Leveling subfloor for hardwood

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Well-Known Member
Aug 11, 2020
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Harrisburg, PA
This winter I cut down two massive Norway Maples. The trunks went off to the mill and I had them cut, kiln dried, and milled T&G for flooring. I pulled the carpet up in my house today knowing I would have to do some leveling, but I didn't expect this much. It has a significant peak in the middle. Using a rotating laser level, I measured down to the highest point (which is about on the 2/3rds point in the room) and got 17.25". From there it all goes downhill.... literally.

I'll try to post a photo with my plotted measurements. I did a CAD of my floorplan, so it should be pretty easy but it will take a hot minute. I know it's easier to bring the rest of the floor up, but two things are preventing me from doing that:
1- why bring up 300 sf if all I need to do is take down 20 sf?
2- if I pour enough, I will lose 1.5" of my already low 82" ceilings, and it will require replacing a custom entry door. Not cheap.... both because of the door, and all the leveling mud I would need.

Main structure was built in about 1900. This part of the house where the bulk of the flooring is going was an addition in the 1920s. The floor is very sturdy. The main beam is a rough-hewn 4x8 supported by a concrete pad in the dirt crawlspace. Subfloor is 3/4 T&G pine.

For the most part, the entire perimeter of the floor is sitting at approximately 18.25" below the line, so 1" too low. There are two really low spots at about 18.75, but I can pour leveler in those.

How does one go about lowering a floor properly? A bonus here is that the east edge of the subfloor doesn't go under any wall. Since it's an addition, it just ends at a sill plate sistered onto the original foundation, so I could pull a fair amount of subfloor without really much effort.

Pics to follow.
In this photo, the red-outlined area is getting the Maple.

Joists run N/S, subfloor runs E/W, maple will run N/S. One beam runs under the highest point in the floor (shown by the zeros). I'm relatively sure there is an additional beam, but hard to see without more demo.

As you can see, if I try to use leveling mud, I'll be filling the whole bloody floor except for one strip near the middle. It's too much to sand down the peak, or I'll have 1/16" subfloors in the middle.

I don't mind if it slopes 1/4" from north to south, but doesn't it look to you like if I could just yank that subfloor in the middle, shave a 1/2" off the joists, and replace with some ply or OSB, I'd be good?


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Were it I, I would rent or purchase some 5ton bottle jacks, a length of 2X12 to set them on, jack up the beam, pull and cut the pier posts shorter, and be done with it.
I have plenty of bottle jacks, I think my concern is access to the crawlspace. How much of the subfloor can I remove without risking structural things happening? Can't really reach it from the basement without taking out the jack-stud wall between the foundation and a load bearing wall above it. This type of foundation work is not really my strong suit. I'm a theater tech director, so I build stuff that LOOKs like the real thing. I used to work for a contractor, but that was new construction.

How much of the subfloor can I remove to get access to it without risking structural problems? Can I just leave the sub under the exterior walls and it will be OK? I don't mind taking out a large-ish section of it to true up the joists and replacing it with ply or OSB, but I don't want to make bad things happen.
So you're saying that there is no access (or very little) to get under this addition, unless you drop in from the top through the subfloor. Not a pro here but taking out the subfloor to expose the joists sound pretty common to me.
I think I would avoid using OSB for a subfloor.
Correct. There is only about 5" in some places between the joists and the dirt.

What would you use for sub? Ply?
Your subfloor is not a structural component. You could remove it all if need be. Shaving down the existing joists makes the most sense. Trying to lower the piers would still leave you with a set crown in the joists and they might not settle down. You definitely do not want to "mud" the floor. Remember, it doesn't have to be perfectly "level" but just flat.
I agree with @mabloodhound.

You have gone to all the trouble of having your flooring made from a tree onsite so the project is clearly a special project. I would take the entire sub flooring up and get a really clear picture of what you have below. With a dirt floor just a few inches in spots you could find any kind of damage to joist or the sub flooring you didn’t know about. Having it opened up would make all your work easier and you could also install some vapor barrier taping the joints and any additional posts and footings you may think would help. Maybe even some heat ducts that were never run or some new needed wiring etc etc.

Were the floors ever cold in that room?

Putting the new sub flooring down can then be glued and screwed and you have piece of mind of what you end up with will be good for the long run.
Thanks for all the input.

I've been working a lot on this as my back pain allows. I have decided to pull up the entire subfloor and do things right.

The first 6' or so near the door was pretty easy. That part was evidently an old patio and the framing is rough cut 2x6 shimmed up of the slab. I removed all of that subfloor, trued up the joists, installed a new perimeter for screwing, and I splurged on some Advantech OSB and some good subfloor adhesive. I have that section darn near flawless. That whole area is now within 1/16" which meant shimming up a few areas under the joist, then adding shims to the tops of the joists in places where they dipped.

The rest of the floor is over the dirt crawlspace and I think I have a plan of attack. The west wall is the problem area. Old termite damage that I anticipated. The first two joists (what I have uncovered so far) have enough damage that I am just replacing them entirely with new 2x. I should have the rest of the sub pulled up today and I'll update more. As far as settling down, I think I'll be in luck. The rim joist on the west wall looks good so far, if not a little crooked as the house has settled. That one I will basically screw a new 2x on the face. The second one I will replace entirely. The south wall is currently at the proper elevation, so no brainer there. The east wall should be simple. It is a rim sill that is nailed to the original foundation. It appears to be sturdy, so it can be shaved.

In this photo you can see the concrete slab part with 2x6s 24" OC. The dirt crawlspace part shows the 2x8 joists on a remarkably annoying 18" centers. The pink are "piers" which are concrete pavers on the dirt, then a concrete block, then usually a scrap piece of 2x, 1x, or in one case 4x leftover lumber to shim up to the joist/beam.

