A shim is a thin, wedge-shaped piece of material used to fill gaps and make slight adjustments between surfaces. Shims help bring doors, windows, cabinets, and other home elements perfectly level or plumb during installation. They bridge small spaces where two objects meet but don’t fit flush together.
I’ve spent 23 years in residential remodeling across San Diego County. During this time, I’ve used thousands of shims on projects ranging from simple door adjustments to complex kitchen renovations. Let me share what I’ve learned about choosing and using these simple but essential tools correctly.
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What Exactly Does a Shim Do?
Shims solve a fundamental problem in construction: nothing is perfectly square or level, and floors slope. Walls bow. Foundations settle over time. These imperfections mean the parts we install rarely fit together perfectly on the first try.
When you install a door frame, there’s always a gap between the frame and the rough opening in the wall. Without shims behind the hinges, the door will sag within weeks. When you set kitchen cabinets, an uneven floor means some cabinets sit lower than others. Shims let you make tiny adjustments so everything lines up right.
The tapered wedge design is what makes shims work. Push a shim partway into a gap, and you fill a small space. Push it further and you fill more space. This lets you adjust things incrementally until they’re exactly level or plumb.
I learned how crucial proper shimming is early in my career. On my third door installation, I rushed through the shimming process. The door looked fine when I left. Two weeks later, the homeowner called. The door wouldn’t latch anymore. The frame had shifted because I hadn’t secured the shims properly. I spent an afternoon fixing what should have been done right the first time.
Types of Shims: Materials and When to Use Each
Walk into any hardware store and you’ll find several shim types. Each serves different purposes based on where you’re using it and what load it needs to support.
Wood Shims for Interior Work
Wood shims are the most common type you’ll see. They’re usually cut from cedar or pine and come in packs of 12-20 pieces. Cedar costs more but resists compression better than pine. For door shims and cabinet work where long-term stability matters, I always choose cedar.
Wood shims work great for:
- Door frame installation at hinge points
- Leveling kitchen and bathroom cabinets
- Window installations in dry areas
- Trim carpentry and baseboard work
- Any interior project away from moisture
Standard 8-inch construction shims handle most jobs. I keep 12-inch wood shims for leveling floors under appliances or working with thick exterior door frames.
The big limitation with wood is moisture. I once used wood shims for leveling under a washing machine. After six months of occasional leaks, those shims turned into compressed mush. The homeowner called me back and I had to redo the job with plastic shims. That mistake taught me to always consider where water might show up.
Plastic Shim for Wet Areas
Plastic shims have become my default choice anywhere moisture might be present. They don’t swell, compress, or rot like wood can. Many plastic shims feature interlocking designs that make stacking easier when you need more lift.
Use plastic shims for:
- Shimming toilet bases to stop rocking
- Bathroom vanities and shower installations
- Basement projects with higher humidity
- Exterior door installations exposed to weather
- Any location where building codes require rot-resistant materials
Plastic shims cost about twice what wood shims do. But for a toilet installation, where a failed shim means water damage costing thousands to repair, the extra dollar per shim is nothing.
Metal Shim and Steel Options
Metal shims are specialized tools I don’t use daily, but they’re critical for certain applications. Steel shims are what foundation repair companies use for slab-on-grade foundation work.
A foundation contractor I know in Texas mentioned buying over 80,000 pounds of steel shims during his career. When you’re lifting and supporting a home’s foundation with piers, you need shims that won’t compress under enormous weight.
These steel shims are typically small rectangles measuring about 3 by 5 inches. They vary in thickness from ⅛ inch to ⅝ inch. Foundation crews stack them on concrete blocks that rest on pier caps, filling the space between the pier and the home’s underside. The shims develop surface rust over time but remain structurally sound for decades.
I use metal shims for:
- Steel door frames in commercial projects
- Heavy machinery leveling
- Structural adjustments where inspectors verify load calculations
- Situations requiring documented thickness tolerances
Concrete Shims for Heavy-Duty Applications
In foundation repair for slab homes, concrete blocks serve as the first layer of shims. These blocks rest on each pier cap and start bridging the gap between the pier and your foundation. After the blocks, workers add progressively smaller steel shims on top until the space is completely filled.
Most people don’t think of concrete blocks as shims. But that’s exactly what they are in this context: the first gap-filling layer in foundation support work.
Specialty Shim Designs
Beyond basic wedge shapes, several specialty designs solve specific problems:
- Horseshoe shims wrap around pipes or cylindrical objects. Use these when shimming something that has a pipe running through it.
- U-shaped shims have cutouts that slip around bolts or fasteners. You can add shimming without removing hardware, saving time on adjustments.
- Flat plate shims aren’t tapered. They’re uniform thickness throughout. Use these when you need to raise something a specific, consistent amount across the entire surface.
