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Doing a complete remodel of our kitchen and I'm at the stage where I am going to take the subfloor out for plumbing work, etc. I also want to plan ahead for the eventual floor install. Photo 1 below shows the general construction of the house:

  1. Foundation wall in basement
  2. 2x8" floor joists overlapping at foundation
  3. 3 1/2" x 3/4" tongue and groove planks as original subfloor (sandwiched under wall plates) at 45 degree angle

The new flooring I want to install is LVP which requires a very level subfloor. I don't think that the current subfloor will cut it, as there are hills and valleys all over due to inconsistencies in the rough-sawn floor joists, and settling over time. The maximum deflection isn't massive, perhaps 1/2" between the high spots and the low spots. However, the rate of deflection is quite rapid, as shown in photo 2. You can see the old mortar that someone tried to use to level the floor before placing plywood underlayment and tile (now removed).

I'm wondering what the best way to create a level surface for new plywood subflooring would be? I have read about many methods including shimming the joists, sistering new joists to old ones, using raised blocking between joists as a level surface for the edge of the plywood, sleeper joists, etc.

I want to avoid self-levelling compound as I believe it will be messy and since I plan on removing the subfloor anyways I'll have access to repair the joists as best I can to do the job properly. I also want to avoid extraneous layers (ie: 2 layers of subfloor, sleeper joists, etc.), to keep as much room space as possible.

My questions are:

  1. Is there a best practice for levelling out old floor joists? (ie: shimming, sistering, blocking, other?)

  2. Is it recommended to leave the sandwiched subfloor underneath the wall plates? (Photo 3)

  3. Any recommended tools (ie: laser level, etc.)?

Thank you for reading and leaving answers!

Kitchen Model

Out of Level Subfloor

Subfloor Sandwich

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  • It sounds like your joists are the uneven bits—the planks should be relatively consistent in thickness. Why not just smack up the low spots from below and shim the joists in those locations?
    – Huesmann
    Commented Aug 7, 2023 at 12:31
  • @Huesmann Thank you! That is an idea. I was going to remove the planks as I need to do plumbing and electrical work also, but a bunch of people have come back to me and said that the planks are not in super bad condition. I might just try to cut sections out where needed or broken, and do as you suggest. Commented Aug 7, 2023 at 15:28
  • I wouldn't cut if I could simply raise them as necessary. Cutting comes with its own set of problems—in the "putting back" part.
    – Huesmann
    Commented Aug 8, 2023 at 13:16

1 Answer 1

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For #1 it will depend. If the joist is a constant 1/4 to 1/2 inch difference, a shim of similar size will work or if just a few small sections. If the joist is wavy then a sister will work better.

#2 I would leave/cut them near the wall. Cut at joist. It would be hard to find the same thickness wood today and much unneeded work. It is holding up those walls.

#3 Tools will depend on what you need(or want an excuse to buy). A good level, which you seem to have. Good saw, hand or power.

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  • Thank you so much! With sistered joists, is it usually best to sister both sides of the joist (to eliminate eccentric load) or is it safe to do just one? Commented Aug 6, 2023 at 21:54
  • It should be safe on one side. You are not repairing a broken/cracked joist, just levelling the floor.
    – crip659
    Commented Aug 6, 2023 at 22:19

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