After taking a look at my subfloor, the floor contractor recommended using Ardex GS4 self-leveling cement to flatten the bumps and valleys out, in preparation for floating a wood floor. Later, after thinking about it, I had some questions:

1) What's the best way to apply this stuff? The contractor said to use a rake with the tines cut to approximately the depth you want and distribute it around before it self-levels.

2) Do you pour the whole floor at one time, or can you do sections?

3) Can it be skim-coated, used at depths less than 1/4", to fill in minor variations? The contractor also recommended using cheaper SD "feather finish" applied with a trowel to smooth minor variations, but if I'm going to cover the floor with the self-leveling, why would I bother with that? The GS4 is alot more expensive, but at a 1/8"-1/4" depth the cost to do the whole floor is in my budget, just not sure if it works at 1/8".

4) Can both products be used with each other, e.g., some areas covered in GS-4 and others with SDF?

I guess I should just call up Ardex, but maybe someone here has worked with these products and knows the differences.
Thanks,
Peter

GS-4...

You must have a gypcrete substrate, I'm I correct?

If not, K-15 is what you want, if your over, ongrade slab.

Big areas need to be pumped. A mixing pump is highly recommended, or you will be in big trouble, quickly.

Perry, not sure what it is. It's like a light-weight cement, only not as dense and hard. It's more like a cemented sand, about 1" thick, that was spread all over what appears to be oak plank floor (I scraped some away in the corners to see what was underneath). Put a little water on it and it scrapes off fairly easily. If it were up to me, I'd take it out completely, but this is in a condo and the board would probably not allow it, plus it'd be a huge mess.

The whole room is about 350 sf. I've scraped off some of the high spots, but it's alot of work doing that, and so I asked the contractor about easier way. Some parts are flat within 1/8", others off by 1/2". Seems like it would be easier and quicker to spread the GS-4 over everything, with just a skim-coating on the flatter parts. I could get some friends to help mix&spread, but I'm not sure how much we'll need, and it's expensive stuff (don't want to buy more than necessary) so I was asking if you can spread it in batches, if you can skim coat flat areas, if using a rake would work, etc.

I agree with floorguy, self leveling cement is very finicky. You have only a few minutes to mix and spread it or it will fail to harden. The amount of water you add is also extremely critical. For large areas you either need several people working together or have it pumped in. You also have to make forms and seal them with caulk.

The coverage is specified on the bag. John Bridges tile setting book has some info it or you can ask at his site.

Self leveling cement was really made for people that don't have experience with cement. An experienced person can level your floor without using self leveling cement quite easily.

That would be me....Any recommendations for an experienced installer in the Virginia, metro-DC area?

I tried to get a guy to do my slab but most installers do not want to do part of a job and I can't blame them. Thing is, a few installers that I called told me that they don't bother with leveling! It's sad, but an installer that levels your floor is probably a cut above the typical rif raf.

I ended up doing mine myself but I did it in a patching style using a screed set. If you use a screed it's not that hard except on the edges of the slab where you have no real reference. You mark the low spots and use a screed larger than the low spot.

If I had to do it again I would make a reference for the screed on the edge of the slab ahead of time like the tile guys do.

At least you should check out your surface with a straightedge or level first and mark all of the low spots to see what your up against.

yeah, the leveling is key. Without it how can they meet the floormaker's specs of 3/16" every 10'? I guess they don't. Maybe they are relying on the underlayment to smooth things out.

I've gone over the floor with a straightedge and rolled a marble around to get a general idea of the low/high spots. Maybe I'll try screeding a spot like you did, see how that goes. Perhaps call a few more installers....

More I think about it all, the more I wonder if the whole idea of floating a floor on top of gypcrete which is on top of wood subfloor is a good idea. The woodfloor subfloor probably expands and contracts at a different rate than the gypcrete, so whatever I patching I do will likely crack where existing cracks are.

This is what I have heard. If you have access to the underside of the subfloor you could shim the subfloor. I have also heard of installers using wood shingles as shims on the top of the subfloor. Why is your subfloor so far off? Is it an old house that has settled?

No, it's a 35+ year old condo, second floor.

Ideally I'd like some kind of ditra-like layer between the wood subfloor and the gypcrete, to allow for shear expansion/contraction, but that's out of the question here as I'd have to rip everything out, and can't do that (fire code, condo rules, expense, mess, etc.).

Actually the gypcrete is in fairly good shape except for wavy sections (+/- 1/2" max) and some thin cracks , considering its age and the wood subfloor it's on. It has a bit of flexibility. Perhaps filling the low spots followed by roofing paper shims will get it within spec. I have a bag of GS-4 that I'll run a test with this week, see how that goes.