I will be installing 9mm stranded bamboo flooring in a second floor bedroom. When we bought the house, there was a separate balcony behind the bedroom which had a 6" concrete floor. The balcony was converted by a previous homeowner into an enclosed room. This room was wasted space, so I removed the walls to expand the size of the bedroom. Because the outer walls of the house were built on the concrete pad, I chose not to remove the concrete pad (to expensive and would lead to more extensive renovations). The end result is the bedroom subfloor is 3/4" plywood, with one corner portion actually being a 6" concrete pad. I have inserted pictures below for clarity.
The plan is to simply have a step up (or elevated area of flooring), with the entire bedroom installed with the stranded bamboo flooring (including the concrete pad). I'm trying to determine how best to prepare this concrete pad as a subfloor.
The concrete pad was not poured level, the sides aren't square with the plywood subfloor and the edges of the pad aren't straight. There is also an uneven 1/2" x 1/2 " channel along the edge of the pad. I used two bags of self leveling compound and improved to level of the floor. There are some still some spots that are roughly 1/4" dips, so I could potentially use a third bag of self leveling compound.
To remedy the lack of squareness on the concrete pad sides, I placed two 2"x4"'s on their sides and got them as close to the sides of the pad as possible (small uneven gaps remain). I placed one 1" x 4" on top of the 2" x 4"'s and put spacers under the 2" x 4"'s to raise the wood (top of the 1" x 4") to be the same elevation as the concrete pad. The 2" x 4"'s also fill the space where the previous wall studs were.
Now the question is - Do I lay 3/4" plywood on top of the concrete pad/wood edge framing? I realize plywood on concrete is generally a no-no, but isn't it that mostly due to concrete slabs in contact with soil (this is a second floor bedroom)? This would then give me a consistent subfloor which would also be level and remove the issue with the gap and 1/2" x 1/2" channel.
If the answer is no, just install the bamboo on top of the concrete pad with wood framing, I do have a small uneven gap between the concrete pad and the 1" x 4" topper. There is also the 1/2" x 1/2" channel on the edge of the pad. I could fill the 6" deep gap between the pad and the wood with 5" of spray foam and then pour more self leveling compound to fill the channel, gap and address any small dips in the concrete pad.
I want to make sure I do this right so I'm looking to you guys with all the experience for direction. Thanks in advance for your help.