How can I shave a sheet of plywood into a wedge shim?

There is a hallway with an uneven floor tilt to one side due to foundation settling more on that side. The settling had since been permanently stabilised.

The goal is to nearly level the hallway by shimming one side about 3/4 inch. A 36" wide sheet of plywood with thickness of 3/4" on one side of the 36" width and 0 inches on the other side would seem to fit the bill. It would be glued to the existing plywood subfloor.

How can this plywood be "shaved" to achieve that 3/4" to 0" gradient ?

• Good answers below to what you should be doing in your situation. To answer the question you actually asked, academically ... just go to youtube and search "How to make shims" and you'll find some good lessons on how to make jigs for that. Commented Jun 2, 2023 at 15:17
• Yeah, no. "The settling had since been permanently stabilised." pictures of that? directly below where we're talking, so that you can put a floor jack post in there snug, and then you tighten it a quarter turn everyday for a month. 3/4" is too small to do anything but that (other than a gut job). Problem is, it kinda has to stay there, or you have to put some kind of structural member. Commented Jun 2, 2023 at 18:32
• Mazura a comprehensive stabilisation of the foundation was performed including helical piers down to bedrock, I am not going into details here. Commented Jun 2, 2023 at 18:44
• Is the subfloor supported directly by the slab, or is it on joists spanning a crawlspace or lower floor? Commented Jun 3, 2023 at 1:10
• @TimSparkles second floor hallway Commented Jun 3, 2023 at 1:40

Glue a 3/4" inch block to one side of your ply wood, place a 1/2" block at 1/3 of of the way, and a 1/4" block at 2/3 of the way and then run the 36" sheet of wood thru a planer, cutting a 1/32" off each time... Might be able to do it faster, but planers eat plywood for dinner so, slow and steady wins the race.

Alternative solution

Purchase self leveling concrete, mix it up as directed then pour it on your floor, make sure you get it to 3/4" on the side that is not level and spread it across the hallway... Let it cure then put flooring over it.

Not the prettiest solution, but it works.

A more expensive solution

instead of using self leveling concrete, use a 2 part, thin plastic resin, it will not grind to dust like the concrete, will bend and flex (depending on the type of resin) with the subfloor, and can be screwed through more easily. It will just cost an arm and a leg. Though this cost can be reduces by putting strips of wood, or some other filler into space to be leveled with resin to reduce the amount of resin required...

Advantages: Can get much thinner then self leveling concrete. Will not crumble. Disadvantages: Cost... Also, also future remodelers will look at what you did and wonder why in the ***** you did this. Will either take far to long to cure, or not long enough... Slight danger of fire... Will make the hallway unusable for a long period of time. Also might summon Bob the god of Builders to smite you for heresy.

• Self-leveling cement only works if your outcome should be level. If you're working with a compromise or an intentional slope it won't. It also depends on solid support or it'll disintegrate. I consider it a last resort. Commented Jun 2, 2023 at 16:27
• +1 for answering the actual question, even if the question and answer are both a little dubious. :) Commented Jun 2, 2023 at 16:31
• @jay613 :)... Question asked how to do X... This is how I would do X if I had to do X. This is the safest way I know how to do this... My original idea was to use a really, really big bandsaw and cutting diagonally But most people (me included) don't have a 36" bandsaw lying around and it is a stupid idea so... That being said most people don't have 36" planers either Commented Jun 2, 2023 at 17:42
• A few solid answers, this one includes the literal answer - which I can not execute on lacking the planer - and alternate approaches that are workable for me. Commented Jun 2, 2023 at 18:28
• I predict that these approaches will make you sad and angry, taking more time and effort than you expect for a less-than-satisfactory result. isherwood's approach will leave you much happier. Removing & replacing the subfloor is easier than you imagine. Commented Jun 3, 2023 at 1:17

I know from hard experience that it's very challenging to get a flat outcome doing something like this. Your existing floor isn't flat or level, and you need to remedy both of those things. A simple sheet shim probably won't do it and will be difficult to create in the first place.

Don't try to create wedge shims for each joist, either. That's a huge pain and never comes out perfect unless you measure thickness at frequent intervals from a straight line between reference lines, then cut very accurately freehand.

Instead, remove the existing subfloor by cutting as close to the walls as you can and start fresh. Pull carpenter's lines from end to end on each side, leveling across between them. Now evaluate the larger situation to decide whether flat and level is appropriate, or whether you should compromise with adjacent floor levels.

Once you have that plane established, sister 2x4s to the joists with 3" screws (and construction adhesive, if you like), aligning them with your reference lines. Now you can simply lay in your new 3/4" subfloor, fill the edges where you have the original subfloor remaining, and move on.

