Floor flattening to industry guidelines is really important. We've all had clients who want to save a buck and ask us to cut corners on floor flattening for the sake of their budget, but that just doesn't work out well. Besides a floor failure, here in California we can have our license up for disciplinary action or, even worse, revocation if we leave an out-of-spec floor behind.
Remedying out-of-spec subfloors is our responsibility, and if the client pushes back on fixing the issue, the best decision is to walk away and live another day with our reputations and our licenses in perfect health. For those of us who work out of stores, the same is true if the store advises you to install the floor and move on with the subfloor out of spec. Even if you believe that legally the store would be on the hook, they could come after the installer on a civil basis, as well.
For my part, I never went to "floor flattening school." I simply had to learn by following the advice of those whose skills far exceeded mine. I'm no expert even now, but I have accumulated a fair share of experience chasing out-of-spec subfloors until they surrendered. As with all of us, I've learned some tricks from others, and even learned new tricks of my own on the job as I wrestled with complicated applications. I have never walked out on a tough subfloor job, but I have walked out on clients who refused to have the floor flattened because of the cost. It's that important.
How do we get to flatness?
I find that breaking down the requirements into different lengths and relatable measurements can be helpful.
If we stay within these tight parameters we are good, but how do we get there?
I can tell you what we do. For grinding a hump, we use an edger on wood subfloors and a 7-inch right-angle grinder with a diamond wheel I like (pictured below) on concrete subfloors. For filling a depression on wood subfloors, I've seen wood shims, roof shingles and drywall shims for simple depressions. For demon-possessed depressions on concrete, we use cementitious products. For challenging wood subfloors, we can use cementitious products in combination with wire lath or fiberglass fabric to add integrity—per the manufacturer's direction, of course.
I used water to keep the dust down when grinding this subfloor—even though I had a vacuum. The spatula was for picking up the slurry created, which I put in the blue container. Of course I had the level there, or else I would have had no way of knowing if I made any progress.
RELATED: How to Prove Subfloor Prep Is Necessary—and Get Paid for It
The following pages have examples from some of my jobs with subfloor prep.
The English muffin concrete entry
This project from last year required me to flatten a concrete entry that was pock-marked like the inside of an English muffin. My intent was to have a finished elevation that allowed for my ¾-inch solid oak floor to be flush with the existing guest bath on one side and the guest closet on the other side.
Fortunately, I had more than enough room to work, and I did not have to excavate the existing concrete. I used Tapcon screws set in the concrete and adjusted each screw by turning it until the required elevation at the top of the screw head was met with a level.
This part was not difficult but was very rewarding because the method yielded a perfect height with the oak floor installed. I used two coats of Bostik Roll-Cote as a moisture retarder.
Then I used a cementitious leveling compound poured over the slab to the finished height where it barely covered the Tapcon screw heads.
I considered the membrane thickness of the adhesive in the final measurement for the height of the Tapcon screws. It all added up to a win—and it came out perfectly level and flat.
No one can teach you this at a school. We have to chase it on our own by reaching out to those of us who have done it many times; from research on the web; and mostly from just pressing in and not letting the task beat you. As I like to say, we wood floor pros are the ultimate McGyvers.
The job where we had to create a subfloor
On this challenging project, we were spared flattening a subfloor—because there wasn't one. What we had was a request to remove existing tile over a raised foundation. The tile installation was based on turn of the century technology, and sit down for the specs—because they are unique, to say the least.
For starters, the tile was over 1 inch thick. The "thinset" was a low-compressive-strength concrete that measured roughly 4½ inch thick—for a total of 5½ inches altogether. It varied a wee bit here and there, but, once removed, we had to match the existing elevations of a wood floor living room on one side and a wood floor dining room and kitchen entry on the other side. After the tile and thinset demo was complete, I remember staring at what was left and thinking to myself, "Arghh … I have a challenge here."
Here is the beginning of where the client wanted a wood floor.
We've all been here before, but we had no idea of how thick this floor was. We knew it was a raised foundation platform, but after 3 inches of digging, there was no end in sight. It was a real adventure.
We finally found the subfloor after almost 6 inches of excavation. There was no subfloor to level, because there was no floor at all. This was Amazon-rope-bridge stuff over a chasm 1,000 feet high. We were on our own with no similar applications to glean information from.
RELATED: Make Sure Your Wood Floor's Subfloor is Clean, Dry, Sound & Flat
We visualized this as a big floor heater patch, and we started with the rim joists to set the elevation.
The laser and 8-foot level were our primary tools here. We used 3-inch screws to secure the rim joists to the existing structure. We put the bubble in the middle of the lines of the level at every instance, because this step was the basis for everything that came after.
We installed 2x4s 12 feet long for the spans to make the elevation work, and kept the level and laser at hand to make every board count.
We used a lot of shims here to set the elevation of each board—foot by foot. Simpson A35 Teco clips secured each board firmly to the subfloor, and the shims were glued and nailed. There was zero deflection in the joists as we walked over them (we walked every board and tested this before we moved on).
We glued each joist before setting the plywood.
We put about four screws in each panel to "tack" them in place. Then, after pre-marking where the joists were (we used a chalk line), we commenced with one screw per square foot to final-fasten each panel. When we were done, we (finally) put the level and laser away. Walking on the platform felt like concrete, and that was an enormously good feeling to end with.
Then our site-milled herringbone was installed, sanded and finished.
A wood floor in a former porch
In this case, we were working on what used to be a porch but was now a room addition. Because it had been a porch, the floor was canted down for rain runoff.
We used sheets of plywood and veneer skins in multiple layers to get it flat, and after nailing and gluing the plywood and skins, the floor was solid.
