Rotten Chipboard floor in kitchen but can't find any joists?
james jefferies 24/10/2025 - 8.02 AM
Hi there, we're laying some new flooring in our new flat. The kitchen floor was lino directly laid onto the chipboard subfloor. The previous owners must have got water on the floor because there are sections of the subfloor that are expanded/rotten.
My plan was to cut out the damaged sections as far as needed, replace with new board, and attach the new board by screwing it into the joists. I cut a small hole in the chipboard to measure what thickness I needed, and as soon as I got through the chipboard, there is about 1 inch of insulation, and then a concrete floor (we're a first floor flat).
So then I cut another small hole in a join between two chipboards, expecting to find a joist, and nothing. Then I cut a hole in a board join running perpendicular to the last one and still no joist?! So what is holding up the chipboard sheets? Is it laid directly onto a compressed layer of insulation? If that's the case how do I remove and secure new chipboard in place of the rotten bits? Or is it easier to just board over the entire thing with a thin layer of ply? I'm apprehensive to do this because it will add a lot of extra labour and cost, and isn't actually fixing the problem just covering it up. There is quite a lot of movement between the chipboard sheets, so I need to stabilise it all before I can put down decoupling membrane and then tile. Any advice would be massively appreciated!
Are you a tradesperson and able to answer this question?
Hello,
The chipboard is laid directly over insulation, forming a floating floor system — common in flats with concrete subfloors. There are no joists; the chipboard is supported by rigid insulation beneath.
If any section of chipboard is rotten or no longer structurally sound, it must be replaced. I recommend cutting out a section larger than the visibly affected area to ensure the new board can be securely bonded to solid material.
Movement between chipboard sheets is normal in floating floors. Before installing any floor covering, the standard practice is to overlay with marine plywood (thickness depends on the final floor finish), laid in a brick-bond pattern. Fix the ply using screws every 9 inches.
For best results, I also recommend gluing the plywood to the chipboard using F3 adhesive before screwing. This ensures a strong bond and reduces movement.
Depending on the final floor covering, additional prep may be required. For a premium finish:
• Feather-edge the joints in the plywood
• Apply a primer
• Use a flexible, fibre-reinforced water-based levelling compound
This system is designed to accommodate minor subfloor movement and provides a stable, long-lasting foundation for tiling or other finishes.
Hope this helps
DSBrown flooring
I’m assuming your flat was built or refurbished in the 90s or after ?
With that in mind I would say it is a floating floor above concrete with an isolation of some order.
Caber floor comes in 2400x600 sections and will have been glued together. You’ll have to find where the joints are and cut out a section to replace the damaged board or boards.
My shout would be to get a joiner in to do the job unless you’re confident you can extract without damaging the tongue and groove. These are important to be retained on boards that’ll stay so that any new board can be stuck down to it with PVA that is D4 rated. Two types exist and I would go with the one that expands to fill any gaps.