- Stackable shims with interlocking features let you build up thickness predictably. These work well for leveling cabinets or setting bathtubs where you need consistent height across multiple support points.
How to Choose the Right Shim for Your Project
Picking the wrong shim is probably the most common mistake I see. A shim that works perfectly for trim will fail under a stone countertop. Here’s how to choose correctly.
Match the Load Requirements
Light jobs like door hinge shims or picture frame leveling need minimal strength. Standard pine works fine because the load is small.
Heavy installations are different. Stone countertops, cast iron bathtubs, and industrial equipment put serious weight on shims. Wood fibers can compress and break down over time under these loads. That granite countertop weighing 500-plus pounds on a few shims? Use plastic or metal designed for continuous load-bearing.
I’ve seen wood shims under vanity tops turn into compressed pancakes after a few years. The entire vanity sagged, causing cracked tiles. Better to use proper load-rated shims from the start.
Consider Moisture Exposure
This is where many projects go wrong. Wood shims in locations that might get wet will eventually swell, compress, or rot.
Use plastic for:
- Any bathroom installation
- Kitchen sink base cabinets
- Laundry rooms
- Basements with humidity concerns
- Deck railings or outdoor elements
- Anywhere condensation forms
Wood is fine for:
- Interior doors in climate-controlled spaces
- Trim work and baseboards away from water
- Kitchen upper cabinets away from sinks
- Window installations with proper flashing
Think About Long-Term Performance
Wood compresses under continuous load. A perfectly level cabinet today might develop a sag in a year if you used undersized shims under areas bearing heavy weight.
I now use plastic shims under any cabinet section that will hold heavy items—stacks of plates, canned goods, heavy cookware. The small extra cost prevents future problems.
Budget vs. Quality Trade-offs
Budget wood shims run $3-4 per pack. Quality cedar shims cost $5-7. The difference in performance justifies the cost for professional work or any installation you guarantee.
For a typical door installation using 8-12 shims, we’re talking about a $2-3 difference between wood and plastic. Factor in a callback to fix a failed installation and you’re looking at wasted time, travel costs, and reputation damage. That $2 savings becomes a $200-500 mistake.
My rule: Use plastic shims anytime there’s even a slight chance of moisture. The small upfront cost prevents much larger problems later.
How to Install Shims: Proper Technique Step by Step
The actual mechanics of shimming look simple: tap a wedge into a gap. But there’s real technique here that separates work that lasts from installations that fail.
Identify What Needs Adjustment
Before grabbing any shims, use a quality level to determine exactly what’s off. Is your door frame leaning? Which direction? By how much? Is the cabinet sitting low on one corner or tilted front-to-back?
I’ve seen people start shimming without knowing what they’re correcting. They end up over-correcting and making things worse. Take time to measure first.
Position the Shim Correctly
Insert the shim narrow end first into the gap. The tapered design means the further you push it in, the more it fills the space and lifts or levels the object.
For shim construction with door frames, place shims at each hinge location and at the latch side. Space them every 12-16 inches along the jamb height. This distributes stress evenly and prevents bowing.
For cabinets, start at the highest point on the floor. Every floor slopes somewhere. Shim the lowest areas until everything reads level. Check both front-to-back and side-to-side.
Use Gentle Tapping, Not Pounding
Use a hammer or rubber mallet to gently tap the shim into position. The key word is gentle. You want the shim snug enough that it won’t slip, but not so tight that it bows the frame or lifts the object too much.
I’ve seen contractors pound shims so hard that door frames bow out in the middle. The door then binds when you try to close it. The frame should remain straight—check with a level as you tap.
Double-Shim for Extra Thickness
If one shim isn’t thick enough, add a second facing the opposite direction right next to the first one. This creates a parallel surface rather than a double-taper, giving more stable support.
Two shims together create a flat platform. This technique works great for cabinets needing significant lifting in one area.
Secure Everything in Place
Once everything is plumb and level, drive screws or nails through the object you’re installing, directly through the shims, and into the underlying structure. This locks everything in place.
For door frames, I use 3-inch screws driven through the jamb, through the shims, and into the wall studs. This creates a rock-solid connection that won’t shift.
For cabinets, I secure through the mounting rail at the back, going through any shims between the cabinet and wall.
Don’t skip securing the shims. Unsecured shims can slip or drop from their position over time. Everything you carefully leveled will shift. I learned this on my fourth cabinet installation when I got lazy and didn’t screw through the bottom shims. Six months later, the homeowner called about a gap. The shims had compressed and shifted.
Trim Flush and Clean
After everything is secured, use a sharp utility knife to score the excess shim length right at the surface. Then snap it off by hand. Make the cut flush so nothing protrudes.
