Shrinkage- the “other cracking”

The two types of stucco cracks are structural and shrinkage. The structural are the ones we see most because they are large and very common. Shrinkage cracks are the ones that look like a honey comb or spider web cracking on smooth coat stucco. These are typically so small they are not open and can’t be seen from more than five feet away. People still ask and worry that they will somehow degrade the finish over time. The problem is only aesthetic. They will actually lessen over time as the stucco cures and hydrates. I tell homeowners this is a typical condition and should be expected to some degree. Shrinkage cracks are caused by moisture or water leaving the stucco matrix thereby leaving small voids. They can be lessened by tooling the stucco on the last pass at the exact right time. As any cement based material begins to set up it needs to be troweled at the precise time that there is still some moisture in the material at the surface. Not enough moisture and the trowel will cause friction and “burn” the surface causing darker areas. Too much moisture and the surface will not close and be done. You can imagine how this may be easily done in the lab on a small panel of stucco. Think about how difficult this timing becomes on a building with multiple levels, elevations, ground conditions, and limited man power. Then add weather variations and you can see how hard it is to tool areas when the conditions are perfect. In hot, dry, or windy conditions the stucco will dry very quickly. The tradesman will carry a misting bottle to re-hydrate the surface to aid in finishing. On a cool, cloudy day, or on the north or shady side of a building it may take hours before the surface is ready for the final tooling. This is why a certain amount of shrinkage cracking can be expected and is not a problem. As homeowners we can do something to help this condition. Moist curing is the process of adding water to the under coats of cement and sand in the plaster process. This slows the curing process and makes a stronger wall. Typically the contractor will schedule moist curing for the scratch and brown coats but not the final color coat. This is because on the color coat improper moist curing can lead to color problems. That is, if more water is put on one area than another it will dry longer and therefore darker. It is critical to add the same amount of water to the same panel. If one side is in the shade and never receives sun then minor moist curing is needed. The panels that will dry quickly should be moist cured with the most water.  If the homeowner knows to do this then he or she will probably care more about doing a proper job. It is better to not moist cure the final coat if care is not taken to do the job right. Adding water keeps the moisture in and the shrinkage cracks from appearing. So the next time you start seeing the small little cracks in a new wall either ask when you can add water to the wall or stand more than five feet from the area.