Depression in floorboards - 10mm across 1.8m span - leveling for laminate floor?

[danlightbulb]

Hi all,

Upstairs bedroom, wooden floorboards.

Due to the way the floor beams have been constructed near to the chimney breast, a section of the floor is dipped by a max of 15mm in one area, but typically more like 5-10mm, over a span of around 1.8m.

I have had the floorboards up in a couple of places, the joists either side of the chimney breast run into the side walls of the house, but those directly in line with the chimney breast are terminated just before the chimney, and are supported by a cross member. This cross member was only 2x2, secured to the joists by nails. Obviously insufficient and so over the many years the house has been here, that area of the floor (approx 1.4x1.4m square) has sagged by upto 10-15mm.

I have added in an additional 3x2 cross member, screwed into the joists either side for extra strength. But obviously the sag remains.

The rest of the room is level enough, but that sag area of 10-15mm I assume is too much for fitting laminate floor on and it will likely bounce or deteriorate.


I was thinking are firring strips the best way to solve this? Lay some strip wood and then plane it out to feather edge in order to fill in the middle depression?

 

[cross thread]

Can you put new wood attach to the old joists at the correct height the refit the floor boards

 

[oldbutnotdead]

Yes you'll get bounce with laminate.
Firring strips will be a pain with all the planing and sanding to get a level.
Easiest would be to use sand or slc to fill in the depression.
Better will be to sister the joists with straight timber (ripped 1" ply is a favourite, rough sawn 1" fence boards work well) then fix the floorboards to your level sisters.

 

[danlightbulb]

In order to shim the existing joists under the floorboards, or sister the joists completely, I would need to lift alot more full length floorboards. They are mostly nailed down with the tongue& groove still intact, except for in some places where wiring or central heating pipes have been put through. That would be alot more disruptive. Alot of boards would likely break as well requiring me to replace them with new.

To use sand or SLC I would have to somehow seal the gaps between floorboards in the area that needs to be filled? Also wouldn't that add alot more weight to that specific area of floor, compared to wood? Are you saying dry sand could be used, if it could be somehow contained within the depressed area?

I need to get the floor level enough to use the 5mm foam laminate underlay Ive bought with the laminate flooring - that itself can absorb some variation so I don't need to get the floorboards absolutely perfect, just roughly level where the depression area is. The depression area is about 1.4x1.2m. With furring strips at 92mm wide x 10mm (reducing to 6mm thick nearer the edges of the area), I would need to fit and plane about 15 widths.

 

[Deleted member 174758]

Build the affected area up with multiple layers of 5.5mm ply laid like the contours on an OS map to build to the required level. Finally nail a single layer of 5.5mm ply over the entire floor (nailed at 200mm centres or screwed st 150mm centres) and trowel a layer of SLC over the top of that. Lot of work - potentially perfect results

 

[danlightbulb]

@JobAndKnock what if I ever need to get under the floor, repair wiring or heating pipes? That approach would be a nightmare for it.

I think Im going to have to try the furring strip approach. Pulling up floorboards to shim the joists will likely result in breakage. If I lay down the strip I can plane it level and feather it out at the edge. Will take time but at least it can all still unscrew if the floorboards ever need to come up.

 

[opps]

@JobAndKnock what if I ever need to get under the floor, repair wiring or heating pipes? That approach would be a nightmare for it.

It needn't be. In the event that you need to cut out a section, you can "butter" up the joining sections with 2 pack filler. Drop some polythene on the floor, spray the underside of the ply with water and "stick" the polythene to it then drop some blobs of 2 pack filler. Push the ply down until it is level. Once cured you can screw it down.

The two pack filler will act as a spacer and the polythene means that it can be removed as required. Think of it as a custom made packer.

I regularly use 2 pack filler for such tasks, it hardens in minutes and the polythene can be pulled away if required.

 

[Deleted member 174758]

@JobAndKnock what if I ever need to get under the floor, repair wiring or heating pipes? That approach would be a nightmare for it.

