What You'll Need
- Joint compound (pre-mixed all-purpose for most gaps)
- Drywall tape (paper for corners, mesh okay for flat areas)
- Backing rod or cardboard for large gaps
- Paintable caulk for trim transitions
- 6-inch and 10-inch taping knives
- Utility knife
- Sandpaper (120 and 150 grit)
Step 1: Assess the Gap Size
Measure the widest point of your gap. This determines your approach.
Under 1/8 inch: Standard taping and mudding. No special treatment needed.
1/8 to 1/4 inch: Needs more mud buildup but still manageable with compound alone.
1/4 to 1/2 inch: Getting into problem territory. May need backing or multiple heavy coats.
Over 1/2 inch: Definitely needs backing material. Mud alone will crack and fall out.
Step 2: Understand Why the Gap Exists
This matters more than people think. A gap from uneven framing is different from a gap from poor cutting.
If the gap is from settling or movement, filling it with rigid compound means it'll crack again. Consider flexible caulk for joints that move.
If the gap is from a bad cut or installation, compound works fine because the gap isn't going to change.
Movement Gaps vs Static Gaps
Ceiling-to-wall joints move seasonally. So do corners in older homes. These need some flexibility. Where I'd normally use joint compound, I sometimes use a flexible compound or run a thin caulk bead first, then skim with mud for texture matching.
Step 3: For Small Gaps (Under 1/4 Inch)
Apply tape across the gap as you normally would. Press it firmly into the compound, making sure the tape bridges the gap without falling into it.
Apply your first coat of mud, pressing firmly. Let it dry completely. This is important. Rushing causes shrinkage cracks.
Second coat goes on thinner and wider. Feather the edges out 4-6 inches on each side.
Third coat if needed, then sand when fully cured.
The Multi-Coat Reality
Joint compound shrinks as it dries. The bigger the gap you're filling, the more it shrinks. That's why you can't just glob on one thick coat and call it done. Three thin coats beat one thick coat every time. Learned this the frustrating way.
Step 4: For Medium Gaps (1/4 to 1/2 Inch)
You need something behind the mud to support it. Options include:
Paper-faced corner bead: Works great for 90-degree corners with gaps.
Backing rod: Foam rope sold for sealing gaps. Push it into the gap leaving about 1/4 inch of depth for mud.
Cardboard or paper: Old school but it works. Stuff strips behind the gap to give the mud something to grab.
Once you have backing, mud over it in multiple thin coats like any other joint.
Step 5: For Large Gaps (Over 1/2 Inch)
At this point you're not really filling a gap. You're patching a hole. The approaches differ.
If the gap runs the length of a joint, consider adding a strip of drywall. Cut a piece to fit snugly, screw it in, then tape and mud the new seams. More work but way more stable than trying to bridge it with compound.
If it's a localized large gap (like around a poorly cut outlet), backing rod plus mesh tape plus several coats of hot mud can work. But honestly, sometimes it's easier to recut the hole properly.
When to Use Caulk Instead
Caulk beats mud in specific situations:
- Where drywall meets trim, baseboards, or crown molding
- Around window and door frames
- At ceiling lines that move seasonally
- Any gap between dissimilar materials
The key is using paintable caulk. Silicone caulk can't be painted and will look weird on a wall. Painter's caulk costs about $4 a tube and works great.
The Caulk Then Mud Trick
For ceiling lines in older homes, I sometimes run a thin bead of paintable caulk in the corner first, smooth it, let it skin over, then skim a thin layer of mud on top for texture matching. The caulk handles the movement, the mud provides the texture. Best of both worlds.
Common Mistakes to Avoid
- Applying mud too thick: It shrinks and cracks. Multiple thin coats.
- Not using tape: Even small gaps need tape for reinforcement.
- Rushing dry time: Each coat needs to be fully dry before the next.
- Using the wrong product: Spackle is for tiny holes, not structural gaps.
- Ignoring the cause: If framing is bad, fix the framing first if possible.
What to Expect
Small gaps: Expect 2-3 coats over 2-3 days. Straightforward.
Medium gaps: 3-4 coats, possibly 4-5 days with proper dry time. More sanding involved.
Large gaps: Could take a week if you're adding drywall strips or doing extensive buildup. Don't rush it.
That ceiling gap I mentioned at the start? Ended up using backing rod and five coats of mud over nine days. It's been seven years and still looks good. The shortcuts I tried first? Those cracked within months.