What Is a Shim and How Do You Use It?

Plain and simple, a shim is a tapered wedge used for making slight adjustments to home elements, such as door frames and windows, to make them plumb and level.

The most common shims are wood (often cut from cedar or pine) and serve well for almost any interior adjustment. Plastic shims are also available and have the distinct advantage of being water and rot-resistant, which makes them ideal for plumbing or exterior adjustments.

You may have come across a pile of these thinly sliced wood wedges among the tools in the garage. While they may appear to be wood scraps from a past project, shims are actually an important element for a variety of DIY fixes and fittings. Learn more about shims and how to use them from Mr. Handyman.

When Do You Use a Shim?

Despite the best measurements and care, once a door or window is put into place, it’s unlikely to be perfectly square. Rather than agonizing over cutting a new frame or trying to nail in an absolutely precise opening, shims are used to make the final tweaks.

Wedging a shim in between a frame and the adjacent stud helps to incrementally bring the element plumb and level. Not limited to just windows and doors, shims can be utilized for squaring up cabinets, leveling a wobbly toilet, adjusting subflooring and floorboards, or eliminating the creaks in outdoor decking.

How to Use Shims

Once a door or window is seated into the opening, place shims in any gaps between the window or door casing and the framed opening. Use a hammer or mallet to gently tap the shim into place, using a level to check that the window or door is moving toward plumb or level as desired.

If one shim is not thick enough for the required adjustment, tap another shim in right next to the first one, but face it the opposite way. Be careful not to drive the shim in too far, as they are difficult to pull back out.

Once the door or window is plumb and level, drive a nail or screw from the casing, directly through the shims and into the framing to secure it. Once the window or door is firmly seated and nailed, use a utility knife to score the excess shim length and break it off.

On a Shim

Shims are imperative for the final fittings of windows, doors, cabinets and the like. While traditional wood shims will serve well for most purposes, anything involving potential moisture exposure (such as fitting toilets, subfloors or exterior elements) call for plastic shims.

A well-placed shim can take a canted door frame and pull it to plumb and level as it should be. While tapping in a shim is a fairly easy task, putting in a window or installing a toilet can be a daunting task. Leave the installation to the pros! Call Mr. Handyman at (877) 685-1377 or request an appointment online for a guaranteed professional fitting and peace of mind.


Learn more about attic fans, another very useful tool around your house, from the HVAC experts at Aire Serv. Aire Serv is part of the Neighborly® trusted network of home service providers.