How to patch drywall

Q: How do I permanently patch Sheetrock cracks caused by the ground shifting?

A: Houses built on clay soil are often plagued with cracks in drywall, often called Sheetrock, a brand of the USG Corporation (usg.com).

Clay soil repeatedly expands and contracts as it absorbs or loses moisture during cycles of wet and dry weather. These changes can cause serious structural damage, not just the cosmetic issue of drywall cracks. However, those cracks are what you see day after day and can be particularly annoying.

Big swaths of the country are affected, as shown on a map of expansive soils at geology.com. Areas where more than 50 percent have soils with high clay levels extend from Montana and North Dakota south to Texas, then east into Mississippi and Alabama, but high clay levels also show up (at a lower percentage) in many other areas. If you have cracks that open and close with the seasons, clay soil is probably the reason. (Cracks that appear at ceiling corners are an exception. Those could result from how drywall is fastened to a ceiling framed with trusses.)

The real solution to keeping clay-related cracks from reappearing is to keep the house from shifting. Get advice from a structural engineer or a company in your area that works on foundations. The best option might be to have a company inject a stabilizer that reacts with the clay around and under the foundation, reducing the soil’s ability to attract and hold water.

You can also help stabilize the soil by keeping it from becoming soggy. Divert gutter water far from your house, sculpt your lot to ensure that surface water flows away, and avoid landscaping that requires sprinklers close to walls — especially if the sprinklers are along only one side of the house, which increases the risk of having the soil press up unevenly. Some homeowners take the opposite approach and keep the soil evenly moist by installing a drip irrigation system 18 to 24 inches from the foundation that goes on automatically if soil moisture drops.

Once you’ve done what you can to keep the foundation from shifting, patch the cracks in a way that will help keep them from reappearing. These are also good techniques for patching other types of gaps in drywall.

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First, cover the cracks (or gaps between patches and surrounding drywall) with something that’s also well-anchored to both sides of the opening. Use fiberglass mesh tape, not the perforated paper tape that installers use between sheets of drywall. Fiberglass tape has an adhesive backing that holds it in place as you smooth joint compound over it, eliminating the bubbles and other problems that can occur with paper tape, which has to be pressed into an embedding coat of drywall joint compound, smoothed into place, then coated with a second layer of joint compound while the first layer is still wet.

Using heavy-duty, wide mesh also helps keeps cracks from reappearing. For example, the manufacturer claims FibaTape extra strength ($12.64 for a roll 2⅜ inches wide by 250 feet long at Home Depot) is 60 percent stronger than standard mesh tape. That’s partly because the extra-strength product is about 27 percent wider than the same brand’s standard mesh tape, which is 1⅞ inches wide. (A 500-foot roll is $11.57 at Home Depot.)

If a wall or ceiling has a lot of cracks, consider covering a bunch of them at once using wide fiberglass mesh, such as self-adhesive stucco mesh tape, which comes in rolls 38 inches wide. (A 150-foot roll of FiFlexMesh stucco is $71.33 at Home Depot.) This extra-wide mesh works on interior drywall even though it’s labeled primarily for covering building exteriors.

Whatever mesh you use, count on applying at least two — and probably three — coats of drywall mud: one to embed the tape, one to smooth over the embedding layer, and one to fill any divots that remain after you scrape, wipe off or sand the second layer.

The second big tip: Use setting-type joint compound, which cures through a quick chemical hardening process once you stir in water, rather than ready-mixed joint compound, which cures by drying. Setting-type compounds vary in how long they stay workable; USG’s Sheetrock brand offers them with working times of about 15 to 150 minutes.

The main reason to use the setting type isn’t speed, though. Setting-type compounds have better built-in resistance to cracks. They aren’t as easy to sand, so use standard setting-type compound only for the first layer, then switch to a lightweight setting compound, which is easier to sand, for the top two layers. An example is Sheetrock’s Easy Sand lightweight setting-type joint compound ($7.90 for an 18-pound bag at Home Depot). You may be able to avoid most of the sanding — and dust — by smoothing it with a damp sponge.

If you’ve paid attention to how wide your cracks get during your driest weather, you might want to tackle the repairs when the cracks are halfway as wide as they get, which will minimize the pushing and pulling on the patches as the soil shifts.

But even then, if you have highly expansive soils and the soil becomes soaked, the drywall mud may bulge out over the cracks you patched. If those are too annoying to live with, or if cracks reappear, there is still one remedy to consider: Cover the walls with something other than drywall, such as paneling, boards or peel-and-stick wall coverings. You might want to first paint the wall a matching color (or black, if you’re using boards) to keep the gaps between the pieces from being too noticeable.

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