Bathroom over a bomb shelter

My house was built during the war by the Post Master General and incorporates what looks like a bomb shelter inside which has been filled in. I want to be sure that the new bathroom I am building is absolutely water tight.

The external walls are 400mm solid engineering brick with no cavity, stripped back to bare brick.

With exception of one wall being block with a lime mix render, plaster, plaster skim, and paint, the internal walls are brand new bare block construction.

There are 2 bathrooms to address, tiled top to bottom, in windowless rooms, but with so much going on, I'll just stick to one room at a time for now!


I'll start with the "bomb" room which is currently completely bare, not even any plumbing yet.

The area used to be an oversized 1800 x 3000 hallway which I have built a block wall the make it into a room. That new wall is directly over the shelter doorway lintel so more than safe!

The Floor. The area is literally the covered in steps down to the shelter which has been filled in with rubble, a DPM sheet inserted and then a really rough 300mm deep concrete slab on top. The original hardwood framing is still around the edges with the iron railings cut off to ground level.

Would I be correct to seal the floor with 1:1 mix SBR, level it with Mapei Ultraplan 3240 Fibre reinforced compound, then use a liquid waterproofing barrier prior to tiling? This room will have bath with a shower over it at the end.

The 3 sides of the bath/shower area are all different...bare brick at the shower end, rendered and painted wall at the other end, and studded block wall to the long side of the bath with all the pipework in it. Do you think I should just invest in simply covering the studwork and bare brick with Marmox insulated tile backer board, or am I skipping a step here? ... do I marmox the brick wall, and cement board/liquid membrane the stud wall? What should I do with the painted wall? Simply heavily key it to get the paint and any extra skim layer of plaster off, add some extra liquid membrane over the keyed "wet area" and tile it?
The new block wall in non wet area could be just scratch coat rendered and tiled, however, with the nature of this house, I really want to avoid any issue with moisture so I’m looking for advice on the best way to deal with this.

Have I planned anything right?!

All sounds like you've done your research and got it all almost spot on... the only thing about your plan I wouldn't trust is the painted wall. Ideally, this should be chipped back or overboarded.

If you only have a small wet area over the bath, marmoxing the whole room is probably a little more than is required. I would personally put cement boards all round, either screwing to the studs or using a combination of tile adhesive and mechanical fixings into brick/block walls and just put a tanking membrane in the shower area and round the bath. I'm not saying you can't use marmox, on the contrary. But it would be a belts and braces approach that would make the whole room perfectly water-tight, providing you put a waterproof tape over all the joints and screw fixings, and put collars over any pipework. These boards also would not require any additional tanking membrane.

Also with the concrete slab being that thick, it may be prone to shrinkage cracks. Also with the wooden frame, you may want to consider priming and levelling as you have suggested, but then installing a decoupler like Ditra matting before tiling. That way if there is any lateral movement between the wood and concrete, or if the slab forms hairline cracks, the tiles will remain unaffected.

Thank you for replying Paul

With the floors,
I have absolute full confidence that the timber is going absolutely nowhere, it's a thin frame extremely heavily bolted down, and the concrete has been down since 1991 at the absolute least, but if the Ditra matting is waterproof, and thin, so won't raise the floor too much, I can skip the liquid membrane on the floor after sealing and leveling.. yes?
The floor in the other bathroom is mostly thick quarry tile and partially onto the bomb shelter "floor" with a raised walk in shower at the extreme end, so I assume I treat the floor the same, including under the shower construction?

With the walls,
In this bathroom... ok, I will go for the cement board, mechanically fixed, and waterproof necessary areas only as suggested.
The only query I have is "chipped back"... do you mean just the plaster (as best I can), or right down to the block to essentially replace the render with cement board? Would you recommend I seal the bricks and blocks before dot dabbing mechanically fixed cement board to them?

The other bathroom is a little different. The external bare brick walls will require more extreme insulation and damp proofing, with the added bonus of a walk-in shower so Marmox all day long in there to the wet walls in entirety. The slate bed damp course in the 400mm external walls has failed so Dry rods are being added outside and it will get the thickest insulated solution I can afford.

If you would just help me dot the i's and cross the t's on this, I would appreciate it enormously.