Shims are the thin, tapered wedges that decide whether a door hangs straight or drives you up the wall. Learning how to shim a door lets you square the frame inside an out-of-plumb rough opening, so the slab swings true and the latch catches on the first try.
This guide walks through the opposing-wedge method step by step, compares wood, composite, and steel shim materials, and shows how to correct the gap problems that make a door stick, rattle, or drift open on its own.
Key Takeaways
- Purpose: Door shims are tapered wedges that square the jamb inside a rough opening so the door does not stick, bind, or swing open by itself.
- Materials: Wood and composite (plastic) shims are the two standards; composite resists rot and water, so it wins for exterior and damp locations.
- Placement: Set shims in opposing pairs behind every hinge and behind the strike plate to build a flat, load-bearing surface that will not twist the jamb.
- Installation: Plumb and shim the hinge side first, fasten through the shims into the stud, then hold a consistent 1/8-inch reveal on the lock side.
What Are Door Shims?
Door shims are thin, wedge-shaped pieces of material used to fill gaps between the door frame (jamb) and the rough wall opening (studs). Because rough openings are rarely perfectly square or plumb, shims allow you to adjust the frame until it is perfectly vertical and level.
How Many Shims Per Door?
You typically use three to five sets of shims per door side. Critical placement spots include directly behind each hinge and behind the strike plate (lock side). Using pairs of shims at these stress points prevents the jamb from twisting.
Best Materials for Door Shims
Not all shims are created equal. Choosing the right material depends on whether you are working on an interior bedroom door or a heavy exterior entry door.
Wood Shims
Wood is the traditional choice. They are usually made from cedar or pine. Cedar is superior because it resists rot and pests better than pine.
- Pros: Readily available, easy to cut with a utility knife, inexpensive.
- Cons: Can compress over time; prone to swelling if they get wet.
- Best For: Interior doors and dry locations.
Composite / Plastic Shims
Composite shims are becoming the industry standard. They are molded from recycled plastics and often come with pre-scored lines for easy snapping.
- Pros: Rot-proof, water-resistant, high load-bearing capacity (won’t compress), easy to snap without tools.
- Cons: Slightly more expensive than wood; can be slippery to stack.
- Best For: Exterior doors, bathrooms, basements, or heavy solid-core doors.
Steel Shims
Steel shims are generally reserved for industrial applications or fire-rated commercial doors where combustible materials are prohibited. They are extremely durable but unnecessary for typical residential use.
Cardboard
Cardboard should only be used as a temporary spacer or for very lightweight interior adjustments. Never use cardboard for exterior doors or heavy loads, as it compresses easily and disintegrates when exposed to moisture.
How to Fix Common Door Gap Problems
Shims solve a range of alignment issues. Whether your door hits the jamb, won’t latch, or swings open by itself (a ghost door), adjusting the shims corrects the angle. Here is how to read each gap and what to change.
Gap Too Big on Lock Edge
If the gap between the door and the frame on the lock side is too wide, the latch might not catch.
The Fix: Place equal thickness shims behind the hinges. This pushes the entire door slab toward the lock side, closing the gap. You can also add thin cardboard shims behind the specific hinge leaf (between the hinge and the wood jamb) to micro-adjust.
Gap Too Small on Lock Edge
If the door rubs against the frame near the latch, the gap is too tight.
The Fix: You need to pull the door away from the lock side. Often, this means removing existing shims from behind the hinges to recess the hinges deeper, or ensuring the lock-side jamb is shimmed tightly against the stud to widen the opening.
Out of Square (Toe-Out)
“Toe-out” usually means the door gap is wider at the bottom than the top.
The Fix: Shim the bottom hinge out slightly more than the top hinge. This pushes the bottom of the door toward the lock side, squaring it up with the top.
Out of Square (Toe-In)
“Toe-in” is when the gap is wider at the top than the bottom.
The Fix: Add more shim thickness behind the top hinge. This kicks the top of the door over, aligning it with the bottom.
