Lay laminate on uneven subfloor

[Mark A]

Hi chaps,

I'm going to lay 14m3 of laminate next week for my sister so I need to get everything in order beforehand. The 22mm T&G chipboard subfloor looks relatively new, but it's nowhere near flat or level - over 8m it drops almost 3 inches, and undulates in all directions. Ideally the chipboard should be taken up and the joists packed out (or entirely replaced; the ceiling below needs reskimming anyway) however I live near Swansea and she's outside Coventry, so I want to get it over and done with in one go.

Any suggestions ow I can go about it? Shim the largest dips with hardboard or thin ply and cover with rigid underlay?

Cheers,
Mark

 

[sunnybob]

Sounds like a disaster in the making.
One of those jobs I would take my time and do it only once.

 

[Mark A]

My sister can't currently afford the expense of rerouting any pipes and cables under the floor, replacing joists, plasterboarding and skimming the ceiling below and laying a new subfloor. That would be the proper way to go about it, but right now I can only work with what's there already unfortunately.

The floors in my own house were even worse, but I was vetoed on the decision to replace entirely. Levelling the existing joists by planing down one end and packing up the other took far longer than outright replacement, so is something I want to avoid.

I will make thresholds between the landing and the connecting rooms so the thickness of the underlay doesn't matter but raising the floor up several inches to level will interfere with the height of the top step of the stairs.

It's quite an awkward situation I'm in, as I live 160 miles away so can't be traveling back and forth for several weekends.

 

[DBT85]

I wouldn't worry about level if the floor is already drunk, flat would be more my target to try and fix a bit.

Just how much uneveness is there over say a 1m run of board?

Would dropping some cheap thin ply down help those undulations?

 

[Mark A]

Most of the floor has undulations up to 3mm over 1m, though in one small area there's a 12mm dip and along some of the joints between the chipboard there are ridges of 2/3mm. Doesn't sound a lot but it'll be enough to make the laminate bounce.

Surely thin ply laid on top would only conform to the shape of the chipboard, raising it up without actually making a great difference when it comes to flatness? I can imagine thicker ply would bridge the dips quite well, but too thick and there's other issues like small steps between rooms and an unequally high top step on the staircase.

 

[n0legs]

Go a day earlier and use levelling compound on the uneven areas.

 

[DBT85]

Most of the floor has undulations up to 3mm over 1m, though in one small area there's a 12mm dip and along some of the joints between the chipboard there are ridges of 2/3mm. Doesn't sound a lot but it'll be enough to make the laminate bounce.

Surely thin ply laid on top would only conform to the shape of the chipboard, raising it up without actually making a great difference when it comes to flatness? I can imagine thicker ply would bridge the dips quite well, but too thick and there's other issues like small steps between rooms and an unequally high top step on the staircase.

Well if course there's thin and there's thin. If the differences in the chip are evident over shorter distances then it will at least control it a little. If they are only really over longer distances then it won't make much difference.

Given the budgetary and time constraints on the job, 3mm might just have to be tolerated. It's not ideal of course but in these situations which some of us find ourselves in, it has to be done. What we might look at and go "ooff I wouldn't want that" most people wouldn't ever know was a problem and even when explained won't understand. One can't always make perfect work if given certain limitations, I think most would understand.

Are you going with the 3mm foam underlayment or the thicker green stuff?

Fancy a bit of planing on the ridges between chip boards?

 

[moretea]

It sounds as at some time in the future, some major remedial works will be required. Until then, your just chucking good money after bad with what you do now, to get the floor down now, any minor levelling can usually be worked out using an extra layer or two of underlay feathered at the edges not perfect, view it as a temp fix.

 

[Mark A]

Thanks for the replies chaps.

Regarding future remedial work - my sister and her partner have no intentions of staying at the house forever, so we all agree there's little point spending money unnecessarily.

I'll check the floor with my 6ft level again when I'm next there to see just how bad it is. If we have to cover the chipboard with ply then it can't be helped.

What's the difference between the fibre board underlay and the thick roll stuff?

Can it be doubled up in places to fill gaps, or will it compress too much?

For smaller gaps I reckon a layer or two of DPC beneath the underlay will do nicely.

Mark

 

[DBT85]

The green square stuff is a bit thicker, I think the normal foam roll stuff is only 3 mm thick though it can be had in thicker too.

Just do the best job you can, as I said they probably wouldn't notice anything if you didn't mention it, and they will probably accept that perfection isn't attainable given the circumstances.

 

[mr rusty]

Unless you know where every service is fastening anything to the chip risks hitting something. You don't have to replace the joists. Chip is fairly cheap. Rip it up and sister the joists with 2x3 fastened to the joists with coach screw and bolts. Lay new chip floor gluing and screwing to avoid squeaky floor. Laminate will then go down like Lego in half the time it would if you fight it all the way across a lumpy floor.

 

[Mark A]

It's done!

A combination of 5mm foam underlay and layers 8" DPC in the worst dips seems to have worked. The floor is still nowhere near level, but it is acceptably flat: the oak-effect laminate's pattern is busy enough to distract the eye from the general wonkyness of the landing, especially now that furniture has been moved back in.

Now what's left to do is fit skirting boards and architraves, and make three oak thresholds, but that's for next time.

Cheers,
Mark