Hot Topics: Leveling an Uneven Floor

Leveling a basement floor.

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Original Post: Leveling extremely uneven floor

Justin Furnas Member

I am working on converting part of my basement into an office. In doing so, I have come to realize that there is an extremely aggressive slope running toward one side of my basement. There are areas where it's easily 4" lower. My original idea was to use leveling compound, but I realized that it's not made for the thickness I need, and is also extremely expensive.

The next solution was to use cement, but I then realized that it's too thick, and wouldn't feather very easily in the spots where it's extremely low. I then tried to scribe some 2x4s, but was unable to cut the scribes evenly, and made a terrible mess. The end result I came up with is this:

Leveling a basement floor.

I took 1x2 furring strips, laid them down flat on the high side, and shimmed them up to level on the low side. There are 2x4 blocks and or shims about every 6" or so, but now I am worried that even with the 3/4" OSB on top of it, the furring strips are not strong enough to support weight.

Would the setup I have work for what I am trying to do? Another alternative may be—although extremely expensive—to use the furring strips as guides, pour new concrete between them, and screed the entire floor level. Considering the room is 8.5' by 14.5' long (the furring strips are running along the 8.5' section), and parts of the floor are 4-6" lower than the rest, it may result in an extremely large amount of concrete to pour, which could get costly. By my calculations, that's about 106 #60 bags, at $5 a bag.

Would the 1x2 furring strips work for what I need it to? (Perhaps if I add some more supports under the higher parts, but I think the 3/4" OSB may offset some of the load and disperse it evenly over more of the strips at once. I centered them at 16" on center.)

Sitting on top of the 1x2 furring strips is going to be the bottom plate of the 2x4 wall. The 2x4s under the furring strips were to level the floor from right to left (in the image), as it had two different slope angles going in different directions.

Marq1 Member

The problem is you're using a material (furring strips) on a floor that should be kept off a floor, especially if you embed into concrete.

You mentioned concrete—that is clearly the best material to use and once cured, self-leveler could be applied to finish it up. An alternative filler for the low spots would be cement board stacked up, mortared in place, and then use a self-leveler.

Pilot Dane Group Moderator

Moisture/water vapor coming up through the concrete slab will cause the untreated furring strips and wood blocks to rot. Trapped underneath the floor, you're creating a dark and damp space which is perfect for destroying wood. You should never use untreated lumber in contact with masonry. The bottom plate of your framed wall should be replaced with pressure treated wood.

Justin Furnas Member

It doesn't matter either way; most of this needs to be tore down. I continued laying the strips out in this manner to see what I ended up with and at the lowest point there's an 8" height difference. There is no way I can scribe sleepers or even do anything to bring that back up to level. I understand that there is supposed to be somewhat of an incline there, but 8" in one direction and 4" in the opposite is just ridiculous and poor craftsmanship.

Also, the lumber sitting on the slab is pressure treated. The furring strips aren't, but they weren't being permanently fastened yet, either. I was trying to get a feel for how high I would need to go.

XSleeper Forum Topic Moderator

If you are so inclined, tearing out the cement floor and repouring a level one is the best and only good solution to this problem. They may have done it this way if the floor occasionally got flooded. Is there a sump or floor drain anywhere? Is that floor not very thick? Have you drilled it or busted through it anywhere?

Justin Furnas Member

Again, there is no way I am spending that amount of money to fix this. There is no drain in the floor. There is a cleanout back there, but no physical drain. My guess is that they did it so that all the water would pool in the low spot, and you could pump it out rather easily. The floor is heavy and thick. When using the powder nailer to tack the ledgers to the floor, one didn't shoot through all the way and chipped the concrete, and you can see at least 4" down in thickness.

The cement is smooth with no cracks, so I can tell you with almost 100% certainty that it didn't settle this way. It was built this way. Concrete doesn't really 'bend' without cracking.

2john02458 Member

If you have enough headroom, you could build a platform like a deck. Use pressure treated wood for the structure. Since you can support below wherever you need to prevent bounce, 2X4s on joist hangers should be enough on 16" centers. The 4- to 8-inch gap at the edges should be enough to allow ventilation and prevent moisture buildup.

Justin Furnas Member

This isn't a terrible idea, although I have vent work, etc. that is causing me some tight headroom as it is now, so I am worried adding another 6 inches or so (2x4 + underlayment + padding * carpet) will make it unbearable. I am also not sure how I would support the bottom of the 2x4s where it's not level, either.

