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I need to build a wall on a basement floor. The wall will need to support significant weight so I want it to be solid. The sole plate will be secured with concrete screws. The floor is a slab; it is bumpy and slightly sloped where the sole plate will be. A 2x4 rocks. The gaps are up to 1/4 inch.

How do I put down the sole plate flat, level, and solidly supported? I don't want to feather patch or SLC a large area. I don't want to grind down the bumps, they are too wide. Can I flatten a strip just under the plate with mortar? Press the plate in place into a wet layer of mortar like setting a bathtub? Bound off a 4 inch strip and pour self-leveling compound?

The slope is across the plate, not along it, so the plate is rolled slightly. The bottoms of the studs would have to be cut at an angle but all the studs will be the same length.

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Use a product called "non-shrink grout" It is available in big box stores and masonry supply businesses. It is a cement product, can be laid out under the plate, not needing to cover any other area.

When you get ready to do the setting of the plate, get tapcons that are perhaps an inch longer than needed. This length is only a guess, they may only need to be a 1/2" longer than originally needed. Tapcons typically cannot handle going into strong concrete no more than 1 1/4". If it goes deeper than that, they usually shear off, so when adding the non shrink to your floor, consider that as part of your figuring the over all length of the screws.

The non shrink will need to go under the entire plate, even the high spots, it does not like going down in a thin layer when doing this type of thing, at least the way I am going to suggest it be done.

Start by drilling the tapcons into the floor through the plate, once started, add shims near the tapcons to get your plate the way you want it. It need to be at least 3/8" at the high places in the floor to then pack the non shrink under the plate to support it. You may be able to set the screws a little deeper after the grout is packed it to help level it all out. You may only have a half hour to do all this so you need to get it close to start with.

The tool that will help pack the grout under the plate is called a "slick". It is a masonry tool for smoothing masonry joints. then again a stick may do the trick too, having an offset handle helps, the slick has this.

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  • This ^^^, and reminding that some hydraulic cements go off really fast, so think about smaller batches. Feb 21, 2023 at 16:49
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    Yes they can, that is the reason for having it even on the high spots, for when it gets too thin, it will flash set. If I remember right, some instructions say to wet the surface. Make it damp, but not standing water in small puddles. If it is shiny, it is too wet, damp surfaces will not shine..
    – Jack
    Feb 22, 2023 at 4:38
  • @Jack Would you recommend against screeding a thin patch 1/2"+ thick and then laying the sole plate on top instead? What you describe sounds more efficient but I'll either be working with a PT 2x4 which could use straightening or sill gasket which would be a pain to pack mortar under. Feb 22, 2023 at 13:07
  • There is a chance it could work, it is worth a try. I imagine it won't take a lot of time or material to do it either way. My concren is if you laid it out and let it set up, then drilled the tapcons in, it may make the non shrink break its hold from the concrete floor. If you did the hole while it is still wet, the vibration from drilling may make the non shrink settle out. perhaps it could be tucked back in since most of it will still be there. Most of this is a guess, you may not have any trouble at all, it is just that I have never done it that way.
    – Jack
    Feb 23, 2023 at 5:56
  • There is yet another way, but not with tapcons, but using epoxied in anchor bolts. This is the way I have done it. Then you drill holes in the plate 1/8" to 1/4 bigger than the bolts used, set the non shrink add sill sealer, then your plate, using the nuts to draw down the plate flat and straight as they are tightened down. Place them no more that 4 ft. apart, closer is fine too.
    – Jack
    Feb 23, 2023 at 6:01
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Plastic shims would work just fine to avoid cutting studs at an angle, but if that bothers you, you can create a short “wall” of cementitious material To put under the bottom plate as has been described. Don’t worry about making it perfectly level. Just cut each stud individually to the correct length. I would use expanding or lead anchors instead of screws for less chance of breakage.

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  • Do you have anchor brand and specific type recommendations? I'm thinking the concrete screws because I know they are decent quality so they shouldn't rust from moisture in the slab or the pressure treatment chemicals. Feb 22, 2023 at 13:18
  • 1/4” Stainless steel wedge anchors. Any brand. The screws sometimes work, but some break or stripped threads don’t grab.
    – Mlew
    Feb 24, 2023 at 5:21

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