Some contractors use a hand saw, which works but makes a mess. I prefer scoring deeply with a knife, then a quick snap. Takes two seconds and leaves a clean edge.
For visible installations, sand the shim end lightly so it’s completely flush. This prevents it from showing through paint or caulk later.
Advantages of Using Shims in Construction
Shims bring real benefits to construction and repair work beyond just filling gaps.
Precise Alignment Control
Shims let you make micro-adjustments that would be impossible otherwise. Moving a door frame 1/16 inch at a time until it’s perfectly plumb requires the gradual adjustment only a tapered wedge provides.
This precision means doors close smoothly, cabinets sit level, and windows operate correctly. The difference between amateur work and professional results often comes down to careful shimming.
Cost-Effective Problem Solving
Instead of tearing down and rebuilding misaligned surfaces, a simple shim often solves the problem in minutes. This saves both time and money.
On a recent kitchen remodel, the floor sloped nearly 2 inches across a 10-foot run. Rather than trying to level the entire floor, an expensive, time-consuming job I used wood shims for leveling the base cabinets. Total cost in shims: about $8. Time saved: hours of floor prep work.
Prevents Future Damage
Proper shimming prevents stress that causes cracking and warping over time. A door frame that’s slightly out of plumb will cause the door to sag and bind. The repeated stress damages both the door and frame.
A rocking toilet breaks the wax seal, leading to water leaks that damage subfloors and joists. A $3 plastic shim prevents thousands in water damage repairs.
Accommodates Building Settlement
Houses settle and move over time. Shims provide the adjustment layer that accounts for this movement. You can add or adjust shims as needed without major reconstruction.
I’ve gone back to homes I worked on years earlier to make minor shim adjustments as the house settled. Takes 15 minutes and keeps everything working properly.
Common Mistakes to Avoid When Using Shims
I’ve made just about every shimming mistake possible early in my career. Here are the ones that cost me time and money.
Over-Shimming Creates Instability
In my second year, I shimmed a door frame that was way out of plumb. Instead of carefully adjusting, I kept jamming more shims in until the frame looked straight.
Within a month, the door wouldn’t latch properly. The frame had bowed from excessive pressure. I had to remove the trim, pull some shims, add others in different spots, and basically redo the entire installation.
Use the minimum number of shims needed to achieve plumb and level. More isn’t better. If you need substantial adjustment, consider whether the rough opening itself needs correction first.
Wrong Material for the Job
The washing machine incident taught me about moisture and wood. But I’ve also seen:
- Wood shims under a toilet that rocked and compressed from moisture
- Plastic shims on a heavy door where slight flex caused squeaking
- Cheap thin metal that bent and failed under granite countertops
Match your shim material to both the load it will bear and the environment where it will live. When in doubt, go one step more durable than you think you need.
Not Trimming Properly
Leaving long shim tails sticking out looks terrible and prevents trim from sitting flush. But the mistake I see more often is trimming too short cutting into the secured portion.
If you cut a shim too short and it’s no longer firmly held, it can work loose over time. Score your cut line carefully at the exact surface level, then snap cleanly.
Ignoring Compression Over Time
Wood compresses under continuous load. That perfectly level cabinet today might develop a sag in a year if you used undersized or poor quality shims under heavy-weight areas.
Think about what loads the shim will bear over years, not just at installation. Where compression is a concern, use materials that won’t compress.
Shimming Only at End Points
On a long door frame or cabinet run, some people only shim at hinge points or cabinet edges. This works short-term, but over months the unsupported sections can sag or bow.
Space your shims every 12-16 inches maximum along any edge or frame. This ensures even support across the entire length.
Professional Insights: What 23 Years Has Taught Me
Some lessons don’t come from manuals. They come from experience and paying attention to thousands of installations.
The 1/8-Inch Rule
In door and window installation, I try never to have a shim exceed 1/8 inch thick at any single point. Beyond that, you risk creating instability or bowing the frame. If you need more adjustment, double-shim with two wedges facing opposite directions.
The Three-Point Check
After shimming any door or window, I check three things before final securing:
- Frame is plumb in multiple spots
- Frame has consistent reveal around the door or window
- Door or window operates smoothly with no binding
If any fail, I adjust shimming before proceeding. Fixing it now takes minutes. Fixing it after trim is installed takes hours.
The Sound Test
When I tap a shim into position, I listen to the sound. A solid, dull thud means good contact. A hollow sound or rattling means the shim isn’t seated properly. This audio feedback helps me know when things are right without constantly checking with a level.
Climate and Seasonal Movement
Houses move with seasons expanding in humidity, contracting in dry weather. I account for this by not over-tightening anything. Leave slight room for seasonal movement, especially on exterior doors.
A door shimmed perfectly tight in winter humidity might bind come summer dryness.
