Chipboard hole in ensuite.

The chip board which was under laminated in our ensuite has some water damage in one small area where the shower was. I want to lay karndean on the floor.

Do I need to rip up all the floor in the room, or can I somehow patch the hole, which I made bigger to find the problem? It's a small area, but the surrounding area has swelled up a bit.

The problem I see with ripping up the flooring is that it appears to go underneath joists/battens into the next room, and it appears to be tongue and grooved chipboard, so would be hard to take out and then re-lay?

Could I put marine plywood over the top and then karndean on top of that?

Any advice much appreciated.

yes will be fine with quality plywood on top. 9mm SP101 or FG1 or similar. Don’t use Any ply manufactured in china.

Would it be better to use cement board? What should I do about the hole and the surrounding area which has slightly bulged? If I cut it out how could I put something in it's place because there's nothing to screw it onto?

How much is the bulge, and would anyone feel the bulge under their feet?it’s close to the wall.

if it is just the hole, you could simply apply filler or make the hole more regular and plug the hole.
Easiest way would be to drill a bigger hole with a hole saw and use the same saw to make a round plug.
Then use a bit of string to drop a bar across the hole, inside, and screw into it from above,then drop in the new disc.
But if you are overlaying with ply it wouldn’t matter

if a bigger area is bulged, replace the area with new rather than somehow shaving the bulge.

Thanks for your message. It's really not that big a bulge tbh. I think I will just fill, sand and lay plywood. Would cement board be better?

Raising the floor level could give you knock-on problems.
Leaving swollen areas in the floor, even with a ply or cement board overlay, might mean you'd have problems laying tile for instance - or installing a new fixture.
Theres also a repair patch on the wall perhaps connected to more water damage, and more water staining further along the chipboard.

Why not cut and remove the chipboard two or three joists back from the wall and open up the patch in the wall for investigating?
Once the floor is up post pics on here for how to do the next steps.

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