Shimming an uneven floor--at subfloor or higher?

Discussion in 'Floor Preparation' started by dave123, Dec 16, 2017.

  1. dave123

    dave123 Member

    Got a 1950s kitchen and attached laundry room. Am down to the plank subfloor, and have found that it is sunken in a few spots 1/2 to 3/4 inch, due to the underlying joists being heavily cut by the original HVAC installer.

    Over the planks will first be 1/2 inch ACX, then 1/4 inch hardwood underlay, then resilient flooring. This will bring the new floor level with the hardwood floors of the surrounding rooms.

    But as far a shimming, would you shim above the planks, then put the ACX and underlay? Or would you attach the ACX, then shim, then underlay put over the shims?

    Because many of the planks are lipped, it seems there'd be less shimming to do if the ACX went on first. But I'm not sure.

    Last edited: Dec 16, 2017
  2. Mike Antonetti

    Mike Antonetti I Support TFP Senior Member

    No way to lift and brace from underneath? Steel?
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  3. dave123

    dave123 Member

    There's at least three separate places that would need to be jacked. From what I've seen and read, jacking has to be done over weeks or longer, and even then often damages the plaster walls and changes how doors close. etc. I'd rather not go that route if there's an alternative.
  4. Mike Antonetti

    Mike Antonetti I Support TFP Senior Member

    Ok, so in the five or so years I’ve been here I don’t think anyone has a good system, basically it’s been asked to install hardwood, leveler to nail into.

    I’m not sure if anything is available but my thought is epoxy and sawdust as filler. All you need is filler for the plywood to rest on and the fasteners would go through filler and into plank subfloor. The epoxy would need to be formulated on the rubbery side of hardness. It would then be screeded to the correct height. I think cement based levelers do not stick and they crumble when penetrated by fasteners.

    If you’re good at shimming, the shims would go first, then the plywood and underlayment.

    I’m not sure about the areas needing filler or how fast they slope but the 1/2” plywood if it would follow the contour then underlay, a patch may be used on top for a sheet vinyl.

    Screws usually push plywood away as screwing down causing gaps between the layers, I usually have to reverse the screws and apply weight nearby to keep them tight. None of it fun!
  5. dave123

    dave123 Member

    I thought a floor like this could be shimmed with asphalt shingles and roofing felt?
  6. Jim McClain

    Jim McClain Owner/Founder Administrator

    What do you mean by "lipped?" Cupped? If the subfloor is not flat, you need to make it flat before you apply any underlayment panels. If the boards are cupped, then that will require sanding/grinding. If after that, you still have serious low spots, then fill those with wood-based materials. You haven't described or shown how large an area(s) that need "shimming" so I can't advise any specifics. You could try mapping the low spots in a topographic way and then layer plywood scraps or cedar shingles or whatever to fill the low spots. Then you can install the half-inch ACX, check for flatness again and fill any remaining low spots with layers of 15# saturated felt. Finally, lay the hardwood panels.

    Make sure your 2 layers of underlayment panels are fastened properly - it will take a LOT of nails/staples. The first layer of underlayment is nailed into the subfloor, not the joists. The second layer is nailed to the first and can also penetrate into the subfloor, but isn't necessary.
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  7. Mike Antonetti

    Mike Antonetti I Support TFP Senior Member

    I wasn’t sure how you planned to shim, I assumed you were going to use 2x material. 1/2-3/4 is a lot of shingles/asphalt paper, usually reserved for 1/4” or less. As Jim said you described the depth but not the area footage or how it relates to the layout.

    Valleys or dips are normally the shape of a pond, so filler is mostly circular.
  8. dave123

    dave123 Member

    There are three areas that are sunken--one at each of the doorways to the kitchen, and one on the far side of the adjoining laundry room. But these areas are small, about 2-3 foot across, centered over where the joists were cut.

    Sounds like people here though shim with wood, not shingles? What do you use for areas that need less than 1/4 inch thickness?
  9. Dan Schultz

    Dan Schultz Certified Wood Floor Inspector Charter Member

    You already know what has caused the issue you have with the flatness of your floor. It needs to be fixed at the root of the cause.

    I know this isn't the answer you're looking for, but I can't bring myself to give any other advice. Yes, I do believe in using a shingle and layers of felt paper for a small area to bring within tolerance, but no way i could recommend that for 1/2-3/4".

    Another way to look at it, you're making an investment in your floors. NOT cheap. You're also adding weight over the joists that are the problem. Don't want to deal with a sticking door? Now or later?

    I absolutely hate do-overs. You are setting yourself up for one.
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