I did find that if I use a 2x6 instead of a 2x4, I can scribe it a little more accurately. I am going to pick up a couple pressure treated 2x6 boards tonight and scribe them, then rip them down with my circular saw to see if that works. I think the biggest problem is I am fighting with slope in two directions, not just one.

EDIT: It's not a significant gain in height (only 2 inches), but is it possible to build the platform using 2x2s instead of 2x4s—or do 2x2s lack the structural support to be used as load-bearers? They need to span a distance of a little less than 8 feet. I could even space them 8" on center so there are more of them, if it will help with support.

I was thinking of maybe using 2x2 furring strips (with a plastic VB underneath), or AC lumber with these hangers:

2john02458 Member

You could use 2X4s laid flat. You would only need to use PT for the ones that lie directly on the concrete (even with a vapor barrier). Otherwise, use standard lumber with PT for the leveling blocks. Provide supports every 4 feet. Use tapered shims (wood shingles) to level.

How big is the room? A sketch showing the slope would help. Note the slope as 0, -4, -8, etc. at the corners and at a couple of places near the middle.

If you are concerned about the floor shifting from side-to-side because of the slope and causing the walls to go out of plumb, you could consider building the walls on the existing concrete floor, accounting for the slope by adding additional sill plate and blocks below the sills as needed, and then building the floor inside the walls (instead of building the walls on top of the floor).

If you are concerned about the height even with 2X4 laid flat, I have some ideas that would gain 3/4 inch by recessing the subfloor into a rabbet in the 2X4 or by creating a sort of inverted tee structure from 1X stock.

Do you need or are you pulling a building permit for this work? An inspector would probably not approve some of these ideas.

I don't understand how you intend to use the stud plate tie.

Commercial grade rubber-backed carpet squares are a great alternative to standard carpeting in this kind of installation and are only 1/4-inch thick.

Justin Furnas Member

No, this is not being inspected and a permit has not been secured. It would've cost me 8 times the cost to do it to code because of all the problems I ran into along the way.

Here is a rough sketch of the basement with the room dimensions, as well as what I had planned to do with the hangers I linked:

A sketch of the plan.

The red circled area is where the fasteners would attach. They would be on the underside of the platform, similar to how joist hangers are. Since 3/4 of the platform will be mostly on the flat surface of the cement floor, and wedges will be cut to secure the 2x2s at the low spot, I don't think using these will be too much of an issue. I had planned to space the 2x2s 8-12" on center.

The numbers on the right of the rough floor dimensions indicate where the worse slope values are, and the light grey lines indicate the direction the slope goes, with the three lines converging at the end point of the slope.

I already have all four walls up, I am just working on the floor, now. I was able to run a ledge level from the front to the back to set the remaining walls on. Now I am just working on building up the foor inside of the room.

I was considering using the carpet tiles, but was worried about sound and padding. I also wasn't sure if they made a somewhat thicker pile in carpet tile, as all the ones I've seen are low pile and don't have much texture to them.

2john02458 Member

So, the floor is level up to the area where the grey lines are? In that case, you can use 2x2s at 16 inches on center on a vapor barrier for the level area and just deal with the sloped area differently.

Is there a wall at the corner where the slope occurs? If there is, you could nail a ledger to the wall to support the ends of the 2x2s over the sloped area. Place a PT 2x4 or 2X6 (ripped down if necessary) underneath to support the span of the 2x2s. Follow the slope (i.e., place the support at whatever angle necessary relative to the wall) to keep it on a constant plane.

Or, think of the floor over the slope as a hatch (removable if you need to get to the cleanout). Build a frame that hangs from the wall and/or rests on the concrete floor and build a hatch cover out of 2x4s supported on ledger strips around the inside of the frame.

Justin Furnas Member

Attached is an image of how I would be using them, except they would be horizontal instead of vertical. They would also be facing down to the concrete, not facing up towards the underlayment. I am also going to use the screws instead of the nails to hold them together.

Those fasteners are essentially just to hold the framing together. The support will come from when it's attached to the walls, and from underneath in the low spots using the custom scribed wedges/blocks to level it out and support it. You still don't believe that would work?

2john02458 Member

It should work to hold the framing together. I was concerned that they wouldn't be supported from below and the ties would bend.

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