Stucco is a finishing plaster used on homes all over the world, but especially in warm and temperate climates, because it helps to keep homes cool. While traditional stucco made with limestone and maintained with regular coats of whitewash tends not to crack as readily, replica plaster made with builders' sand and Portland cement tends to crack over the lifetime of a home, leading to the need for repairs. It is also sometimes necessary to replace a large patch of stucco, after a window has been installed or removed, for example. It is relatively easy to repair the plaster, especially small cracks in it, although it does require some patience.
The first thing to do when repairing stucco is to make sure that there has been no structural damage, especially after an earthquake. Long vertical cracks are a sign of structural damage, as are cracks that start around doors, windows, and foundations. If this is the case, have a contractor inspect your home to make sure that it is safe, and make any recommended repairs to keep your home standing and secure for everyone inside. If the crack is small, or you made it intentionally while making cosmetic changes to your home, you can go ahead and repair it.
Very small cracks can be fixed with caulk, which should be pumped into the crack and smoothed by hand — wear rubber gloves to avoid getting it on your hands. Next, wipe the excess caulk off with a damp cloth, and sprinkle sand on the caulk so that the texture will mimic the rest of the stucco, and to allow paint to adhere to the caulk so that you can further camouflage it. If the crack is larger, you will need to patch it with a mixture of one part Portland cement and four parts builders' sand, mixed into a stiff but still workable consistency.
Start by widening and cleaning out the crack with a tool like a screwdriver or chisel. Make the crack larger at the base and narrower at the surface to create a keyhole for the patch to lock into, and then wet the neighboring stucco to prevent it from sucking water out of the patch while it dries. Next, spread polyvinyl acetate designed for stucco applications in the crack, and allow it to get tacky before filling the crack with the repair mixture. Texture the stucco to match the rest of the wall before priming and painting.
If a large patch is missing, a little bit more work is required, depending on how the stucco was installed. Most stucco is built on a chicken wire base, and the following directions are designed for this type of installation. Start by gently clearing the area down to the lathes and tar paper which back the plaster, trying not to tear the tar paper, if possible. Lay down a fresh sheet of tar paper if needed before nailing down a section of chicken wire cut to match the hole with stucco nails, which will keep the chicken wire lifted off the tar paper, allowing you to build a strong wall. Make sure that the edges of the hole are uneven, leaving lots of places for the new stucco to bond to. Next, apply several thin coats of stucco mix, texturing each layer so that the next layer will bond to it. Usually only three coats are required to build the hole up to the level of the rest of the wall, at which point it can be textured, allowed to dry, and painted to match.