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:
- Foundation wall in basement
- 2x8" floor joists overlapping at foundation
- 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:
Is there a best practice for levelling out old floor joists? (ie: shimming, sistering, blocking, other?)
Is it recommended to leave the sandwiched subfloor underneath the wall plates? (Photo 3)
Any recommended tools (ie: laser level, etc.)?
Thank you for reading and leaving answers!