We are about to lay engineered wood floor in our lounge but after ripping up the existing carpet have found that a large area of the floor is not level. I have lasered the room and marked out the height differences and it appears to be sagging in the middle of the room, probably due to the joints. The biggest height difference is 10mm.

The existing floorboards are nailed down and in good condition. I have tested a section of the engineered wood and the level difference is too much and I know it will cause issues, even with underlay. We don’t want to overboard with ply as we want a level threshold with the hall. The current floor is a suspended timber floor on the ground floor with 20mm floorboards.

Is the best option to rip up the existing floorboards, sister the sagging joists to make them level and then fit new floorboards/ply. I don’t think I’ll get the existing floorboards up with damaging them.

How much levelling are you talking about – can you use something like this?

ARDEX FA 20

Edit: The standard mix is suitable for applications from 3mm to 10mm. For thicknesses between 10-20mm on to cement/sand screeds or concrete, add up to a third by volume of ARDEX Coarse Aggregate.

10mm isn’t much but I don’t know if you can’t build it up with more than one layer, worth enquiring I would have thought. Also you might find other similar products which allow greater thicknesses. Worth looking at that option imo as it has to be a lot easier, quicker, and cheaper, than stripping it all and fixing timber to the joists to give a level finish.

Generally 3-10mm are approx 8m2 but in places a screed would just fall through the gaps in the floor boards.

in places a screed would just fall through the gaps in the floor boards.

Can you skin it with eighth ply?

Yes, could do for some of it.

Is this the better option rather than addressing the joists?

Do you have insulation between the joists already? I can feel the cold coming up from our ground floor rooms. I keep contemplating lifting the boards to add insulation, another thing to think about when considering options.

Well it has to be a lot quicker and cheaper!

I don’t understand what you mean by “sister the sagging joists to make them level”. The only way I can think of getting a level finish with the existing joists is to fix level timber on the side of them.

in places a screed would just fall through the gaps in the floor boards

How big are the gaps? Caulk the gaps. This doesn’t necessarily mean with some stuff from a tube can be with some old rope tapped in. Old school on boats is hemp and linseed pushed in with a calking chisel but it doesn’t need to be water tight just enough to stop the screed falling through so some odd bits of rope poke in would do.

Yes, I mean bolting a timber to the side of the existing joists to bring the floor level.

No insulation in the floor, just a big vented void below.

I did similar in my bathroom but by adding a bit more material to the top of the joists. 306 laser level aligned to the highest point then went through putting a bit of 2×3 on top, marking where the laser was, then cutting on the bandsaw. Boards back over the top (insulate while you’ve it all up). 1930s house so it was about 20mm off at worst.

Had this recently on our wonky 1930’s place.  Ended up pulling all the boards, fitting wedges ontop of the joist to level, then kingspan and ply sub floor and feather edge the joints  Its not perfectly level but its good enough for flooring.

If you start pulling stuff up give yourself time to deal with the fun you will find.  We had some woodwormed joists presume where the old sink had leaked continusly and an old steel gas(?) pipe that was notched into the joists but somehow was higher than the actual joists so was causing a ripple across the whole floor.  Right bugger cutting that out and removing it.

Also worth checking the condition of any pipes and wiring whilst you are at it.  You don’t want to be needing to replace those after you;ve put your nice flooring down

Depending on the room it might give you some options for doing some stuff – in our utility I built a 5″ reinforced platform under where the washing machine and tumble drier were going to be to try and cut down on some of the noise / vibration.  Also a great time to fit a floor safe in a secret squirrel spot

The biggest height difference is 10mm.

Sorry I hadn’t read your OP carefully enough, 10mm really isn’t much! “Bolting” extra timber to the existing joists sounds like a bit of an overkill in IMHO. Just stagger screwing with decent screws some 2-4inch timber should be ample. The problem isn’t that the joists are weak but that they aren’t perfectly straight.

If it is just 10mm I would just resolve the issue with some self-leveling gear. Deck it all out with eighth ply first if you are worried about gaps between the floorboards.

10 mm?  does that count as a wonky floor*?  I’m suprised the flooring will not accommodate that.  Taking up all the boards will be a difficult job and may well uncover more issues.  I’d use the self levelling stuff personally I think

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.* my living room floor has nearer 100mm of height differnce across a 4.5m room

I leveled out an uneven floor with self leveling compound. There weren’t many big gaps between the boards but where there were I used pipe insulation to fill the gaps.

Deeper dips were filled with off cuts of Hardie backer board and screwed down. Then I poured the compound in layers slowly filling in the dips.

Worked pretty well.

For 10mm I’d look at overboarding the main area with thin ply then feathering in the edges using that self levelling. I’ve only ever used self levelling on a concrete floor, so no idea if the stuff on floorboards is any good or not.

Or just change direction and go back to carpet.

Or if the boards look acceptable, clean them up, fill the gaps and throw a few rugs down.