Rotten Chipboard flooring

11 Apr 2014
Reaction score
United Kingdom

I have ripped out the shower in my flat because we discovered that it had been leaking for a fair while. The chipboard flooring underneath is rotten and needs taking up. The problem is that it runs under a stud wall.

I haven't cut anything back yet but I think that underneath the floor boards there is a layer of what looks like concrete about 1 inch thick. I can't tell at this stage if that runs all across the floor or just in strips. It is hollow/insulated under that. I can see it where ever the floor is cut (pipework, soil stack etc). Texture wise it is very much like a gypsum based product, crumbly and paper backed.

Anyone got any idea what this layer is?

Am I safe to cut the T&G floorboards back to the the edge of the stud wall and replace it with new T&g laid on top of this layer? Is this layer weight bearing?

Sponsored Links
My guess is that the "gypsum based product, crumbly and paper backed" board is just that - a gypsum board (very much like plasterboard) as a fire stop between you and the flat below (assuming you aren't the bottom flat). However, they've made a right pigs ear of fitting it - there shouldn't be any non-filled holes in it if it is a firebreak. 1" of plasterboard (or rather, the fireboard variant) is supposed to provide a 1 hour fire break - ie long enough to evacuate a building.

Your experience shows the problems with chipboard. It's supposed to be water resistant but if continually wet will swell, go soggy and sag (or even just collapse altogether !). Plus it tends to be used in a "lay the whole floor and then build the walls" mode so the floor panels go under the walls which (as you've found) is a right PITA. But developers/builders love it because the large sheets go down quickly (== cheaply) and they don't give a toss about future owners wanting to get underneath for maintenance.
Thanks for the reply. So, in terms of patching up and making good do you think that i should cut the chipboard at the base of the walls and replace with ply on top of the fire break?

The good news is that the holes in the floor don't lead anywhere important.... Only to the gas boiler!!
I'm not a builder ...
The main thing is making sure the edges are supported. So if (for example) there's a stud wall running parallel to a joist (and probably sat on top of the joist) then you'll find it difficult avoiding having a sheet that's overhanging with nearly a joist spacing unsupported. I think you;d need to put a load of noggins in to support the board in that case.

Other than that, yes pull out the old board (going under the edge of the stud wall as best you can) and fit some fresh. I'd consider going with either "real timber" or WPB (water and boil proof ?) board rather than chipboard that's going to fail again if you have another leak.

Hopefully this has bumped it up and someone will spot it and correct me if I'm wrong.
Sponsored Links
The real difficulty is knowing what is underneath the stud and where the joists run because of that layer of gypsum. Cut a couple of small test holes I guess and hope that it is a double joist!
Why dont you post your pics the right way up and indicate what the viewer is looking at - then perhaps you might be helped a bit further.

What i've read so far is guesswork, it sounds pretty good and it might be spot on but its still guesswork. Just saying.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local