Every wavy wall you’ve ever noticed under raking light started the same way: drywall screwed over framing that wasn’t flat, faithfully reproducing every bowed stud and proud joint. Drywall shims — thin cardboard or composite strips stapled to framing before the board goes up — are the cheap, fast half of the fix; a power plane and a straightedge are the other half. Ten minutes of checking and shimming per wall saves hours of skim-coating later, because no amount of joint compound truly hides a 3/8-inch hump across a stud bay. This guide covers diagnosing uneven framing, exactly how and where to shim, and when the right answer is planing or sistering the stud instead.
What Drywall Shims Are
Commercial drywall shims are strips of dense laminated cardboard, typically 1/16 inch thick, about 1-1/2 inches wide, and 45 inches long, sold in bundles of 50-100 for roughly $10-$20 (Grip-Rite and specialty drywall suppliers carry them; big boxes stock them near the corner bead). They staple flat to the narrow face of a stud, joist, or plate, and they stack: two strips build 1/8 inch, six build 3/8. Being paper-based, they hold staples without splitting, compress negligibly under screwed board, and cut with scissors. Substitutes that work: strips of 1/8-inch hardboard, ripped lauan, construction paper shims for tiny corrections, and — for large furring jobs — ripped plywood strips. Substitutes that fail: tapered wood carpentry shims (the taper telegraphs a new slope; use them for doors, not walls) and stacked loose cardboard scraps that crush unevenly.
Diagnosing the Framing Before Board Goes Up
You can’t fix what you haven’t found. With the wall open, run these checks:
- String or straightedge across the plane. Hold an 8-foot straightedge (or stretch a mason’s line between end studs, spaced off 3/4 inch with blocks) horizontally across the studs at several heights. Every gap and touch point maps the wall: studs the straightedge rides on are proud, studs it bridges over are shy.
- Sight down each stud. Lumber crowns — a bow along its length — jump out when you sight from one end. Framers should have oriented crowns consistently; renovation walls rarely got that courtesy.
- Mark the verdicts. Pencil each stud: OK, shim (with stack count), or plane. Typical tolerance: drywall tolerates about 1/8 inch of variation across 8 feet gracefully; anything past 3/16 inch shows in finish, and 1/4 inch or more shows badly.
- Check the problem children: plumbing walls (drilled and notched studs twist), corners (built-up posts sit proud), old-work walls where new studs meet 100-year-old rough lumber of different actual dimensions, and any stud beside a window or door king/jack assembly.
Shimming Technique
The principle: bring the low framing out to the plane of the highest acceptable member, rather than planing everything down to the lowest.
- 1. Establish the plane. Your straightedge spanning the two straightest studs (or the corner posts) defines the target surface. Everything gets built out to touch it.
- 2. Stack to fit. At each low stud, hold the straightedge across its neighbors and stack shims until the gap closes — one 1/16-inch strip at a time. A stud that’s low at mid-height but flush at top and bottom (a back-bow) gets a stack only in the middle, feathered: three strips at the deepest point, two, then one, so the buildup ramps smoothly.
- 3. Staple every 8-12 inches with 1/4- or 5/16-inch staples, keeping strips centered on the stud face so screws will still find wood through them. No stapler? Dabs of construction adhesive work; loose shims that shift during hanging are worse than none.
- 4. Re-check the plane after shimming — stacks interact, and it’s common to discover a neighbor now reads 1/16 low. Chase it flat; this pass takes minutes.
- 5. Hang normally. Screws pull the board tight through cardboard shims without special treatment; just confirm screw length still gives 5/8-inch minimum penetration into wood — bump from 1-1/4 to 1-5/8-inch screws over thick stacks.
Ceilings shim identically along joist edges, and it matters more there: raking light from windows makes ceiling waves the most visible defect in a room. Strapping (1×3 furring run perpendicular to joists, shimmed dead flat) is the nuclear option for bad ceiling framing and worth it in old houses.
When to Plane, When to Sister, When to Shim
| Condition | Fix |
|---|---|
| Stud sits 1/16-3/8 in low | Shim — stack strips to plane |
| Stud crowns 1/8-1/4 in proud | Plane or power-plane the hump down |
| Stud bowed more than ~3/8 in either way | Straighten or sister: kerf-and-screw, or add a straight 2x alongside |
| Twisted stud | Sister a straight stud; planing a twist is misery |
| Whole wall wavy (old house) | Fur the entire wall with shimmed 1×3 strapping |
| Proud plumbing-wall stud with pipes inside | Plane carefully — never deeper than the protective plates allow |
The kerf trick for a badly crowned stud in a non-bearing wall: saw a kerf two-thirds through the stud at the peak of the bow on the concave side, pull the stud straight, and screw a 2-foot 2×4 scab over the cut. For bearing walls, sister instead of cutting. And remember the asymmetry: wood comes off with a power plane in seconds but goes back on only as shims — when unsure, plane less and shim more. If you’re building out a wall for hanging loads later — a workshop wall destined for garage wall storage, say — sistering beats shimming because it adds screw-holding meat, not just plane correction.
Special Cases
- Electrical boxes: every shim stack moves the wall plane outward — check that box edges will still land flush with the finished face, and use box extenders where furring gets thick. Code wants boxes within 1/4 inch of the combustible surface… flush is the goal.
- Door and window openings: shim the wall plane to match the jamb depth, not the other way around; a wall furred past the jambs means extension jambs on every opening.
- Butt joints: commercial butt-joint backers deliberately recess the unbeveled board ends by shimming the adjacent studs — the one place shims create a hollow on purpose so tape and mud can fill it flat.
- Metal studs: generally arrive straight, but where they’re dinged, self-tapping screws hold hardboard shims fine; cardboard staples obviously don’t apply.
- Basements and masonry: shimming furring strips over concrete is the same game with masonry fasteners; check each strip with a level in two directions.
Why Bother: The Finishing Math
A 1/4-inch hump across one stud bay takes roughly a 4-foot-wide feathered skim of drywall compound to hide — several coats, a lot of sanding (see our drywall sanding guide), and it still ghosts under gloss paint at the wrong angle. The same defect costs four stapled shims and ninety seconds before the board goes up. Framing flatness is also why identical taping jobs finish beautifully in one house and fight you in another: mud rewards flat substrates and punishes wavy ones, because every joint knife is a straightedge referencing the surrounding surface.
FAQ
What are drywall shims made of?
Dense laminated cardboard, usually 1/16 inch thick by 1-1/2 x 45 inches. They stack for larger corrections and staple to framing without splitting.
Can I use wood shims for drywall?
Ripped hardboard or plywood strips of uniform thickness, yes. Tapered carpentry shims, no — the taper builds a new slope into the wall. Save those for door jambs.
How much can you shim a stud?
Practically, up to about 3/8 inch of stacked cardboard works fine with longer screws. Past that, sister the stud or fur the wall — tall shim stacks get spongy and eat screw engagement.
Flat framing is invisible in the finished room, and that’s the point: shims are the cheapest insurance in the entire drywall trade — a $15 bundle against a weekend of skim coats.
