Solid Hardwood, Engineered and Laminate Flooring - Level a sublfoor with paper: how exactly?

Could someone describe (in painstaking detail if necessary) the method whereby you level a subfloor with paper? All I know is that you don't want more than an 1/8" difference over 6 feet and that you fill in the basins with pieces of paper.

What thickness paper do you recommend?
How do you determine how much you feather each successive layer back from the edge of the layer underneath?
If you are also laying down a foam, can you shim by sliding pieces in between the flooring and the foam?
If your laminate has a cork backing, would you still recommend the foam?
If you use foam should you always expect to see a bit of flex of the flooring when people walk over it?

My wife and I laid down a bunch of marmoleum laminate over the weekend after we thought we had levelled the subfloor properly. But after a day away from the floor, walking on it we discovered several soft spots of flex that we had either completely undetected or had lost the ability to detect during the installation.

We had levelled with roofing felt and then laid down underlayment that consists of styrofoam balls in a plastic sheet sandwich. After a few rows of laminate we started to notice areas of excessive flex on the leading row and so we slipped pieces of felt in between the flooring and the foam. We thought that was working....but it also seems like no matter where someone walks on the floor there is a little bit of flex, which we think can't be avoided with the deformation of the foam. Does this happen with all laminate on a foam underlayment? If we can see no spread of the seams now just after installation, can we believe that the seams will not shrink or get worked open over time? Probably not, I would imagine.

Long, I know. Answer what you can.
Thanks,
Evan

I am not a pro and no one is replying to this so I'm going to ask you a few questions that came to mind...

What does the manufacturer's instructions say? What you've done to level the floor and throughout your installation...were those the manufacturer's instructions?

Just curious

Yeah, the silence is interesting....

There are no instructions with the flooring concerning the levelling of a subfloor. It is not even stated what the flatness should be. I have assumed it should be the less-than-1/8"-over-8' that I have read about elsewhere and was told by an installer of 20 years. But I did confirm with a Forbo (manufacturer of Marmoleum) technician that levelling a floor with roofing felt was an acceptable method.

That said, this is not really a marmoleum question. This method of levelling is an industry standard for at least laminate, maybe all types, that, as far as I can tell, is not clearly documented anywhere online.

For others, I now describe the method that my wife and I came upon while puzzling this out: Starting in one corner, I laid a 6' straightedge on the floor perpendicular to the direction of the floor joists (most of the sagging is parallel to the direction of the joists). My wife then took a piece of 20lb felt and slid it under the straightedge marking on the floor with a sharpie where the felt touched the straightedge. We proceeded thusly down the length of one sheet of subfloor in steps of 16". We then drew a line connecting the marks and cut some felt to fill the spot (it's like building a contour map). Then, we repeated that process on the felt marking it with a piece of soap (the sharpie didn't show up) and cut a smaller similarly shaped piece of felt. We laid down 3 or 4 layers until we couldn't slide the felt under the straightedge. After last night, we have two discernible basins filled. The trick is going to be making a flat surface connecting those two infills, which are separated by about 4'. And it is slow work. We decided to avoid sliding pieces of felt in over the foam once we start installing the flooring. We don't think that really shims anything.

Anyway, we hope it goes well from here on out.

AKEvan, Im pretty green at this and have my own simular problems, but have you considered the leveling compound? Have you noticed any crackling noises as you walk accross the felt area of the subfloor?
Karl

Karl, thanks for the reply. No, we have not really considered using a levelling compound. One, I have read that it's not really recommended until you have dips greater than about 1/4" to fill, and two, the installer at the shop said he hates the stuff and wouldnt' recommend it to a DIYer.

No, we don't hear crackling noises. What would that indicate?

Your floor may be locked in from gaining moisture content after it was installed, and now it is heaving, leaving you with soft spots where it is heaving.


Straight edge on the substrate. Fill in under the straight edge. Or you lay a row and insert felt in the places that need it down the row.

I just think it tends to crackle more when there is alot of flexion. It seems that way with the pergo I am laying. There is a leveling compound made by DAP which takes longer to dry (6 hours). It has a slight rubbery feel to it, but I like it because its premixed and gives me time to think about the application. Most of the other stuff dries almost as soon as it hits the floor.
I would definitely try Perrys method first. Felt is much easier to manage and he knows what hes doing. Im not always shure that I do.
Karl

Thanks everyone, here's my latest assessment: If you use a foam underlayment, the floor will flex a little.

We have put back in nearly as much floor as we tore out after being more systematic about the levelling. Areas of previous excessive flex still flex, though not as much. And over broad areas where we know the subfloor and levelling is in good shape, it flexes. I think as newbies (not to mention the ones who purchased the thousands of dollars of flooring) we were simply not sure of how much flex to expect and it kind of freaked us out. But now we are not going to worry about it.

But this weekend we pay the true cost of a DIY installation: working to finish up an indoors project when it's a 3 day weekend and the weather is awesome. brother.