How to Use Wood and Plastic Shims

Wood shims on a window frame

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For homeowners routinely engaged in remodeling and repair projects around the home, certain tools and materials quickly become must-have items. Some, like cordless drills, are expected must-have tools. Other tools and materials might be completely unexpected. Shims are one such item.

Shims, whether wood or plastic, are so valuable to home remodeling projects that you should always have a package of them on hand.

What Shims Are

A shim is a thin wedge made of 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.

Where to Use Shims

What's often surprising is how many 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 a use. When installing cabinetry, for example, shims are indispensable for leveling base cabinets along the floor or adjusting wall cabinets so they are perfectly plumb.

  • 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 kitchen cabinets during installation

Sizes of Shims

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 as long 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.

Even if the shim's thick end isn't thick enough, that can easily be fixed. Shims can be doubled up or tripled up to increase the thickness.

Wood Shims vs. Plastic Shims

Wood Shims
  • Thin wedges made of softwood like pine

  • May be damaged by water

  • Remove excess by sawing off or scoring and snapping

  • Can be glued together

Plastic Shims
  • Thin wedges made of plastic

  • Impervious to water

  • Remove excess by snapping off at score points or by sawing

  • More difficult to glue together than with wood 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 sawed off with a miter saw or 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

Insert Shim

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.

Assess Shim Depth

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. It is especially difficult to back out a shim that has been driven in too far, because your pliers have nothing to grab onto.

Fix Work Material in Place

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.

Cut Excess Shim

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.

Tip

When using shims on door or window jambs it is usually best to use two opposing shims. Place the thin edges of each shim on either side of the jamb and slide them over each other until the desired thickness is reached. This gives a flat or square surface to nail or screw to without tweaking the jamb.

Can You Make Your Own Shims?

It is possible to cut your own shims with a table saw or electric miter saw. Simply place wood in the saw with the grain of the wood parallel to the saw blade. Cut off the wedge. Make sure that your fingers are not close to the saw blade.

While you can cut your own shim 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 properly and safely is no easy feat since the pieces are so thin and getting the taper just right is complicated. For most homeowners, the low cost, easy availability, and uniform size of manufactured shims make them a better option than homemade shims.