The two western joists over the dirt are being replaced, so they will go at the correct height. The eastern joist, the plan is to either reposition it or shave it.. Then if I cut the nails between the E/W beam and the eastern joist, the entire deck should be "hinged" on the south wall and floating on the piers. In theory (he say hopefully), then it's just a matter of re-shimming or using some 4x4 to make stubby leg posts and reconnecting it all.

picture 1 shows the pier. Paver, block, shims.
picture 2 and 5 show one of the joists with termite damage (glad I tore into this... don't worry, it's old damage. Termites are long gone and I treated anyway)
picture 3 is looking southeast. I think if I cut the nails between the beam and that rim joist I can let the others drop and then shave the rim.
picture 4 looks north. I went ahead and installed two pieces because my house is so small I have nowhere else to put my couch.


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One more photo just because. This shows how far I had to drop the joists over the concrete slab.


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Good job. That's how you conquer an old house. Shims and shaving!
Looks good. :thumb:

Don't forget there are many different steel hangers you can use in joining some of this new and old.

Also this is the only chance you will have to be in this space, if there is anything you think you need to do.
I have some Simpson joist hangers. Might need more.

So I encountered two things that I would love some help with.

1) the west wall (an exterior wall) had the most sag in the floor. I suspected termite damage from a really old infestation (none there now, and the exterminator guesses many decades old).

In this photo, you'll see what's left of a 2x4 that was nailed onto the sill and supported (somewhat oddly) with 1x down to the dirt. This was the nailer for the subfloor. On this wall, the subfloor doesn't go under the wall for some reason... maybe a slapdash repair, who knows. The foundation appears to be concrete, but can't really tell how thick. Outside is lath and stucco. The 2x8s behind the 2x4 are mostly trashed as well. Part of me is thinking that if they're trashed, they're obviously not supporting any weight, but it seems like there are 2 or 3 of them sistered in there. So the other part of me is thinking if there is just barely enough non-eaten wood left in them, maybe they are feebly supporting the wall, and removing them will cause the house to collapse with me in it. How do I proceed with this corner? Do I rip out the eaten wood and replace it? How do I know what is supporting the wall without taking these out to see?

2) The joists on this side of the room are spaced at a frustrating 18-20". Fine for 1x6 boards, not so great for 4x8 sheets. I could cut all the floor sheathing to a length that matches the spacing, but that would make one continuous seam in the middle of the room. I could add one joist at 8' from one of the walls, but that would also make a seam the whole way across. I could add TWO joists; one 8' from the left wall and one 8' from the right wall so I could stagger the sheathing. What's the "right" way to do it?

Well I’m glad you took the advice to dig into it deeper.

I think you have a pretty good line on what needs done and your vid does a good job of showing what you are up against. I would be getting rid of anything that is even slightly compromised.

The big question I have is the areas where the load supporting walls are actually contacting the foundation and how blocks and shims had been added as additional support points.

I think I would be scribing a line one foot up on the walls and cutting away the drywall/plaster down to the framing to see what’s left of the sill support. At least in the areas where the bugs did a lot of damage.
Additionally the places you pointed to where the shims on the blocks look to be notched into the rim joist. I don’t think they were done that way, I think it is the weight of the house compressing the bad wood and the house is lowered in that area. You can check and see if that compares to your level numbers you measured.

If you try and jack that back up or not just leveling the joists isn’t enough until you stabilize that from future movement. Sometimes correcting it back all the way isn’t the best plan because over the years other work has been done that will crack if you try lifting the house an inch or so. But you can’t let it to continue to fall more.

To me it looks like someone tried to do some of that with placing those blocks and nailing in supports and shimming etc. Only now all that looks like it is not doing much any more.
Thank you. From what I am seeing, the blocks aren't additional support, they ARE the support. Some kind of pier/beam foundation that is supported with a paver and a block, then shimmed to height.

Here's where I really need guidance. This drawing is an elevation view, so as if you're outside laying on the ground looking at the corner of the house.
Getting the old 2x8s out isn't an issue. They're primarily sawdust. My concern is that the sawdust may be providing just enough support to prevent the wall from crumbling. I have a plan to repair it, but not support it during the repair. Basically take it all out and replace with PT lumber, but if I take out the compromised rim joists, will I risk having the whole corner of the house fall off? I would be removing about 12' under the E/W wall and another 12' on the N/S wall.

Can you folks take a gander at the only thing I can think of and tell me if I'm crazy?

In the next photo, the yellow would be 2x6s screwed to a stud in the wall, and under a ceiling joist or rim. I would likely use some HeadLok screws I have an I wouldn't be shy. The red squares are bottle jacks on a paver with a scrap of ply. Once supported, I could rip out all the compromised wood and replace it. This idea at least gets the support jacks out of the plane where I need to replace lumber. Does that sound safe?

Important to note. According to the wonderous rotating laser, the corner you're looking at is about 1" lower than the highest point in the floor, (which was about where I'm standing for this photo) but since I lowered that high point about 1/2", the corner is actually only about 1/2" lower than my target joist height.

So that begs the question. I could
A) replace both the rims at their current height, replace the joists, and painstakingly shim the entire room (sounds like a recipe for a noisy, weaker subfloor)
B) replace both the rims at their current height, but notch the top of each span joist so I can clear the sill and install 1/2" higher to my target height
C) use the bottle jacks to lift the whole corner 1/2" and sneak the rim under it at target height before installing new span joists.
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Here is a not-to-scale elevation looking east showing my target heights. The part over the concrete slab I already dropped the framing down 1/2" toward the middle, so that section over the slab is kosher at 19" and level. Now I need to drop or shave the other cross beam and bring up the south end about 1/2".