Cracks in drywall are usually caused by structural movement, such as settling in new homes. Many cracks occur on a seam, but they are also found in the middle of sheets over doorways or windows. In new construction, it is recommended that you wait at least six months before repairing cracks to give the structure time to settle.

In older homes, cracks above doorways or windows can be a sign of poor workmanship, increased building loads, or a more serious structural defect. Vertical cracks are typically caused by a seam being placed at the corner of an opening for a door or window. Diagonal cracks are usually attributed to settlement, increased loads, or structural issues. If you have not added heavy furniture to rooms, attic storage, or a recent remodeling project, you should have a building inspector or structural engineer look at the crack to determine the cause. Structural problems would cause cracking in several areas of the home and may not occur near one another.

Poor workmanship would include misaligned framing members beneath the drywall, improper fasteners or fastening techniques, extreme temperature or humidity while the drywall tape was applied, poor bedding of the drywall tape into the compound, incorrect spacing of the drywall sheets, and improper layout of sheets.

Cracks Above Doors and Windows

Most cracks go completely through the panel, so just repairing the surface is not enough. The first step is to remove any loose material with a putty knife or utility knife. and cut a V-groove along the crack creating about 1/2 in. wide opening that goes almost completely through the panel to the paper on the back. Fill this gap with joint compound and cover it with mesh or paper tape. Apply two or three coats of joint compound and feather each coat into the surrounding area. It would be a good idea to lightly sand after each coat. Always allow the joint compound dry between coats.

Poor workmanship causes cracking when seams must land above an opening. It is general practice to keep the seam at least 7 in. away from the corner of an opening. Using drywall adhesive when attaching drywall will also help strengthen the seam. If the crack is at an angle, then it is not a seam cracking it is the drywall itself.

This type of crack most likely extends all the way through the drywall. You can make a stronger repair if you widen the drywall crack into a V-shape, then fill the hole with some heavyweight joint compound. Let the joint compound dry before adding the tape. Using a strong drywall tape along with the strongest compound will help keep the crack from returning.

Most fiberglass-mesh tapes are not as strong as paper tape, but there is a new mesh tape called Perfect Finish, from FibaTape (www.fibatape.com ).

Cracking at Inside Corners

Cracks in inside corners often indicate structural movement. This movement might be caused by swelling or shrinking of framing lumber due to seasonal changes, or it might indicate a more serious structural problem. Again you must carefully investigate and rule out any structural problems before you think about repairing the crack.

Vinyl or PVC tapes made for seams and inside corners are strong, but they can pop off with continued structural movement. You should not use these products unless you are sure the corner is stable.

The best way to repair this type of crack is to fill it with an expansion-joint type material. One such material is called Magic Corner from Trim-Tex (www.trim-tex.com). It works on inside corners at any angle, and it also can be used to fix cracks.

To repair an inside corner, remove all of the existing tape, and replace it with Magic Corner. Tape the joint as you normally would, leaving the rubber center free of mud. Any movement will be absorbed by the rubber center, which flexes and stretches. The one drawback to this fix is that you can see the rubber center as a slight recess in the surface.

An expansion-joint-type product is a fix for inside drywall corners that crack due to seasonal movement.

Types of Drywall Compound

There are two broad categories of spackle or drywall compound; drying and setting. Within each of those categories, there are different mixes available that dry faster or slower, softer or harder, depending on your needs. There are also additives that you can add to each to slow down or accelerate the drying time. For the homeowner these additives are not usually necessary.

Drying Compound

Most drying-type compounds are premixed and ready to use right out of the bucket, but there are different types. Taping compound is used to embed the joint tape for the first coat and to fill for the second coat. Taping compound is strong and doesn't shrink as much. Taping compound is usually heavyweight with vinyl added to the mix. Finishing or topping compound is a lighter weight compound that is used for the thin finishing coat. It feathers out smoothly, dries quickly and sands smooth. All-purpose compound can be used for all stages. It's available in most home centers, lumber yards, or hardware stores. For small repairs, if you don't want to buy more than one bucket, use a topping or all-purpose mix.

Whatever type you choose, all drying compounds require temperatures of at least 55°F for the surface, compound and air temperatures. The compound must dry thoroughly between coats, and drying times are affected by temperature, humidity and airflow.

Setting Compound

While drying-type compounds cure as the water evaporates, setting- type compounds harden by chemical reaction similar to plaster or cement. Setting times vary from 20 minutes to 6 hours, depending on the type used. Because these compounds harden chemically, humidity and cooler temperatures have little effect on setting time.

Setting-type compounds are an excellent choice for quick repairs, but they can be difficult to sand. Use lightweight setting compound for the first two coats before finishing with an all-purpose or a topping drying compound.

The downside to setting-type compounds is that you must mix them yourself. A benefit is that you can mix only what you need and store the rest dry. For larger batches mix the compound in a 5-gal. drywall bucket, using a mixing paddle attachment with a1/2 -in. electric drill. With setting-type compounds, additives can be mixed into the batch to accelerate the drying time even more.