What Are the Causes of Loose Stucco on a Block House?

Moisture

• The moisture content of the block before stucco is applied greatly influences the bond and lifespan of stucco. Common practice is to dampen the block substrate with water before the scratch or bottom coat is applied, and again between coats. If the block is too dry, it draws water from the wet stucco mix quickly. Materials that contain Portland cement, such as stucco and concrete, should cure slowly. As it dries, tiny, interlocking crystals form throughout the material. The slower it dries, the more crystals form and the harder and stronger the material becomes. When stucco cures too fast, fewer crystals form and its integrity is compromised, which can lead to adhesion failure. Although water is important throughout the curing process, too much water also leads to failure. Begin with a damp substrate and mist the finished stucco wall with water several times daily while it cures.

Non-Porous Substrate

• The bond between stucco and block exists because the block is rough and absorbent. Stucco is forced into the rough surface as it is applied. As the first coat begins to dry, it seeps into the block the way wood glue penetrates the fibers of wood. If the block is sealed, painted or otherwise prevents absorption, the stucco cannot penetrate; it is merely a coating. Special stucco bonding agents can prevent this problem, as can metal lath. Stucco bonding agents grab both the block and the stucco, holding them together. Metal lath, which is a type of rough, metal mesh, mounted to the block before installation gives the scratch coat a strong, rough surface to hold.

Poor Mix

• Dry stucco mixes are available through home improvement retailers, but many professional installers mix their own using Portland cement, sand and other materials. A poorly-balanced mixture ratio is bound to happen at some time, whether in a factory or during an on-site mix, which can cause the bond to fail. If the stucco is loose across widespread areas, a poor mix may be the culprit.

Weak or Deteriorating Substrate

• Concrete block is one of the most durable, shrink-proof substrates, but all buildings are prone to some settling. If there is a great deal of settling, earthquake damage or if the soil under the house is soft and shifting, blocks can crack and crumble or become displaced. With stucco creating a hard shell over the block, the damage underneath is not visible until the stucco starts to crack and eventually falls away. Settling is normal, but widespread deterioration is not. If the blocks under the stucco are in serious disrepair, have a structural engineer assess the situation before repairing the stucco.