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Flattening very uneven subfloor
Here’s an issue I was searching for solutions for before joining here. I saw a few ideas on YouTube and posted in a couple other flooring related forums.
When our house was built, the foundation perimeter was made at least 1/2 inch higher than the central support beam running across the middle of the basement, and nothing was done to fix that as the walls & floors were added. Even if there was some settling (I don’t see cracks in walls), I can see the beam connecting to the foundation wall at the lower height. Thus, the floors immediately bow down towards the center from the outside walls. Then, they used engineered trusses (2x4s connected with metal spike plates) and sighting along them in the basement, I can see them warp up and down; they are not flat nor simply bowed down towards the center. So, the subfloors go down towards the center plus have varied height even side to side for adjacent trusses.
Adding any form of cement seems like too much — I’ve got some places where the subfloor is nearly an inch lower than next to the walls, while almost the entire floor is some distance lower.
Some people suggested trying to add various layers of thicknesses to bring up the entire floor to be flat, then add new 3/4″ t&g subfloor. With how non-flat the wavy parts are, this sounds like a nightmare to achieve.
Other things I saw showed people doing this by raising the height only above the trusses via layers of various thicknesses of plywood strips or other material, then adding the new 3/4″ subfloor glued & screwed through the new “truss” height. This is the method I’m currently planning to do unless I see something that really shows it to be a bad idea.
The biggest downside, other than the work involved, is that years ago the kitchen contractor never bothered to flatten the kitchen floor before laying tile, so the transition from the kitchen to dining room will end up with almost an inch rise.
While writing this, I realized this might not be a question but a comment on what I’m dealing with. OK, maybe looking for feedback on whether there is a solution I haven’t considered? I also can’t really see ripping out the existing subfloor and flattening the trusses via shims – at a minimum, that would need some sort of sistered truss next to the ones below the side walls, or there would be nothing to support the edges of the new subfloor, and there is no room for that in the basement because of wiring, pipes, etc.
Anyway, this is my biggest headache of the project, where I’m trying to replace carpeting on 2 stories of the house + the connecting stairway.
Oh — then on the 2nd floor, I would probably need to do the entire floor all at once. If I flattened each bedroom individually, then do the hallway, I imagine that would likely end up with a different height transition in each BR doorway.
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27 Replies
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my email is – floormechanics@gmail.com
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Thanks for the comments. I’ll try to answer a few things quickly. Can try pics later, but I’m still not sure where I would upload video, unless I actually simply emailed it as an attachment.
1) Age of the house: built in 1983. I have no idea if the whole tract is like this, or we got unlucky.
2) Situation by the walls: The outside walls are the highest spots, then it drops almost immediately to a lower height. Example: the floor at the side wall along the front-back truss is at some height, then it drops down the half inch or so to the first adjacent truss 24″ OC. After that, the high varies, but would never come back up to the height at the wall.
There is also lippage, for lack of a better term, where some of the subfloor plywood meets. Before taking up the 1/4″ ply under the carpet, I actually thought they had put extra 1/4″ ply in some places, but no: it was lippage and other issues. Maybe no T&G at some seams?
3) Are we OK with leaving it nonflat: I’ll discuss with my wife, but I think we want to flatten the floor. Example issue: China cabinet along the wall needed to be shimmed in the front previously so that it didn’t tilt forward. Anything with wheels would roll away.
4) Use a different plank than what we have in mind: I’m concerned with scratches with our cats’ claws and such. I had a really hard time making any marks on the Floorte Elite, whereas most other samples were easy to scratch.
I’ll try to at least get some pics at floor level and so on, but that will be later.
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Again – I appreciate the feedback on this. Yes, it’s a mess.
In terms of whether it was always like this, or it might be settling and getting worse:
Built this way? I know the center beam was installed below the foundation top because I can see it in the basement. There is 1/2″ ply on top of the foundation that does not match the beam’s height. A level from one truss above the foundation to the next shows the drop too. I agree with you that the builders probably didn’t know what they were doing and/or didn’t care. There has been so much over the years that previous contractors I hired would scratch their heads and wonder why something was done like it was. Electricians would regularly tell me things were fire hazards & then fix it.
Is it settling? I don’t think so, but can’t prove it of course. There is only 1 place in the house where the sheetrock has a small crack & that’s in 1 doorway on the 2nd floor. The kitchen has been tiled for almost 10 years now and the grout lines are still solid. The floor flexes if you jump on it, but it is also nailed, not screwed. The screwed down kitchen seems more stable.
The laser level I have really shows how variable the subfloor is.
Oh — a couple pics are attached. If you want anything specific, please let me know, but I don’t know if there’s much more to say 🙂
One shows where the center beam meets the foundation; the other shows an 8′ level sitting on the floor, from the wall out. It hits on both ends with a large gap below, and you can see that the bubble is out of level.
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heh, good to know that truss system is considered to be very good 🙂
When I say warped, I can see them going up and down a bit when I sight along them in the basement (or put a level against them). It looks like what happens to 2x4s when they dry and you want one nice & straight.
Anyway – I think the warping affects the flatness only, not whether it can support the floor.
I’ll be back in touch with other questions at some point.
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Discussed with wife – we want the floor to be flatter. Really kind of annoying when furniture is tilted.
Plus one more question, please: In my wood shop, I would use titebond II wood glue. Something tells me that isn’t really what I should use to glue the stack of strips. Do you have a reccomendation? I saw a construction adhesive in some videos, but not sure if it also applies here.
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Use Pl 400 construction adhesive or something similar.
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Well, that’s interesting. While adding screws to the existing subfloor, I found that many of the nails came out fairly easily. So the 1/4″ underlayment was attached to the 3/4″ plywood with 2+” ring-shanked nails, the subfloor used smooth nails.
