a tray that looked sound. Replacing the fittings and leaving the substrate as found means the next failure is only a matter of time, and it will not be your ceiling it lands on.
Where the stack allows it, keeping wet rooms stacked is genuinely the easier engineering, and we will usually recommend it. What we will not do is force a layout to align with a flat we have not surveyed. The connection that matters is to the stack and at a workable fall, and that is measured on your floor rather than inferred from the plan below.
The work this applies to
Where this comes up
Related guides
- Renovating a mansion flat: lifts, risers and common parts
A purpose-built mansion block is not a large flat in a converted house. The building itself sets most of the terms.
- Licence to alter: kitchens and bathrooms in leasehold flats
Most flats in North West London are leasehold, and most kitchen and bathroom work in them needs the freeholder's written permission before anything is taken out.