It was quite a puzzle to figure out how to install red oak floor in a 3-bedroom basement while maintaining maximum ceiling heights, over a wavy, tilted, funnel-shaped concrete slab.
You might think: just use self-leveling concrete to flatten the entire subfloor and call it good. But I couldn't, or else some rooms would be left with a 5.5ft. ceiling height and others with an 8ft. ceiling. So instead of flattening everything together, I flattened each room to itself. This left each room's floor higher or lower, and tilted at a different plane than the one next to it.
To marry these differently skewed rooms together, I made custom red oak transition strips (including one that had to twist):
But let's back up. To flatten each room to itself was quite a trick, because I couldn't use "level" as a reference. "Level" had to get thrown out the window. "Flat" became my goal. I explained it to my family like this: think of someone picking up only one corner of a nice sturdy dinner table. The table's surface will remain flat, but it will no longer be level.
To find the flat plane that would maintain max ceiling height I used custom built criss-crossing screeds, referencing the highest single spot in the topography of the wavy concrete surface, and tilted and slid the screeds around until nothing bonked the underside.
To create a strong subfloor at this newly discovered optimum plane, I used 2x4 joists, each meticulously custom-scribed to mate with the unique bit of wacko concrete floor it was being was nailed to.
Cavities between the custom joists were then filled with clean, dry, sand, tamped, and carefully feathered to meet the high spot:
Natural building techniques used:
Hardwood red oak, rather than toxic engineered PVC floor. .
Subfloor flattened using compacted sand and lumber rather than carbon intensive self-leveling concrete.
Finished with Rubio Monocoat, a non-toxic, zero VOC wood finish.