How to Use Wood or Plastic Shims

Wood shims on a window frame
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For DIYers routinely engaged in remodeling and repair projects around the home, certain tools and materials quickly become must-have items. Nearly every DIYer comes to depend on some of the same tools routinely used by pros, such as levels, caulk guns, cordless drills, pry bars, work lights, 5-in-1 tools, and channel-lock pliers. Most DIYers also keep a variety of standard materials on hand, such as wallboard patching compound, a variety of caulks and glues, electrical wire connectors, and assorted plumbing fittings. These tools and materials are used so frequently that it makes much more sense to have them on hand rather than visit a hardware store or home center each time they are needed.

And nearly every DIYer will find a use for another perennial favorite: shims made from wood or plastic. If you are a new homeowner who has just purchased a fixer-upper home with plans for remodeling it to perfection, you're well-advised to buy a few packages of both wood and plastic shims right off the bat, because you're certain to use them.

What Are Shims?

A shim is a thin wedge made wood or plastic, traditionally used for incrementally positioning and adjusting building elements before securing them in place. The most common construction use of shims is to wedge window or door units in place within their framing during installation. After a door or window unit is placed into the opening, shims spaced at intervals around the frame are gently tapped into place with a hammer. Careful placement of the shims will incrementally move the window or door unit until it is both level and plumb, wedging it in place as nails or screws are driven to secure the unit to the framing. Once secure, the shims are trimmed off flush with the framing.

But you will be surprised by how many other uses you will find for shims during routine home maintenance projects and repairs. A couple of packages of shims on the shelf in your home workshop will always find uses. When installing cabinetry, for example, shims are indispensable for leveling base cabinets along the floor or adjusting wall cabinets so they are perfectly plumb.

Common Uses for Shims

  • Plumbing or leveling windows during installation
  • Plumbing pre-hung door units
  • Leveling key sections of subflooring or floorboards
  • Adjusting exterior decking boards to flatten the surface
  • Taking the squeaks out of interior flooring by inserting the shims between subflooring and joists from below
  • Controlling noise on staircases
  • Leveling and plumbing cabinets during installation

Commercial shims purchased in packages are usually between 7 1/2 inches and 9 inches long and 1 1/2 inches wide. Contractor-grade shims can reach lengths as much as 12 inches to 16 inches long. The thicker end of the wedge may be around 3/8 inches thick, tapering down to 1/16 inches thick at the narrow tip. Shims can be doubled up or tripled up to increase the thickness.

Wood Shims vs. Plastic Shims

Shims are made generally from wood or composite plastic. Wood shims tend to be made from cedar or pine, while plastic shims are usually made of composite blends of post-consumer recycled plastic. Some composite shims may have wood material blended with the plastics. Composite plastic shims often have score lines that allow you to snap off the shim at chosen lengths; wood shims must be manually scored with a utility knife, then snapped off. Wooden shims and composite plastic shims cost roughly the same.

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 moist 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 use when you need to stack two or three shims together. The friction helps shims hold together better without slipping, or wood shims can even be glued together. By contrast, plastic shims are slippery and can become dislodged. Plastic composite shims cannot be adhered together with standard wood glue, although polyurethane glue can be used.

One disadvantage of plastic shims is that they are pre-scored to snap at certain locations. Wood shims, on the other hand, can be scored and cut off wherever necessary.

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. A rubber mallet may be the best tool to use when driving shims.
  2. Make sure that you have pushed the shim in far enough, but not too far. It is easy to become too enthusiastic about shimming, which can cause frames to bow or lift cabinetry out of level. Once driven, it is hard to back out the shims.
  3. Nail the work material in place. With window and door installation, this is usually done by driving casing nails or screws through the jambs into the framing right through the shim locations.
  4. If using wood shims, lightly score the wood with your utility knife as close as possible to the workpiece. Bend the shim at the scored line to snap it off. Alternatively, you can cut off the shim with a multi-tool. If using plastic shims, bend back the shim and snap it off. It is usually not necessary to score the shim since plastic shims already are scored. A multi-tool can also be used to trim off a plastic shim.

Are DIY Wood Shims Worthwhile?

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 inexpensive that it is rarely worth the effort to cut your own. Cutting shims on a table saw is no easy feat, since the pieces are so thin and getting the taper just right is complicated. For most people, the uniform size of manufactured shims makes them a better option.