Levelling subfloor in upstairs bedroom

Question

Hi guys, first time posting here.

I'm having a bedroom in a council rented house completely gutted and the subfloor is in very bad state. Damaged and very dirty, I can see noticeable dips in the floor, it's nothing crazy bad but certainly wouldn't be suitable for a hardwood floor to go on top of my naive brain would think.  

I'm wondering how much or how I'd even go about getting it fixed? Would the whole subfloor need replaced? I've always had wonky floors  in my house and I'm sick of it, since this room is gutted I want it done proper from the get go so I'm not rolling about the room like I'm on a rollercoaster.

Since it's so dirty I thought just replacing the whole floor might be worth doing instead of cleaning it and then getting it *fixed* but I've no idea what that really entails. Any suggestions welcome.

Mark · Answer

Hi if its a concrete floor it may be possible to use a self levelling compound there are plenty of You tube videos to help you for guidance but try and use a  flexible deep fill if floor is out more than 10mm use a level onto floor 1st and chisel out any high spots  then clean and prime with SBR

Arman · Answer

ou’re absolutely thinking along the right lines — if the subfloor is badly damaged, dirty, and noticeably uneven, now is definitely the time to sort it properly while the room is gutted.

Here's the honest breakdown:

If the dips are minor (say, less than 3-5mm over 2 meters), you could sometimes fix it with a combination of cleaning, sanding, and using a self-levelling compound over the existing boards or plywood.

BUT, if the boards are really damaged, rotten, sagging, or filthy (which it sounds like they are), the best and most proper way is to rip out the old subfloor and replace it.

You’ll want to check the joists underneath too. Sometimes in older council houses, the joists can twist or sag slightly — if that's the case, you might need to sister them (attach new wood alongside the old) or pack them to get a truly level base.

Steps I'd recommend for a proper job:

Pull up the existing boards (chipboard, old ply, or original planks — whatever it is).

Inspect the joists for level and integrity.

Plane, sister, or pack the joists to create a dead flat level surface.

Lay new 18mm or 22mm moisture-resistant tongue-and-groove chipboard flooring (standard for upstairs subfloors).

Screw it all down tight (no nails — screws prevent future squeaks).

My opinion:
If you’re already gutted, replace the whole subfloor. It’s not a massive cost compared to the peace of mind you’ll get — and hardwood floors NEED a dead flat surface, or you'll regret it with creaks, gaps, and warping. Cleaning or patching a badly damaged subfloor is a short-term fix, not a proper solution, especially for nice flooring like hardwood.

Quick cost guide:

Materials (boards, screws, glue, maybe some timber for joists): around £200–£400 depending on room size.

Labour (if hiring someone): £600–£800 depending how tricky the joist work is.

If you DIY, it’s very doable if you're reasonably handy.

Would you like me to sketch out a simple plan you could follow if you want to DIY it? (It’s easier than most people think if you take it slow.)

Martin · Answer

Hi Ross, sounds like you’re doing the right thing getting it sorted properly from the start.

If the subfloor is badly damaged, dirty, and has visible dips, you’ve got two main options:

1. Replace the Subfloor Entirely
If it’s chipboard or old timber and beyond saving, best to rip it up and start fresh. That way you can:

Inspect joists underneath for rot or damage

Re-level properly using new timber or plywood

Avoid future headaches (especially if you want hardwood or engineered flooring on top)

2. Overlay & Level (Only If Structurally Sound)
If the subfloor is still solid and just uneven, you might be able to:

Clean and treat the existing boards

Use self-leveling compound (if suitable for upstairs timber)

Overlay with plywood or backer board for a flat finish

But to be honest — if it's as rough as you say and the room is already gutted — I’d lean toward full replacement. It'll cost more in labour and materials but saves messing about later.

Expect to pay around £40–£70 per sqm depending on your area and condition — but get a proper quote so there’s no surprises.

Hope that helps mate

George · Answer

Hi there 
It all depends on how much the floor is uneven if it’s sagging in the middle it’s the jousting that is hanging from constant weight carrying. If you are going to replace the old floor boards , you can screw strengthening on to the side of the original joists and straighten it at the same time then cover it with chipboard flooring

Adam · Answer

without removing the joists my advice would be to plyline the floor and use a self leveling compound

Levelling subfloor in upstairs bedroom

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