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That’s how they used to do it before staplers became a thing.
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I knew they used to nail the subfloor, but I didn’t know they were smooth nails. (again: house built in 1983)
Also: I had debated whether to screw down the existing subfloor, but then many of the nails I ended up removing were from screwing the subfloor down & that left nails above the wood, so that means the plywood wasn’t all the way down.
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yes, for sure screw or staple it and re sink the nails or pull them.
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It’s been months, but I’m still slowly at it. Then kids came home from college & didn’t want me ripping up their rooms, etc. Anyway, to continue with uneven subfloor issues:
To recap: I have engineered truss floor joists, as seen in a photo above. These trusses are supposed to be strong, but they are not straight/flat. Putting a long level on the visible ones in the basement shows how they go up and down over their lengths; they are not only bowed down from weight sitting on them.
Example: the latest room where I removed the carpet has a 1″ drop from one outside wall to the other inside wall, and a 1/2 inch drop the other direction. And it isn’t a flat drop; the floor curves in various ways.
1) Q: Do I need a structural engineer to come inspect my house?
There are gaps under wall base plates that are 1/8 to 1/4 inch in some places – I can slide thick cardboard under the wall from one room to another. Even some exterior walls have gaps under them. I think the house is structurally sound & this is just because of the truss variation. There are no cracks in walls where things have settled/changed over the years, so I’m guessing this is because the pre-built wall sections were put up & the “wall box” didn’t touch in some places along the length of the base plate.
Part of this was found when a floor squeak remained after screwing all the subfloor down & removing popped nails. The squeak was under the wall base gap. Oh, and also squeaks in spots where tongue & groove subfloor was not used, so the edges moved against each other. Some plywood is marked on top: “other side up”. The builders of my house were lazy and slapped it together in 1983. Tract housing.
2) I was originally going to try flattening the entire 2nd floor so I could have an unbroken floor, but there is so much variation, I think it would be better to flatten each room and the hallway separately. Each doorway might have a slight height transition, but I can’t see trying to flatten it all into 1 single plane. The hallway at the top of the stairs will already add 1-1.5 inches to the top step/landing as it is, so trying to flatten it to other rooms would mean it would be even higher.
Q: Does all this separate flattening sound bad? The doorway transitions should be no more than 1/4 inch or so, so it isn’t like they would have huge steps like a couple places in the 1st floor.
3) The biggest height variations will be into rooms that I’m not redoing, such as the hall bathroom. Since the hallway will now be higher, I believe I’ll have to remove the marble threshold and add some new threshold when done to do the height transition. I guess there is no real question on this one; just commenting in case you read it & have something to say about it.
This house is a bit frustrating, in case that wasn’t obvious. I’ve had a few choice words in some of the empty rooms.
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Thanks again for the feedback on these issues. I went ahead & called our home inspector, who is an engineer also, to do the structural inspection. It sounds like things are stable, but it would give some peace of mind to get confirmation about the situation.
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I’m still at this. Slowly. I’m not doing this full time, but the joist shimming to flatten the floor is even slower than I thought it would be.
I’m going to end up with transitions in every doorway, because there is no way I’ll manage to rip up and flatten the entire 2nd floor in one go. So, each room gets its own height, which leads to the question.
The attached pic shows a doorway from the bedroom I’m working on to the hallway. I’m trying to decide how far to carry the shimming and the subfloor that will go on top of it. I figure the transition piece will go under the door itself. But would the two subfloors continue and meet right at the door stop, so the room’s subfloor continues all the way under the door itself?
Then, later, I’m not sure if the flooring needs to have a gap for the transition (I assume so, but not 100% sure since this is the first time I’ve dealt with it). But, then I’m not sure if the flooring on both sides ends a bit short or if one goes all the way to the edge of the door by the stop.
Do these questions make sense? Need clarification of what I mean, or do you already have comments you could provide?
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BTW: this is what the initial joist shimming looks like. I still need to check for actual flatness and adjust any rows as needed. The far wall in #2 is the high spot on an outside wall. Looking towards the closet in pic 1, the inside wall to the right falls as much as 1 1/4″ below level, back to about 1″ at the door.
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After crawling around with my 10′ level, we’re talking no more than 1/16″ height difference for some joist shims now. Specs for flooring say no more than 3/16″ over 10′, or 1/8″ over 6′. A couple places I might sand an edge down or add a tiny bit more shim, but adding the new Advantech subfloor *might* affect it as much, I’m guessing.
Oh, and here’s a pic showing the drop from the outside wall to the first joist, against a level level.
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Hey Andy. I am out of the office until tomorrow and I would like to answer this using video. I will get this to you first thing tomorrow.
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No rush from me 🙂
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Great! Thanks for the t-molding/transition explanation and the shimming confirmation comments.
The subfloor will be the 3/4″ Advantech subfloor product — I had debated filling in that spot under the door, but with your explanation, it seems I can mostly leave it. If the 3/4″ subfloor hangs over just a bit, then the 5/8″ flooring is on top of that, the slight lip overhang should be fine from your description. And now I know how to plan for that part – thanks again! 🙂
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Oh — feedback, btw: sometimes your mouse cursor wasn’t shown, so the “over here” & “over there” parts were not always clear, but I figured it out. Only commenting so you know in case it affects any other videos.
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Whew! Finally got this room’s subfloor all glued & screwed down and all the shimming works: there are hardly any gaps as I move my 10 ft level around. Also ripped out the closet door frame/jambs/whatever it is called because I found them to be fairly non-straight too. The vertical framing under that is so tilted inwards that you can put another 2×4 piece at the bottom before it gets straightened out.
Oh well – I’m at least very happy that the tedious shimming job worked out in the end!
Closer to getting to the flooring!
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looks great!
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