Hardwood over sloping subfloor?

Old 10-15-20, 02:01 PM
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Hardwood over sloping subfloor?

Hi guys!

My wife and I recently purchased a home, and we are trying to do as much renovation work as possible ourselves. One of the things we are doing is installing new gorgeous looking red oak hardwood flooring in mocha finish. I am placing a 2 mm underlayment layer for moisture and acoustic insulation.

In doing the initial assessment of the subfloor after removing the existing carpet, underlayment, etc., I spotted a couple issues in our first room. I am listing below what I have noted (good and bad) in hopes that will give you info to help me answer the question:

- Subfloor is generally flat. with a 6 ft level, I noted the room is generally level (under 3/16 in of difference in a 6 ft run). I have sanded some of the high spots and the floor is generally under the 3/16 in tolerance.

- In beginning to install the hardwood floor, I am running the boards perpendicular to the joists (parallel to the long dimension of the room.

- However, I have noted on the other side of the room the subfloor seems to be sagging. I am saying it seems because with the level, the floor appears to be even in both directions at the center of the room, and is quite higher at the edges of the room (1/2 in, maybe 3/4), to the point that the subfloor in the hallway (hallway is parallel to the joists) is sloping (1/2 in to 3/4 in in the short direction, roughly 37 inches span, or 2 degrees in the worst spot). Nothing crazy, but it is not even. A similar thing happens in the closet but in opposite direction.

- More on this... in between both walls, there is a guest bathroom. The subfloor within the bathroom is, again, reasonably flat in both directions. There are no signs of rotting of subfloor or structural systems (I have exposed the subfloor in the bathroom and there is no signs of water damage. I am going to remove the bathtub, but again, I haven't seen signs of water damage from the adjacent, exposed areas.

- Now some analysis... the way the floor is sagging makes no sense. There is a structural wall underneath, and the floor would be sagging perpendicular to the floor joists. The foundation is fine, since the outside wall (load bearing) and the column supporting the other side are the highest points.

I have created a few diagrams describing the situation. I considered evening out all the subfloor to make it perfectly flat. however, evening all the floors out will require redoing the bathroom plumbing, since I would have to raise that floor to match the now elevated subfloor in the hallway.

My question is: what do you guys think? Do you think it is acceptable to simply sand a fraction of an inch the higher areas to make the slope more gentle and live with it? Meaning, try to ease the transition so the wood planks fit together and the boards interlock.
Old 10-15-20, 03:42 PM
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You can largely ignore level. All you need to worry about is flat. So lay a 6ft level the direction the planks will lay. You should never be able to slip in more than an 1/8" thick shim under the middle of the level. If you can, the floor is bowed too much... and you may want to shim it if it will help. This will give you a headache as you install because the planks will only bend so much. You don't want flooring suspended in midair with only nails supporting it.

So yes, sand any high spots if you can, but you need to check in both directions to make sure you are not doing more harm than good. Everything often heads downhill to a bathroom due to the heavy cast iron tub filled with 200 lbs of water and a 200lb woman.

If it helps, you can put transitions across doorways if the floor seems to tip at or near a doorway.
berni1984 voted this post useful.
Old 10-15-20, 04:28 PM
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So what you are finding, one of the more you look the more you find situations, is the result of what we call "rough construction"!

If carpet was installed non of this would not be discussed, the flatness that XS states is the key, if the floor slopes up or down, who cares, as long as it's flat the flooring will go down, everybody sleeps well tonight.

For the floors I have installed I don't care if they slope, I have never sanded high spots, I simply use felt or shingles (in extremes cases) to level, sorry, get the floor flat!

Believe it or not hardwood is somewhat flexible and minor deviations of flatness are no issue!
berni1984 voted this post useful.
Old 10-16-20, 06:09 AM
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Thank you for your comments!

Yesterday I began installing the underlayment in the remaining areas, and I noticed (you guys are right) that other than a hump near the start of the hallway, the slope is consistent and the floor is sloping, but level. I had to use hardwood floor planks to check the flatness of the subfloor, since my level is too long, and the planks are simply laying on top of the plywood.

All I did was to use a very coarse wood file to ease the transition from the low spot in the main area of the room to the high spot at the beginning of the hallway, and no wood piece seems to be floating in the air. Maybe a little, but definitely not more than 1/8 in to 3/16 in.

I will post pics when I'm done. Thanks for your help!

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