• This is the right way to do it... But I think that most DIYers would prefer something that doesn't involve removing the subfloor. Commented Jun 2, 2023 at 17:57
• @WestCoastProjects, you might be surprised. I'd consider this the most time efficient approach. If you create your uber-wedge you may just find yourself fiddling with it to get things flat and solid longer than you'd take to do this. Commented Jun 2, 2023 at 19:06
• i have in mind quickly laying down 3/8" plywood across a bit less than half the width (16 inches ) which leaves 3/8" or less across the entire width and then putting self leveling [mapei/sika] to finish it off. Commented Jun 2, 2023 at 19:29
• It sounds nice in theory. In practice it's messy and frustrating (partly due to space constraints in a hallway, and how do you finish the ends where you encounter other floors? It's rarely anywhere near as simple as that. (It gets lots of votes from people who've never tried it, though. :D ) Commented Jun 2, 2023 at 19:31
• I've not done SLC but i get the sense that it actually levels better if it's deeper so you would be disrupting it if you put down that extra plywood. Something to look into. It's not water, it's pretty thick stuff and you have to work it to make it level out. That's why I suggested the patch below: you're working it anyway, you may as well work it flat yourself and know what you're in for. Commented Jun 2, 2023 at 19:32

Build it up, don't shave it down. Depending on how smooth and level you want to get it, do some or all of the following steps:

• Set yourself up with a temporary reference for level down each side of the hall, and a straight edge to use between your references. This way you can tell where you have high or low spots.
• Buy 1/8 and 3/16 underlayment plywood from your favorite big box or hardware store.
• Start with the 3/16. Cut pieces to fit the lowest areas. Glue and staple them to the existing subfloor. When you're done filling in, every part of the subfloor should be within 1/8 of your reference.
• Now do the same with the 1/8 ply, except it's okay to go a 1/16 over your reference mark. Once you have that all in place, shave down the high spots with a plane or a sander. OR
• Get to within 1/8 or less everywhere using pieces of the 1/8 ply. Then do a full overlay of 3/16 underlayment play over the top of your newly leveled subfloor, stapling or nailing it down as appropriate.

That's the approach I would take (and have taken) when confronting low spots in an otherwise solid subfloor. Basically, build it up a layer at a time. Cutting pieces to weird shapes is far easier than trying to shave down a piece, and for most flooring types the subfloor only needs to be 'pretty close' to provide a solid base for the floor. For sure you can't tell where I had to build up layers under the hardwood floor I did this on, and it's still holding up just fine seven or so years later.

• This probably works, but it's an exercise in tedium and will result in a huge amount of dust. I'd be surprised if the outcome is truly flat in most cases. Commented Jun 2, 2023 at 14:03
• I like this concept. These thin underlayment plywood sheets are unfamiliar to me - so this approach had not occurred to me. I'd add a self leveler compound as the last stop. I already have good experience with on concrete. On youtube I have seen the identical compounds being used atop plywood just as in this case. Commented Jun 2, 2023 at 14:13
• @isherwood personally I didn't do the shaving down step, as little ridges being out by 1/8 was no big deal for the floor going down. It worked really well. We were cutting and sanding reclaimed hardwood anyway so already had to be prepared for dust. The reason I suggest working with 3/16 and 1/8 sheet materials is that they're really easy to cut freehand, which is part of what makes the project workable. And OP: you can pair this sort of build up with the patching suggestion someone else made. Use the pieces of plywood to build up the bulk of the areas, do the final smooth with compound.
– KMJ
Commented Jun 2, 2023 at 16:36

Shaving a 3 foot wide sheet of plywood into a shim is not a realistic solution. You could probably get it done but there are easier and better ways of doing it. For one thing the plies are going to start delaminating and chipping as you're working the plywood and it's going to be a colossal pain to get the taper right over 3 feet. And that floor is probably not a straight gradient so you'd end up with lumps.

The better way is to use a patching compound, for example Mapei's Planiprep SC. There are a lot of manufacturers making their own analogous products so take your pick but you want something that feathers down to skim coat, can be laid down as thick as you need (3/4"+) and probably is modified for flexibility if your floor has some bounce. Patch any big holes in the floor, and either build up the patch or lay a straight edge along the low side and screed it. You can grind it down some if you overshoot (but do wear a respirator).

You could also use self-leveling compound but that seems trickier and has more gotchas. For a small-ish area like you're working with I would go (and have gone) with the manually applied patch compound.

• I agree. I'm always skeptical when folks toss around self-leveling stuff as a solution. It doesn't work well as often as it does, for various reasons. That said, I'm averse to troweling about anything due to the skill it requires for a flat finish. Commented Jun 2, 2023 at 19:09
• @isherwood That's why I mentioned screeding. I always ended up troweling on small patches to raise low spots and grinding down the bumps where I put on too much. Commented Jun 2, 2023 at 19:20

You could make a square router plate that rides on two pieces of wood. As you across the room, lower it a little bit at a time. Once done, use a self leveling compound to even it out more.

• My only experience with routers was with doing cabinet making decades ago . Can you provide more details on your approach? I do think a self leveling compound is going to be part of the solution. Commented Jun 2, 2023 at 14:02
• Look up "flatten boards with router" and you should see a similar sled you can move the router back and forth on. Commented Jun 2, 2023 at 20:40
• OK this is an interesting technique. I'm going to go an easier route but its neat to see a variety of approaches Commented Jun 3, 2023 at 1:41
• I can't put elves into the evil category in my mind. I have the model of `Glorfindel` `Earendil` and `Galadriel` firmly in place. Commented Jun 3, 2023 at 17:18