How to level a concrete Subloor?

Question

I'm installing Laminate onto a very uneven concrete subloor with radiant heating. I'm looking at self levelling compound for some of the larger dips and am also considering putting a 3/8 OSB Subfloor on top of the Concrete and then 6mm Underlay/Moisture barrier. The hope is that the combination of OSB Subloor and Compound will even out the floor. Any issues with this approach? Thanks

Answer 1

If the dips are quite bad I would recommend floating a new thin slab over the concrete, maybe 2" or so, it will leave a perfectly smooth and new floor if you hire a good concrete finisher.
Self leveling compound on its own will work, but it will never be perfect, you will feel it under the laminate.
Installing U-Lay subfloor over concrete is best, but self levelling concrete has a tendancy to chip and crack especially when the coating is thin. This shouldn't be that much of an issue though with laminate (tile is another story), just make sure when you glue the sheets down that you spread a good layer of glue over the floor before putting down the plywood (don't cheap out, it will make all the difference in the end by reducing "pops").
Make sure you use a proper moisture barrier under your laminate and Tuck Tape -ALL- the joints so it is sealed and no moisture can come through. This is very important because it will GREATLY reduce cupping and swelling since laminate is a MDF-type product which is very sensative to moisture.

Answer 2

Self leveling compounds do work for up to 1/2 inch. Check on the instructions on the bag as some leveling compounds are not to be used in thicknesses of over 1/4 inch. Some will also require you to pour in stages. Because of your radiant heat a good leveler would be better than a wood underlayment. However if you are going to install a new sub-floor anyway, I would not use OSB as an underlayment, but rather a plywood. The cost of plywood may be a little bit more than OSB but it will give you a smother surface under a floor. By the time you have leveled the floor and put down a layer of sub-flooring you may find that it might just pay to bypass the sub-floor after you have poured a leveler and install a floating laminate floor. They are more expensive but better in the long run. The radiant heat should come up through any floor you install, but you may find the floor doesn't "feel as warm" as it did before. I have radiant heat in my home. With the constant heating and cooling of your floor (seasonal changes) you may not wish to put down any product that can not contract and expand (a glued laminate floor tile for example -this may end up with the seam expanding). Sheet goods would a wiser choice if you go with a glued down product.

Answer 3

CHEMREX makes a self leveling compound that will work up to an inch and they provide a number for you to call so that if the area you want to level is thicker than that they will tell you how to do it with there product, I wouldn't put the osb down because it's not moisture resistant and then you will have to try and get it screwed into the concrete and take a chance on screwing up the radiant heat. I would level it out, put down you moisture barrier that you have picked to go with the laminate wheather it's the blue foam, grey, or heavy rubber for sound. then lay you laminate.

Answer 4

Most of the answers are suggesting that since you have some warping you need to do more than self leveling concrete. You would have to put the concrete on so thick I doubt you could do so. Best is to either go with rigid concrete stuff that comes in 4X4 and is used for underlayment for tiling, or go with the marine plywood as suggested. Either way, if you have flex and don't get it rigid, self leveling compound wil not solve your problem, it will only crack on you as you put weight on it. Drywall after any flooring repairs/fixes.

Answer 5

This sounds perfectly reasonable as long as the osb is left in the room for a couple of days to stabilise it before fixing and it is firmly fixed so it doesn't buckle.

Answer 6

use DASH PATCH best thing to use -