How to Use Shims in Different Applications
Understanding general shimming techniques is one thing. Applying it to specific situations requires knowing the unique challenges of each application.
Shimming Wood Door Frames
Door installation is where most people first encounter shims. The rough opening is always slightly larger than the door frame. This gap lets you adjust the frame until it’s perfectly plumb.
Place shims at each hinge location. These points bear the door’s weight, so secure shimming here prevents sagging. Also place shims along the latch side of the frame every 12-16 inches.
Insert shims from both sides of the frame so they meet in the middle. This creates a more stable connection than shimming from one side only.
Check that the frame is plumb before driving screws through the shims into the wall studs. A door frame that’s even slightly off plumb will cause binding and uneven gaps.
Leveling Cabinets on Uneven Floors
Kitchen cabinet installation tests your shimming skills like few other projects. Floors are rarely level, and cabinets must be perfectly level for countertops to sit right.
Start by finding the highest point on the floor using a long level. This becomes your reference point. All other cabinets will be shimmed up to match this height.
Place shims under cabinet corners and at points where the cabinet back meets the wall. As you shim, check the level both front-to-back and side-to-side.
For base cabinets that will support stone countertops, use plastic shims. The weight of granite or quartz is substantial, and wood shims can compress over time.
Installing Windows with Proper Shimming
Windows need shimming on all four sides to prevent the frame from twisting. A twisted window frame means the sash won’t slide smoothly and you’ll get air leaks.
Shim the window at the corners first, getting those points roughly level. Then add shims every 12 inches along all four sides.
Check the window with a level and square before final attachment. The goal is maintaining the manufacturer’s specified gap all around usually about 1/8 inch so the window operates correctly and you can seal it properly.
Over-shimming windows is a common mistake. Too much pressure will blow the frame and prevent the window from operating smoothly.
Need Help With Your Project?
Most shimming work is DIY-friendly if you have basic tools and patience. But some situations benefit from professional help.
Large kitchen remodels with expensive materials at risk deserve expert installation. Exterior door installations require weather-sealing expertise that takes years to develop. Foundation work and structural shimming need engineering knowledge and specialized materials.
At San Diego Home Remodeling, we’ve handled everything from simple door adjustments to complete kitchen renovations across San Diego County for over two decades. Our team knows proper shimming technique for every application from door hinge shims to foundation support work.
If your project involves valuable materials, requires building permits, or you simply want the peace of mind that comes with professional installation, our home remodeling team is ready to help. We guarantee our work and use only appropriate materials for each specific application.
Contact us for a consultation. We’ll assess your project and provide honest guidance on whether it’s a good DIY candidate or benefits from professional expertise.
Conclusion
Understanding what is a shim and how to use them properly separates work that lasts from installations that fail. These simple tapered wedges solve the fundamental problem in construction: nothing is perfectly level or plumb.
Whether you’re hanging a door, leveling cabinets, or setting a toilet, proper shimming technique ensures stability and prevents future problems. Choose the right material for your environment: wood for dry interior work, plastic for moisture-prone areas, steel for structural applications.
Take your time during installation. Use the minimum number of shims needed. Secure them properly. Trim them flush. These small details add up to professional results.
I’ve spent 23 years learning these lessons, sometimes the hard way through callbacks and mistakes. But that’s how expertise develops through real experience solving real problems. Apply these techniques to your projects and you’ll get results that last.
FAQs
What is a shim used for in home improvement?
A shim fills small gaps between objects to make them level or plumb. Common uses include door frame installation, cabinet leveling, window adjustments, and toilet stabilization. Shims compensate for uneven surfaces and ensure proper alignment during construction or repair work.
What types of shims are available?
The main types are wood shims (cedar or pine for interior work), plastic shims (moisture-resistant for bathrooms and wet areas), metal or steel shims (for heavy-duty structural work), and specialty designs like horseshoe shims, U-shaped shims, and flat plate shims. Each type serves specific applications based on load requirements and environmental conditions.
Can I reuse shims from previous projects?
Generally no. Once a shim has been compressed and fitted into place, it typically doesn’t maintain its shape and integrity. Wood shims that have been tapped in place often split or compress. However, plastic or metal shims that were only test-fitted might be reusable in less demanding applications.
How do I know if I'm using the right shim thickness?
The shim should fill the gap without requiring excessive force to insert. If you’re pounding hard to get the shim in, it’s too thick. If it slides in easily without creating a snug fit, it’s too thin. The correct shim slides in with gentle tapping and holds firmly through friction once in place.
What's the difference between cedar and pine shims?
Cedar shims resist compression better than pine because of their grain structure and density. For load-bearing applications like door frames or cabinets, cedar is superior. Pine shims work fine for light-duty applications like trim work. Cedar costs more but provides better long-term performance where it matters.
