What Are Wood Shims and How Are They Used?

Wood Shims on a Window Frame
Wood shims are often used for leveling and plumbing a window during installation. Jodi Jacobson/Getty Images 

When remodeling or improving your home, some tools and materials are long-standing favorites that can be used in many different situations. Items like speed squares, cordless drills, pry bars, work lights, 5-in-1 tools, and lineman's pliers will find their way into your hands over and over again, throughout your entire do-it-yourself career.

And then there are those perennial favorites: wood and plastic shims. Anyone who has just purchased a fixer-upper home and is intent on remodeling it to perfection is best advised to buy a pack of both wood and plastic shims right off the bat. But what are shims used for and why are they so incredibly valuable?

What Are Shims?

A shim is a thin wedge of wood or plastic used for incrementally positioning large building elements before securing them in place. Wood shims tend to be made from cedar or pine. Composite plastic shims are sometimes made from post-consumer recycled plastic products.

Two projects that heavily use shims: window and door installation. After the door or window unit is placed into the opening, the shim is gently tapped into place by a hammer. The gradual insertion of the shim incrementally moves the window or door unit until it is both level and plumb (vertically straight).

Shim Sizes

A common size for wood shims is between 7 1/2 inches and 9 inches long and 1 1/2 inches wide. The thicker end may be around 3/8 inches thick, tapering down to 1/16 inches thick. Shims can be doubled up to increase the thickness.

Contractor-grade shims can even reach up to 12 inches to 16 inches long.

Wood Shims vs. Plastic Shims

Shims can be made either of wood or composite plastic. Composite plastic shims have scores that allow you to snap off the shim at the correct length or close to the correct length. Wood shims need to be manually scored with a utility knife. Wooden shims and composite plastic shims cost roughly the same amount.

One advantage of plastic shims over wooden shims is that plastic is impervious to water. So, if you are shimming up cabinets or subflooring near sinks or other water-producing areas, plastic shims will be a better choice. Also, for any exterior work such as decks, siding, or roofing, you should use plastic shims.

One advantage of wood shims over plastic shims is that they are easier to build up when you need to stack two or three shims. The friction helps shims hold together better, and they can even be glued together. By contrast, plastic shims are slippery and can become dislodged. In addition, wood glue will not stick to plastic composite shims.

One disadvantage of plastic shims is that they are pre-scored to snap at certain places. If the score line is farther out than you need, you will have to tap the shim a little farther into the workpiece.

How to Use Wood or Plastic Shims

  1. Gently tap the shim into the opening with a hammer. Shims are very delicate, so be careful when tapping.
  2. Make sure that you have pushed the shim in far enough, but not too far. It is easy to become enthusiastic with shimming and end up bowing out your work material.
  3. Nail the work material in place.
  4. If using wood shims, lightly score the wood with your utility knife as close as possible to the workpiece. Quickly pull back the shim and snap it off. It should snap on the score line. Alternatively, you can cut off the shim with a multi-tool.
  5. If using plastic shims, pull back the shim and snap it off. It is not necessary to score the shim since plastic shims already are scored at pre-determined places.

Common Uses for Shims

  • Plumbing or leveling windows during installation
  • Plumbing pre-hung door units
  • Leveling key sections of subflooring or floorboards
  • Shoring up or bringing sections together when installing exterior decking
  • Taking the squeaks out of interior home flooring by inserting the shims between subflooring and joists (from below)
  • Controlling noise on staircases
  • Leveling kitchen cabinets during installation

Can You Make Your Own Wood Shims?

While it is possible to cut your own shims with a table saw (and this may be done in a pinch), pre-cut shims from a store are so low-cost that it is rarely worth the effort to cut your own. Manufactured shims are uniformly sized.