Yes, it will crack apart. If your subfloor isn;t flat, then you need to remove it and shim the joists until the subfloor is flat.

Thanks for the suggestion, and I'm sure that would be best, but the subfloor isn't coming up for this. So, I'll have to have something done on top.

So, since cementitious products really are a problem, is there anything better than layering roofing felt, which seems to be a common approach? How about for filling small spots, e.g. a missing ply 1"-2" by a couple of feet?

This post was edited by attofarad on Fri, Jan 18, 13 at 22:06

Most self leveling products will cure to a very high PSI (pounds per square inch). Most trowelable cementious floor patches, such as Mapei Planipatch, Ardex SD-P, also cure to a high psi rating. It is almost impossible to attach through the patches and into the floor beneath. However, I have used Ardex SD-P to fill "bird baths" in plywood substrates and been able to shoot through to the subfloor...and in those places where it became difficult...I just switched to hand nailing through with hardened spiral flooring nails. Using hardened spiral flooring nails does require drilling a pilot hole in dense wood species.

SLC is NOT a substitute for a properly prepared subfloor. If you have 2" of missing subfloor, then your subfloor needs to be redone and leveled at that time. Applying moisture (SLC) on top of a unknown type of ply as the subfloor is a sure fire way to create more humps and bumps as the moisture migrates it's way into the ply and delaminates it. Moisture and wood do not co-exist as happy bedfellows. I've seen the edges of ply swell into humps like a paperback book dropped into a tub, thus creating even more problems, as well as central portions of the ply delaminate, creating weak spots that creak and groan as well as eventually sag. If the ply was a known exterior grade with proper exterior glues, small areas of dips can selectively be filled, as Glenn says above. But you do NOT apply SLC wholesale across a plywood subfloor unless you are a hack who is doing a job for someone who doesn't care because they are getting ready to move from the house.

Good points from GreenDesigns on applying self-leveling cementious products over plywood. A trained professional can successfully float the product over a plywood substrate, but the typical DIY'er will skip the necessary primers, lathing and resilient additives, because they add so much to the cost of the job. But, they are essential for a successful lift over any plywood or wood substrate. Skip ANY of it and you can experience the problems listed above.