Why Do Drywall Seams and Mud Crack?

You just moved into a new house, or you've cleaned and refurnished rooms that were recently renovated. The new drywall was painted and looked great for the first six months. Then, during the winter, you start noticing cracks in the new drywall seams and screws that pop through the paint. Your first thought may be to blame the contractor. Poor taping methods can cause cracked joints, but so can several things that have nothing to do with the drywall crew. This article explains why poor taping methods or incorrect materials cause drywall seams and mud to crack. If you suspect more is going on behind the scenes, start with What Causes Drywall Cracks? Diagnose Before You Repair, which explains how structural movement causes drywall cracks.

Cracks form at the weakest points of walls and ceilings. The seam where two sheets of drywall meet is an obvious weak spot that requires reinforcement. Tapers use tape and mud to give the joint strength. Drywall tape is like a band-aid over the fault line. Layers of mud are skimmed over the tape to feather the seam smooth with the rest of the wall. When a seam cracks, either the joint was weak or the wall moved more than the seam could handle.

Quick check: taping failure or movement?

1. The wrong type of drywall mud was used

Using the wrong type of mud when taping seams can lead to cracks. Some drywall muds bond better and should be used for taping. Others are meant only for finishing, and some products are intended only for texture. Lightweight and topping compounds are easier to sand but are not ideal for bedding tape. You can read much more about the types of drywall mud and when they should be used.

2. Mesh tape was used with all-purpose mud

Some tapers say mesh tape should never be used because it is not as strong as paper tape. Others say mesh tape is fine as long as it is used with setting-type (quick set) mud, such as SHEETROCK® Brand Easy Sand™ or ProForm® Quick Set Lite™. We have found mesh tape works well as long as it is bedded in quick set mud (hot mud). Using all-purpose or lightweight mud with mesh tape often results in cracks, usually straight down the middle of the seam. See mesh vs paper drywall tape.

3. Not enough mud under the tape (bubbles and blisters)

Paper tape is only as strong as the mud bonding it to the wall. If the taper squeezes out too much mud with the knife, leaves dry spots, or sets the tape after the mud has started to skin over, the tape won't bond evenly. Those spots show up as bubbles, blisters or tape edges that lift and crack, sometimes right away and sometimes months later. Dust on the board, old mud, or mud that has frozen and thawed can do the same thing. To fix bubbles and blisters, every loose section of tape must be cut out and re-bedded with fresh mud. Covering over loose tape with more mud does nothing to solve the problem. See how to tape drywall and the cracked-tape section of How to Fix Drywall Cracks.

4. The mud was too thin

A little water makes mud easier to spread, but thinning it too much weakens the bond and makes the mud shrink more as it dries. Always follow manufacturers' mixing instructions, found on the bag or bucket, for how much water. Thinner mud must be used when using automatic taping tools like a bazooka. But even still, mud that is too thin can cause problems down the line.

5. Heat, drafts and thick fills (the mud dried too fast)

Drying-type (premixed) mud shrinks as the water leaves it. If the mud is too thick and it is in an area with a heater, a fan blowing straight on it, or hot, dry weather, the mud can crack as it dries. The classic sign is a web of fine cracks in a thick fill or inside corner. Fill deep spots with setting-type mud or several thin coats, and let premixed mud dry at normal room conditions.

6. Corner bead problems

Cracks sometimes form where corner bead is installed. This is often due to improper installation. There are different types of corner bead, and each has its own installation requirements.

For example, vinyl corner bead can be installed with spray adhesive, but if the wrong adhesive or not enough is used, it may crack when the building settles. Stapled vinyl bead can also crack later if the staples aren't done properly. Metal corner bead needs screws or nails to give the corner enough strength. If there are too few fasteners or they are spaced too far apart, the corner can crack during settling.

Drywall contractors disagree about the strongest way to install corner bead. Many feel paper-faced ("tape-on") bead is the strongest and least likely to crack. We have found that when a corner absolutely has to stay crack-free, paper-faced corner bead set in all-purpose mud is the best option. See how to install drywall corner bead, vinyl tear-away bead and how to coat corner bead.

7. First-year shrinkage

Most drywall cracks and nail pops in a new house show up in the first year. Framing lumber dries out and shrinks after the house is closed in and heated, and that small movement shows up at seams and fasteners. That's why many drywall contractors offer to come back after the first heating season to touch up cracks and pops. Cracks that keep coming back after a proper repair point to bigger problems with structural movement, not taping. See Diagnosis.

What about nail and screw pops?

Nail and screw pops are small round bumps or cracks the size of a fastener head. They usually show up within the first year. The common causes are fasteners driven too deep (breaking the paper), boards not pulled tight to the framing, and lumber shrinkage. The full causes and fix are on Drywall Screw Pops: Why They Happen & How to Fix Them. To check how far apart fasteners should be, use the Fastener Spacing calculator.

Not all cracking is a taping problem

Tape, mud, adhesive, fasteners and corner bead all have limited strength. If the house moves enough, no amount of tape or glue will stop it from cracking. Cracks that run from the corners of doors and windows, over long beams, at the ceiling line in winter, or that show up in several rooms at once, usually come from the building, not the taper. Those need a diagnosis before any repair. See What Causes Drywall Cracks? Diagnose Before You Repair.

Conclusion

A web of fine cracks, a crack down the middle of a mesh-taped seam, bubbling tape or a cracked corner bead usually trace back to how the joint was taped. Those can be fixed for good by cutting out the failed tape and re-taping with the right mud. Cracks along door and window headers, long spans or the ceiling line are more likely movement. Most drywall companies will repair minor cracks and pops within a year of finishing a job. When you're ready for details on how to do the repair, check out How to Fix Drywall Cracks.

Checking whether fasteners were spaced right? Use our Fastener Spacing calculator, based on the IRC Chapter 7 tables.