How to Prevent Hardwood Flooring Problems with Avi Hadad

Have you ever wondered what those thin, wedge-shaped pieces of wood are that builders slip behind door frames or under cabinets? Those are shims—small but mighty tools that make a big difference in construction and woodworking.

Whether you’re hanging a door, leveling a cabinet, or installing flooring, shims help you achieve a precise, professional finish.

In this guide, we’ll cover everything you need to know about shims:

• What they are
• The different types
• Their common uses
• How to use them properly

Let’s get started.

What Is a Shim?

A shim is a thin, often tapered piece of material used to fill gaps, level surfaces, or provide support between two objects. While shims can be made from various materials—including plastic, metal, and composite—wood shims are among the most common, especially in carpentry and construction.

Wood shims are typically wedge-shaped, meaning they’re thicker on one end and taper to a thin edge on the other. This design allows you to slide them into a gap and adjust the fit by pushing them in further or pulling them back.

They’re inexpensive, versatile, and easy to use, making them a staple in any toolbox.

Types of Shims

Not all shims are the same. Depending on the job, you might reach for a different type. Here are the most common ones:

Wood Shims

Wood shims are the classic choice, especially cedar and pine. They’re affordable, easy to cut or snap to size, and widely available. Cedar shims, in particular, are popular because they resist rot and hold up well over time.

The downside? Wood can be affected by moisture, causing it to swell, shrink, or warp. That’s why checking the moisture content of your wood shims (and the surrounding materials) matters—more on that later.

Composite Shims

Composite shims are made from plastic or other synthetic materials. They’re resistant to moisture, so they won’t rot, swell, or shrink like wood. This makes them ideal for outdoor projects or areas with high humidity, such as bathrooms or basements.

They tend to cost a bit more than wood shims, but their durability can be worth it in the right application.

Metal Shims

Metal shims are used for heavy-duty applications that require a lot of strength and precision, such as in machinery or structural work. They come in very precise thicknesses and are much stronger than wood or composite shims. They’re not something you’d typically use for hanging a door, but they’re essential in industrial settings.

Common Uses for Shims

Shims are surprisingly versatile. Here are some of the most common ways they’re used:

Hanging Doors

One of the most common uses for shims is hanging doors. When you install a door frame, it’s rarely perfectly plumb (vertically straight) or level. Shims allow you to fine-tune the position of the frame, ensuring the door hangs correctly and opens and closes smoothly.

You slide the shims between the door frame and the rough opening, adjusting until everything is aligned. Then, you secure the frame with nails or screws driven through the shims.

Leveling Cabinets

Whether you’re installing kitchen cabinets or bathroom vanities, shims are essential for getting everything level. Floors and walls are rarely perfectly even, so shims help you compensate for those imperfections.

By placing shims under or behind the cabinets, you can ensure they’re level and secure before fastening them in place.

Installing Windows

Like doors, windows need to be plumb and level to function properly. Shims help you position the window correctly within the opening, ensuring a tight, weatherproof fit.

Flooring Installation

Shims can also come in handy during flooring installation, particularly for subfloor leveling. If you have a low spot in your subfloor, shims can help bring it up to the right height before you lay down your finished flooring.

Furniture Repair

Wobbly table? A shim can help. Shims are great for stabilizing furniture that isn’t sitting evenly, whether due to an uneven floor or a slight defect in the furniture itself.

How to Use Shims Properly

Using shims might seem straightforward, but there are a few tips and tricks to get the best results:

1. Use Them in Pairs

For leveling and support, it’s often best to use shims in pairs, with the tapered ends facing opposite directions. This creates a flat, stable surface and allows for more precise adjustments.

2. Don’t Over-Shim

It can be tempting to stack multiple shims to fill a large gap, but this can create instability. If you have a large gap, consider using a thicker material as a base and then fine-tuning with shims.

3. Secure Them in Place

Once you’ve got your shims positioned correctly, secure them by driving a nail or screw through them. For a clean finish, you can score the shim with a utility knife and snap off the excess.

4. Check for Moisture

Because wood shims can be affected by moisture, it’s a good idea to check the moisture content of both the shims and the surrounding materials, especially in humid environments or outdoor applications. This is where a tool like a wood moisture meter comes in handy.

If the moisture content of your shims is significantly different from the surrounding wood, they may swell or shrink after installation, throwing off your careful leveling work. By checking the moisture content ahead of time, you can avoid future problems.

Small Tool, Big Impact

Shims may be small and simple, but they play a crucial role in achieving professional results in construction and woodworking. Whether you’re hanging a door, leveling a cabinet, or repairing furniture, knowing how to use shims properly can make all the difference.

And don’t forget: when working with wood shims, moisture matters. Using a wood moisture meter can help you ensure your shims—and your project—stand the test of time.

Last updated on July 2nd, 2026