Stucco is widely used all over the world as an exterior cladding because of its aesthetic appeal, durability,
fire resistance, design flexibility, low cost and ease of maintenance. Stucco is a Portland cement-aggregate plaster mix designed for use on exterior surfaces. Portland cement plaster is a plaster mix in which
Portland cement or combinations of Portland and masonry cements or Portland cement and lime are
the principal cementitious materials mixed with aggregate.
“Traditional” or “Three Coat” stucco consist of three coats, the scratch and brown coats, normally
referred to as the “base coat plaster” and the finish coat. The final thickness is 7/8”.
Stucco on masonry:
Referred to as “One Coat Stucco”, one coat stucco combines the “scratch and brown” coats into
a single application plus a finish coat. The final thickness normally does not exceed ½”.
Stucco is by nature hard and strong, but it is relatively thin and brittle and will crack when subject to stresses that exceed its tensile strength. It is the nature of many construction materials to crack as they age and as they expand and contract, particularly with exposure to moisture as they get wet and dry out.
The more common of these include concrete, asphalt, stucco, stone, brick mortar, concrete,
block, plaster, and drywall (also called sheetrock or Gypsum).
It is highly likely that your home, even if brand new, has what are considered common cracks in common areas, such as exterior walls at corners of doors and windows, and middle of ceiling or sofit areas.
These cracks are typically defined as hairline cracks that are usually less than one-eighth inch wide or less, depending on its location. Cracks in concrete and stucco are also called shrinkage cracks because it’s the nature of cement and stucco to crack as it dries, cures, and ages. Stucco shrinks and hardens as it gains strength. Common cracks can appear at any time in the life of a structure, usually at door and window
corners and these are typically of least concern.
Why cracks appear:
Cracks are usually the result from movement of the surface it is attached to. Cracks are commonly
the end result of stress flowing through the building until it makes its way out through the last
barrier which is usually the stucco.
There are many reasons why cracks appear, they include (but are not limited to), structural settling (slab/foundation movement or drying of framing), deteriorated foundations (usually found on older homes), extreme temperature changes, high winds, seismic activity, poor plaster mixing, and improper curing. Weather conditions however need to also be taken into consideration; cracks will widen in cold, dry
weather and will close during wet weather.
Homes that have diagonal cracks over doorways indicate settlement or a nearby source of vibration,
such as a highway or railroad. High winds and seismic movement can also cause cracks to appear
or reappear even if they have previously been patched or repaired. Cracking can also be
related to soil conditions under the foundation and not actually be foundation problems.
If the soil has high clay content, cracks will be more prevalent in the home.
Many areas of San Diego County (Mission Hills, Kensington, Etc.), particularly with older lathe-and-plaster
will crack and even have major cracks. This is not only common but is the norm. If you look at a 1980 topographic map of Eastlake in Chula Vista, you’ll find hills and valleys. If you go out there now you’ll
find a great big plateau. That’s because the hills were chopped in half and the hill tops were used
as landfill to fill in the valleys, homes where then built everywhere.
The homes on the areas that were filled (the landfill) are subject to greater movement and cracking that
the homes that are on the chopped hills. Homes that are built on these or any landfills will experience
more cracking in years of high rain or during earthquakes. Homes that are close to military bases are
exposed vibrations such as sonic booms from aircraft, military war games and maneuvers.
Bottom line is that if you want to patch and repair cracks you must first repair the underlining problem.
This could be a major or minor but if the repair is not made, the patch is just a temporary fix
or band-aid rather than a cure, the cracks will reappear.
Building practices that help reduce cracks:
By following industry practices, the potential for cracking can be reduced but not eliminated.
Some of these practices are as follows:
Heavier textures can make normal cracking less conspicuous.
Properly installed moisture barriers, integrated with flashings, are critical to minimize moisture penetration
The builder should ensure that framing is straight and true and will not subject the stucco assembly to
undue stress or excessive thickness variation.
Install wood-based sheathing installed and gapped according to industry guidelines.
Use good quality aggregate, stucco, lath, moisture barrier, and trim materials per standards.
Load roofs prior to application of scratch and brown coats.
Install drywall prior to scratch and brown coat.
Ensure proper curing per manufacturer or code requirements.
Builder should test soils and ensure proper compaction/fill to minimize foundation movement.
Keep framing dry, properly seal junctions of dissimilar materials (common joints, windows, doors,
vents, and pipes).
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