But it is exactly what us done on commercial jobs, and as a technique is not unlike cement boarding bathroom or kitchen floors which are to be tiled. Granted we normally do this sort of work after a rewire and/or replumb (if necessary) and is really the only way to go about doing the job if you want a perfectly flat floor. Properly installed electrics and plumbing goes on for years without problems (I had my bathroom floor up for the first time last year - after 28 years) so why do you think yours services are so bad? And things are so bad, why not deal with them first? Because ripping up a laminate floor is no fun either (especially relaying it afterwards)

TBH furring strips beneath floors are a complete nightmare - the thin ends are liable to split and they are never sawn right. Far better to partly strip back the floor boards and then sister the joists with something like 3 x 2in, then relay the boards pnto that - never, ever plywood rips because screwing into the edge of plywood can cause it to split and it is also far too easy to miss the edge of an 18mm board and either have no fix or a partial fix - in both cases the floor will eventually start to squeak

 

[danlightbulb]

I am really reluctant to lift the floorboards more. The area to be 'filled' is 1.2-1.4m wide but to get under to shim or sister I would have to be lifting 3-4m long intact boards. They are old, will likely split, all the T&G will break.

At least with wood strip, I can lay it down it sections, screwed and glued to existing floorboards in the same direction, spend time planing it out level with a decent length jack plane.

I understand the thin edges might split but it hasn't got to look good it's just got to fill in a depression.

Guys, will what I prefer to do work, if I spend the time on it?

 

[danlightbulb]

Ok been trying a few things tonight after a trip to B&Q.

I have shimmed one of the floorboards which was already cut before so was easy to access. Its lifted the lowest part by around 10mm. Then I have got some 9mm ply from the store, and approx a 400mm square piece of that will almost fill in the centre section to another floorboard I can lift and shim.

That makes the centre part fairly level, but Im left with a section around the edges of this that needs to be leveled as well, i.e feathered back from 9mm where the ply stop, to almost zero thickness at the other end. I have some 5mm ply but that still leaves a small step obviously.

 

[danlightbulb]

Ok here is where Im at with it.

The middle is two layers in places, the outer edge being 3mm. Is 3mm still too much of a step change to fit the laminate floor onto? If so, I need to try and feather it out even thinner somehow, with a material of say 1mm thickness.

I have considered overboarding the whole room in 5mm ply. I can see it would take away those step changes being an issue, but it would raise the height of the floor, which is already being lifted quite a bit with 5mm underlay and 12mm laminate. Also it would be around £150 of additional ply sheeting.

 

[oldbutnotdead]

You'll be using underlay with the laminate presumably? Get a few of the green thicker underlay tiles and a roll of the thin stuff. Use bits of the tiles for your ramps, cover the whole lot with the thin stuff, give it a go. You'll still get a bit of movement over the steps.
TBH those floorboards look rather nice, I'd have left them bare with rugs.

 

[danlightbulb]

TBH those floorboards look rather nice, I'd have left them bare with rugs.

They aren't nice over the whole room, alot of mismatched colour, damaged edges, cut t&G, plus the levelling issue. Also its noisy with them bare.

Heres the whole room:

 

[oldbutnotdead]

Good luck with it. Given you don't love the boards and you're burying them in laminate (which is also noisy, by the way) lifting them and sistering the joists would be more work but give a much better outcome.

 

[danlightbulb]

Good luck with it. Given you don't love the boards and you're burying them in laminate (which is also noisy, by the way) lifting them and sistering the joists would be more work but give a much better outcome.

I think trying that would create even more issues for me. There is not only a low spot in the centre of the room, there is a hump in the bay window area too, followed by another depression right near the window. Ive been working on that hump today, trying to plane off some material.

And by putting my 6ft level across the room in various areas, I also think the whole floor slopes slightly from front to back.

If I lift boards to get the centre level, I will still need to pad out the bit near the window and try and shave off the hump in that area too. Plus alot of the boards are old, dry, they are bound to break up alot so even more cost in getting several square meters of new floorboards down.

If I wasn't living here, and it was a full on refurb job including rewire, new heating etc, then yeah I would probably lift the boards, solve the joist issues with new joists or a steel in there, take out the remaining chimney breast etc. But we're talking mega work there and mega bucks too.

I just need to get it decent enough to take the flooring, but I want to do the best job I can with what I have.


Im tempted to overboard the lot now with 5.5mm. Will cost about £150 for 6 sheets. Now that I have sorted most of the bad depressions, will this give a better job than just trying to pad out with more thinner material or underlay?

I'll do it if it is the right answer, but I don't want to buy the material and then laying it down shows up even more humps and bumps.