What Is a Shim? How to Use One the Right Way

A shim is one of those small, cheap tools that quietly holds half the things in your house level and plumb. If you have ever wondered why a contractor stuffs little wood wedges around a new door before screwing it in, this guide is for you. Below we explain what a shim actually is, the types you will run into, and exactly how to use them without making things worse - with some honest notes about why Tampa homes need them so often.

What a shim actually is

A shim is a thin, usually tapered wedge used to fill a small gap so you can make something level, plumb, or solid. One end is thin, the other is slightly thicker, so you can slide it in and adjust how much it lifts simply by pushing it deeper or pulling it back. That taper is the whole trick: it lets you dial in a gap of a sixteenth of an inch or less, which is far more precise than trying to stack flat scraps of wood.

Shims are not structural. They transfer a load across a gap and hold a part in position, but they are not a substitute for proper framing, footings, or fasteners. Think of them as a fine adjustment, not a fix for something that is genuinely broken or sagging.

The common types of shims

• Wood shims (usually cedar or pine): cheap, easy to snap or cut, and the default for doors, windows, and trim. They can compress slightly over time and are sensitive to moisture.
• Composite and plastic shims: water-resistant and dimensionally stable, which makes them a smart choice in damp spots like bathrooms, laundry rooms, and anywhere near an exterior slab.
• Tapered cabinet or appliance shims: similar idea, often sold for leveling base cabinets, dishwashers, and washers.
• Specialty shims: U-shaped horseshoe shims for posts and beams, and flat metal shims for tight mechanical gaps.

For most homeowner jobs, a bundle of wood shims and a small pack of composite shims will cover you. In our humid climate we reach for composite shims anywhere water or steam is in play.

What you use shims for

Shims show up in more places than you would guess. The most common uses are:

• Setting a door frame plumb so the door swings true and the latch lines up.
• Installing windows square and level inside a rough opening.
• Leveling base cabinets so countertops sit flat and drawers do not drift open.
• Leveling appliances like dishwashers, refrigerators, and washing machines so they run quieter and drain correctly.
• Steadying a wobbly table, chair, or piece of furniture on an uneven floor.
• Filling small gaps under posts, railing bases, or trim before fastening.

How to use a shim, step by step

The technique is the same whether you are hanging a door or leveling a cabinet. The goal is to support the part evenly without bending or bowing anything.

• 1. Check first with a level. Find out which way the part is leaning or where it dips before you touch a shim. Mark the low side.
• 2. Insert shims in opposing pairs. Slide one shim in from each side so the two tapers point opposite directions. Pushed together, they form a flat, parallel surface instead of a slanted one. A single shim alone tilts the surface; a pair keeps it true.
• 3. Tap gently to fine-tune. Nudge the pair deeper to raise the gap, back them out to lower it. Small taps with a hammer or by hand are all you need.
• 4. Check constantly with the level. Adjust, check, adjust, check. Do not trust your eye - a quarter bubble off is easy to miss and obvious once the door is hung.
• 5. Fasten through the shim. Once it reads level and plumb, drive your screw or nail through the framing and the shim together so nothing shifts later.
• 6. Score and snap the excess. Run a utility knife along the face of the jamb or cabinet, score the shim a few times, and snap it off flush. For composite shims, a few passes with a saw or knife cuts them clean.

Mistakes to avoid

The biggest error is over-shimming. If you cram a shim in too far to force a gap closed, you can bow a door jamb inward, which makes the door bind and the reveal uneven all the way down. The fix is counterintuitive: back the shim out a hair rather than pounding it in harder.

• Always shim behind hinges and behind the strike plate on a door - those are the spots that carry load and need solid backing.
• Use pairs, not single wedges, on any surface that needs to stay flat.
• Do not over-tighten the screw into a shim until you have confirmed the level reading, or you will lock in the error.
• Skip wood shims in standing-water zones; reach for composite instead.

The Tampa angle: humidity and out-of-square homes

Two local realities make shims especially handy here. First, our humidity. Wood doors and frames swell in the wet summer months and shrink when the AC runs them dry, so a door that closed fine in January can stick in July. That seasonal movement is exactly why interior doors often need re-shimming or adjustment more than once, and why we lean on composite shims in bathrooms and laundry rooms where moisture never really lets up.

Second, many older Tampa block homes - the 1950s to 1970s concrete-block houses you see all over Seminole Heights and Temple Terrace - have openings that are no longer perfectly square. Decades of slab settling and CMU walls shifting slightly mean a rough opening can be out of plumb by a noticeable amount. Shims are how you set a new door or window true inside an opening that is not. You are not fixing the wall; you are giving the door a level, plumb home regardless of what the framing is doing around it.

When to call a pro

Shimming is a friendly beginner skill for small jobs - a wobbly table, a slightly off cabinet, a sticky interior door. But some situations point to a bigger problem that a shim only masks.

• A door or window opening that is badly out of square or has gaps wide enough that shims alone will not bridge them.
• Floors that slope noticeably or cabinets that need an inch or more of correction, which can signal settling worth a closer look.
• Anything load-bearing - posts, beams, or framing carrying real weight - where guesswork is not safe.
• Repeated door sticking that returns every season no matter how you adjust it, which usually means the jamb itself needs to be reset.

When the gap is small and the part just needs to sit true, a shim is the right call. When the structure itself has moved, get eyes on it before you keep wedging.

Sticky door, uneven cabinets, or a wobbly install you want done right? Call Fenelon Handyman Services at (786) 509-5555 for honest, local help across Tampa Bay. Get a free quote.