Any suggestions / ideas? Only thing I can think of is aluminium checker plate. Ceramic tiles obviously but this is on top of ply flooring.

Ceramic tiles, job done.

Could do cement board topped with vinyl (or similar). Vinyl for waterproofing, cement board for fire. Not sure thats any easier than tiling though.

Rick's cement board plus vinyl idea is sound, but whatever you finish it with I would tank the floor first and run the waterproof layer up the walls a little. If a cylinder or a valve lets go overnight, that keeps the water contained in the room instead of soaking into the ply floor underneath. Just make sure the seams and the door threshold are properly sealed, because that is where the water always finds a way out.

Vinyl isn't fire resistant is it?

Tiles seems the pragmatic solution.

But as said above, water will travel a long way, under the door, through cracks etc.

If you are looking to contain any quantity, eg a bucket full, then it needs a threshold and 'tanked'. An industrial job. Fibreglass?

13 hours ago, Alan Ambrose said:

Any suggestions / ideas? Only thing I can think of is aluminium checker plate. Ceramic tiles obviously but this is on top of ply flooring.

If the ply is held down very well, tank the floor and up the walls about 50mm or more, then just pour in some 2-part SLC.

About 10-12mm of it (minimum) if a cylinder is going in there, 15-18mm if you can, and then paint it with garage floor paint.

On my current clients build I've done A1 cement boards to walls and ceilings; "dry" MVHR plant room was intumescent caulked and painted, "wet" plant room is same but will be tanked as much as possible up the walls, and down to meet the SLC (poured over the recess in the concrete (MBC raft)), then a 2-part garage floor paint/ other to protect the tanking.

There's a scupper in there to discharge any water direct to the 4" foul.

Why does it need to be fire resistant? Or do you mean fireproof?

Ditto water. Proof or resistant?