I recently finished installing solid hardwood over a new 3/4 plywood subfloor on a concrete slab, and while it looks nice, the installation process wasn't ideal due to variances of up 3/16 to 1/4 of an inch over 8 to 10 feet. I originally wanted the surface flattened, but the concrete contractors were persistent that my installation method of using tapcons to secure the plywood to the slab would fracture the leveling cement. In the end, I decided to just install over the slab as-is.
There's still another area that needs hardwood, and I really want to make sure the area is flattened prior to installation. I spoke to some other concrete contractors who said they could grind down the highest areas, but again, said that using surface leveler is not recommended since it will fracture in some areas if I try to drill and screw through it.
After talking some more, here were my options:
- Use surface leveler, and use tapcons to adhere the plywood to the concrete (at your own risk)
- Use surface leveler, and float the plywood over the concrete
- Don't use surface leveler, and just grind down the high areas (not perfect)
- Don't use surface leveler, and use tapcons to adhere the plywood to the concrete. Shim the plywood using 2x4 shims in low areas, and sand the plywood in high areas.
The second option seemed more viable, but there's one issue: I already bought 3/4 plywood for the other room, so I'd have to buy 1/4 and 1/2 plywood to match up to the existing 3/4 plywood I already laid down. I don't know how feasible the fourth option is, but I would be able to use the plywood I already have.