Background:
I'm an engineer by day and a home improvement junky by night. These days money is tight and my wife and I came across an offer on Craigslist that gave us the ability to have new flooring for a low price and sweat equity. I recently purchased 1800 ft^2 of basketball court constructed out of hard maple, 2 1/4" x 3/4" thick in 4'x8' panels rough cut on sleepers. The plan has been to disassemble the boards, sort them, put a new T&G on those that need them and layemdown! (Sand in place, refinish)... Though disassembling the flooring was a bit of a challenge, what lies ahead seems tougher.

Problem:
I decided to start slow and only work one room at a time. After pulling up the carpet and padding, clearing the floor of staples and debris, I checked the first room for flatness. I found that at a distance of 4' from an exterior wall toward the center of the room (and all along the wall's length) the 1 1/8" ply subfloor falls off 7/16". The floor joists are 4' OC and run parallel to the exterior wall mentioned. The point at which the slope starts is outside of the last joist. When I measure the floor further away from this transition area toward the opposite wall, the floor measures slightly out of level but as mentioned very close to dead flat. I'm planning on laying the flooring perpendicular to the fall off in slope to prevent individual finished boards from teetering off of uneven filler used in the problem area... I've heard that with a subfloor construction like mine I shouldn't have any issues with laying boards parallel to the floor joists but I have not done the calculations yet. Any advice here would be helpful too!

Some Figuring:
It seems like either the stem wall has sunk over time or at the time of the initial build (1974) there was a height difference that was left as is. I went under the house through the crawl space and probed the structural member on top of the stem wall and it does not seem to be rotted (not soft on probing nor visibly crushed by weight). I also noticed that the subfloor is glued so shimming isn't an option, I don't think.

Potential Solution:
I've done some reading and it's seemed that using 30# felt and roofing shingles was a possible approach for less significant out of flat conditions, SLC is a bad option for nailing through, and installing the flooring over the subfloor as laid is a bad idea. I'm considering simply placing 3/16" thick plywood in multiple layers creating a terracing situation in the sloping area and filling in the shallowest areas with 30# felt as needed.

Any suggestions? Hand planing or sanding the perfectly good rest of the flooring to within 3/16" of flat seems like an awful lot of flat flooring to remove. Is using the plywood as listed above the best way to go? I've also heard that pulling up the flooring at the exterior wall and putting in a 2x8 as a new ledger with new ply subfloor would be a good way to go too, but that sounds like $$$. Any ideas? Thanks for taking the time to review my post.

Brian

I had this problem with a house that had a slate floor that was installed level and I had to install wood flooring up to it even though the sub-floor was not level. What I found worked well is using layers of quarter inch plywood. I put shims next to the slate so that is was 3 quarters of an inch difference. I then put a straight edge from the high point to the slate and took small pieces of plywood and marked where I could fit one piece of plywood. Then I marked where I could put two pieces on top of each other under the straight edge and where I could put three.

I repeated this process until I had lines resembling a topographical map. I then cut the first piece of plywood with my jigsaw to the shape of the line closest to the slate. I then cut a second layer the shape of the second line from the slate. Once I had the third and final layer in place I installed the floor and I knew I had something that would hold nails and would not have moisture problems down the road. I also knew the floor was nailed to something solid since I nailed each layer in place.

I know some installers recommend floor leveling compound or asphalt roofing shingles. To me it using leveling compound would be like nailing to particleboard in my opinion and would shatter if the floor expanded or contracted if it didn't shatter when it was nailed. Asphalt shingles would have even less holding power.

In the pictures below I got to the job and the slate had already been installed and it was installed level. The wood floor had to butt up to it and in front of the door the slate was 3/4 inches thick so that was no problem. In front of the fireplace the thickness of the slate had grown to 1 and a half inches because the sub-floor was not level. This was a problem I had to solve.



Installing the transition wood up to the point where I had to start leveling.



The quarter inch plywood in place in layers.



Continuing with the installation.

I did a little picture drawing in Paint to illustrate the process. It's not to scale and I am not the best drawer in the world but I think this will help you and others.



I cut the quarter inch plywood (luan) into 2 inch by 2 inch spacers. Once I had the level resting on the high part of the sub-floor (b) and four spacers to make it level at the wall (a) I then tested to find out where three spacers would fit and marked the floor. Where 2 spacers would fit and marked it and then where 1 spacer would fit. If you want more support you could use eighth inch Luan and add more layers using the same technique.

I repeated this marking with the level in different areas. I cut the first piece of plywood at the wall where I could fit 4 spacers. The second layer of plywood was the size of a to c. The third piece of ply was from a to d and the third piece from a to e so it looked something like the image below.



As far as the gaps you were concerned about the hardwood can span that distance. It will be supported at points a,c,d and e. I nailed each layer as I went so it is all solid plywood. When you staple or nail the floor you will be stapling into plywood.

I also had one job where I actually jacked the corner of the house that was low with 20 ton hydraulic jacks. Not something I would attempt again.

I hope this might be a practical solution for your problem.