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I bought a fixer-upper knowing little about how much of a headache I was creating for myself.

I've recently removed the hardwood flooring that was lain directly on the joists (no subfloor) of my second floor bedroom. I'm now laying a subfloor and will install hardwood flooring on top of it.

After screwed (yes - screwed) down most of my plywood, I noticed that my joists were uneven. As you go from one joist to another, they drop by as much as 1/2-3/4 of an inch. This is noticeable and I would like to correct it. Fortunately, I can easily unscrew my plywood (ha!)... but now what?

I'm leaning towards putting shims between the joists and the plywood subfloor, but I don't see much talk about doing that online. What kinds of shims should I put on the joists? Do they run along the joist or perpendicular to it?

Backtracking a little, the joists seem to be low because they were partially cut (I have no idea why) where they would otherwise rest on the bearing wall. I've put some blocking underneath them to provide some level of contact between them and the bearing wall, but I can't (and frankly don't really want to) raise them to be level. It has crossed my mind that I should replace them. Advice would be appreciated here, too.

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It would be a straightforward process to shim all the low joists along their lengths. One would go about this by finding the highest joists and then shimming all the rest to be even with those highest joists. Of course this requires removing all of the existing subflooring.

It would require also that the highest joists be checked lengthwise to see if their top sides are straight. They may be high because they have bowed up during their aging. If they have bowed up then it would be necessary to adjust those joists first before tackling the lower joists.

There are multiple techniques to find the shim up amounts for each joist. Probably the easiest is to place long known straight members across the high joists at right angles. The bottom of the straight member represents the top edge of the shim that needs to be installed.

Another technique is to use a nylon string line stretched across the top of the joists and having its height adjusted so that it just skims the top of the highest joists. The string can be attached to vertical 1"x2" boards that are temporarily nailed or screwed to the face of the outermost joists across the room.

It would require multiple strings or straight members across the length span of the joists to get the reference level for each shim.

A really good way to make the shims is to acquire nice straight members such as 2"x4"s (or 2x6s) that are the same thickness as the joists themselves. One by one a 2x4 can be clamped to the side of a low joist with its top side adjusted to the string or cross reference straight member. Once carefully adjusted and clamped in place the 2x4 can be scribed with a line that follows the very top of the existing joist. A power saw is then used to rip the shim from the 2x4 to make the shim piece that attaches to the top of the joist. (Take extra care in cutting carefully and along the correct side of the line). The shims can be installed in place with construction adhesive and small finish nails or pneumatically shot brads.

The straight 2x4 material can be used to make two shims from the two straight edges. A more economical way is to use just one side of the 2x4 and then run the remaining piece through a table saw with the straight edge against the fence to restore the piece to two straight parallel edges to get it ready for the next shim marking.

  • Thank you for your response! I'm going to wait for more answers before I declare it the winner. Do you suggest avoiding using 1" width cedar shims as they are more narrow than the joists themselves? – androidnotgenius May 7 '18 at 13:42

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