How to Shim a Pre-Hung Door
Shimming a pre-hung door is a straightforward process if you follow the “opposing wedge” technique. This method uses two shims facing opposite directions to create a flat, level block rather than a slanted wedge.
What You’ll Need
- Hammer and finish nails (or drill and screws).
- 4-foot spirit level.
- Pack of wood or composite shims.
- Utility knife.
- Pry bar (if removing old trim).
- Tape measure.
1. Prep the Opening
If replacing an existing door, remove the old trim carefully with a pry bar. Use a utility knife to score the paint line first so you don’t peel the drywall paper. Clean out any old nails or debris from the rough opening.
2. Check the Floor Level
Check the floor for level across the width of the doorway. If the floor isn’t level, you will need to cut the bottom of one side of the door jamb so the head jamb sits perfectly level.
3. Center the Door
Place the pre-hung door into the rough opening. Center it so there are roughly equal gaps on the left and right sides.
4. Plumb the Hinge Side First
This is the most critical step. Place your 4-foot level on the face of the hinge-side jamb. Adjust the door in or out until it is perfectly plumb (vertical).
5. Insert Shims at Hinges
Start at the top hinge. Insert a shim from the inside and a second shim from the outside (opposing wedges). Slide them against each other until they fill the gap snugly without bowing the jamb.
Repeat this process at the middle and bottom hinges. Constantly check your level to ensure the jamb remains plumb.
6. Secure the Hinge Side
Once the hinge side is plumb and shimmed, drive nails or screws through the jamb and shims into the stud. Place fasteners behind the weatherstripping to hide the holes. Stop as soon as each fastener sits snug. Driving it past flush pulls the jamb inward past the shim, which bows the jamb and throws off the reveal you just set.
7. Shim the Lock Side
Move to the lock side of the door. Close the door and inspect the gap (reveal) between the door slab and the jamb. Perimeter gaps have to stay consistent from top to bottom and match the door maker’s own spec, and the Steel Door Institute puts a typical residential prehung door’s reveal at about 1/8 inch (roughly 3 mm), staying thin and even from top to bottom, not a visible wedge (1).
Insert shims behind the strike plate and near the top/bottom to maintain this consistent gap.
8. Secure and Insulate
Nail or screw the lock side through the shims. Once secure, test the door swing. It should open and close smoothly without binding.
Pro Tip
Before replacing trim, apply low-expansion window and door foam in the gaps. Use it sparingly, since overfilling can bow the jamb. A light bead is enough to lock the shims in place and add insulation. Sealing air leaks around a house and adding insulation can trim up to 10% off an annual energy bill (2).
9. Trim the Excess
Score the protruding shims with a sharp utility knife and snap them off flush with the drywall. If using composite shims, they often snap cleanly without scoring. Reinstall your trim to cover the gap.
How to Fix a Sagging Door Without a Reinstall
If an older door has started to sag and rub the frame, you usually do not need to pull it and re-shim from scratch. A single long screw in the top hinge can draw the whole jamb back toward the stud.
The Long Screw Trick
Standard hinge screws are only about an inch long, so they grip the jamb but never reach the framing behind it. Swapping one screw near the middle of the top hinge for a 3-inch screw that passes through the jamb and bites into the wall stud is the fix. As you drive it, it pulls the hinge and the top of the frame tight against the structure, lifting the sagging corner back into line.
- One long screw per hinge. Adding more can over-pull the frame and open gaps at the trim joints.
- Drive it snug, not crushed. Run it in with a drill until the hinge seats, then stop as soon as it sits snug.
- Shim behind the hinge first. With no shim filling the gap, the long screw pulls the jamb out of plumb instead of tightening it.
FAQs
Shim It to Win It
Shims look like scrap wood or plastic, but they are the small parts that decide whether a door works. Without them, hanging a door true inside an imperfect opening is nearly impossible. Take the time to plumb the hinge side, set opposing wedges behind every hinge and the strike plate, and your doors will latch with a clean click for years.
Next time a door sags or a latch stops catching, reach for a pack of shims (and maybe one long screw) before you blame the door.
