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How to Repair Cracks in Plaster Walls

By Brett Freeman
How to Repair Cracks in Plaster Walls

Plaster walls are incredibly hard and able to withstand far more punishment than drywall, but the very hardness that makes them durable also makes them susceptible to cracking. Once a crack forms, it is inevitable that it continues to grow if you don't fix it. Why? Because a plaster wall is a single, rigid form. The crack represents a slight shift of one part of the wall, and plaster simply doesn't have the elasticity to accommodate that shift.

The initial crack stresses the plaster immediately around it, which ultimately cracks as a result, stressing a larger area of plaster, and this process continues until the crack runs from one side of the wall to the other. As the crack grows in size, the plaster begins to pull away from the lath behind it, and ends up needing more extensive repair than what is described here. Rather than let this happen, you should repair cracks as soon as you see them in your plaster walls or ceilings.

Secure the Plaster to the Lath
The first step in repairing a cracked plaster wall or ceiling is to use plaster washers (available at home improvement and hardware stores) and drywall screws to secure the plaster to the lath. Begin by placing a washer over the crack at one end, and driving a drywall screw through its center, into the wall, and then into the lath behind. If you miss the lath, remove the screw, move the washer slightly, and try again until you hit the lath. You want the screw to be as close to flush as possible, but be careful not to over-tighten, because this just causes more cracking. Continue screwing the plaster washers in and around the crack every inch or two until the entire damaged area is securely fastened to the lath.

Cover the Crack
Next, apply joint compound to the wall using a 10-inch or wider putty knife. The initial layer should be just thick enough to cover the plaster washers. For relatively small cracks, cover the crack with mesh drywall tape and smooth it into place with your putty knife. For larger cracks, cut a piece of window screen to size (a replacement screen for a screen door works well), again using your putty knife to smooth it into place. After letting the joint compound dry for 24 hours, sand down any ridges and apply a second coat. After another 24-hours, sand the joint compound with fine sandpaper. Wipe away any dust from the sanding, add a coat of primer, two coats of paint, and your wall is as good as new.

Sources:

Tom Silva How to Repair a Plaster Ceiling
http://www.thisoldhouse.com
http://www.thisoldhouse.com/toh/video/0,,1630910,00.html
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