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Community and Q&A

Framing walls on an uneven concrete slab

zmorgan03 | Posted in General Questions on

I am about to start framing up walls for my woodshop. The existing slab slopes from back to front a total of about 4″ over the length of 28′-0″….yes, 4″. The concrete crew did a terrible job. I need some advise on the best/most efficient way of dealing with this slope in the slab when framing the walls. I feel a tight connection with a PT sill plate to the slab is crucial. So, I could:
1) Frame walls square on ground and then deal with space between PT seal plate and bottom plate of wall with large shims. Doing this though, I worry about a good connection to the sill plate.
2) Cut each stud individually to length so that top plate is level. Doing this requires the top plate (or string) up first and level, so that I will have something to measure to….seems it will work, but I worry about the consistency of measuring to something that spans 28′-0″
Sorry for the lengthy question….any tips/advise would be helpful!

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  1. SaulK | | #1

    Option 2 is the way to go. You can use a builders level or laser to shoot each stud height indiviually. Setting up a stringline would work, but I would still check your line with a level/laser to make sure it's not sagging in the middle. This way you are able to get a good positive connection (i.e. anchor bolts) from your plate to your slab, and it's easier to seal against air infiltration.

    If you do set a stringline, use a braided line. It's stronger, and you can pull it a lot tighter than your typical twisted line.

    Honestly I'd check your contract with the concrete company, assuming it's a new slab. That's bad enough, they should have to jack hammer it out and replace it.

  2. GBA Editor
    Martin Holladay | | #2

    Option 1 will also work (a pressure-treated sill plate and an untreated bottom plate, with shims in between). Three caveats:
    1. Make sure that you have a shim under every stud.
    2. Tie everything together with well-fastened wall sheathing.
    3. Consult an engineer if you are in a seismic zone or have any special concerns. The engineer's opinion is more important than mine.

  3. wjrobinson | | #3

    Have the concrete company come back and overpour the slab level. Otherwise you still have a floor with quite a slope.

    If the sloped floor doesn't bother you then they could lay one row of any size block using a level string this time for the top edge of the blocks. The shim mix will need to have stone, larger stone as you get to the 4" thickness.

  4. STEPHEN SHEEHY | | #4

    If the slab was placed recently, get a lawyer to send the concrete firm a threatening letter. 4" is way, way beyond acceptable.

  5. Expert Member

    As long as the slope is consistent you can still build the walls on the ground using differing height studs without having the top plate in place. We do something very similar laying out gable ends on subfloors.
    - Cut two studs, one higher than the other by the amount the slab slopes.
    - Attach them to the ends of the top plate laying on the slab, making sure they are at 90 degrees.
    - Snap a chalk line from the bottom of each showing the sloped bottom plate and cut all your studs to that line.

    However, as others have said, that's just the start of your problems. Everything you do from then on will be affected, from the drywall and sheathing, to the siding and any cabinets you might want to put in. Starting from a sloped base puts you behind the eight ball from the very beginning.

  6. user-5010253 | | #6

    Zane , set up a transit Find high spot I find a horrible foundation requires shims that start at 1/2” and go to 1/16”. Rip 4 shims for every size from 1/2” to 1/16” cut them to a length a little shy of the sill size. Take a hammer tacker staple sill seal to pressure treated sill when you’re shimming put shim directly on foundation to bottom of sill seal. After leveling at every anchor bolt find you wall layout and put a shim under every layout and bearing point. Take a picture of your shim job and send it to the concrete terrorist. Lol. Let sill settle after you put walls up then use spray insulation at every gap between foundation and sill.

    1. Expert Member
      MALCOLM TAYLOR | | #7


      Zane's post is five years old. If he hasn't managed to frame the walls by now he's probably got bigger problems than the uneven slab.

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