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How to Cut Super Shims for Leveling an Uneven Floor, Ceiling, or Wall

September 17, 2018

A jig and a few tricks to help you make shims of any length to straighten crooked framing

Old houses settle over time. Floors and ceilings that may (or may not) have been level at one time can present significant problems when it is time for you to install tile or cabinets, or if you just need to level the darned floor.

There are probably as many ways to level a floor as there are out of level floors being leveled by someone, so here's Matt Jackson's take on making precise, extra long shims.

Step by step process for cutting a six-foot ling shim from a 2x4

  • Figure out how much taper you'll need.
  • Find a straight board. If you don't have one on your jobsite, cut one with this jig.
  • Use the straightedge in the previous video, or make a new one, to cut the shims.
  • Drive a flat head screw into the push block at the end of the straightening jig. In this case, Matt is looking for a shim that goes from 15/32 to nothing over four feet one inch, so the screw sticks out 15/32 from the block.
  • Cut the straight 2x4 to the length of the required shim.
  • Set the 2x4 into the jig, screw it tightly into place.
  • Set the fence to the distance of the jig and 2x4—making sure to set that distance to the far side of the blade.
  • Cut the shim.

To make a second shim, just run the 2x4 through the table saw squaring off the tapered edge. Cutting slices off a jobsite 2x4 can compromise the equilibrium of the wood, which can cause the wood to bow slightly, so you may need to square up an edge again.

How to cut a 12-foot shim

  • To cut a 12-foot shim, cut a cruddy 2x4 to six feet long.
  • Remove the taper screw and reposition the far hold-down stop on the jig to accommodate the longer, 6-foot 2x4.
  • Run the cruddy 2x4 through the table saw to make a perfectly straight edge.
  • Set the jig aside and cut the opposite edge of the 2x4 on the table saw to get two straight edges.

To make a 15/16 to nothing taper over 12 feet, cut two six-foot shims that taper 15/32 to nothing. Replace the flat head screw into the same hole at the same depth and insert the six foot 2x4 into the jig. Measure the combined width of the jig and 2x4 blank, making sure not to cut off the first few inches off the shim.

With a six-foot shim in hand,

  • Remove the 2x4 blank from the straight edge jig and set the jig aside.
  • Mark the thick end of the 6-ft shim onto the thin end of the blank (butt the ends together and scribe.)
  • Set rip fence to put the blade at the near side of the line (so that the scribed portion will be the off-cut.
  • This leaves a tapered offcut that perfectly extends the first six foot shim to yield a 2-part 12-foot shim.

If a two-part shim is unacceptable, you can pocket screw the two pieces together to form a one-piece shim.

—Matt Jackson is a master carpenter, remodeler, SketchUp Wiz, YouTuber, and contributing editor to ProTradeCraft. He lives and works in Rapid City, South Dakota.

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