Micro Cracking on Stucco Type Surfaces of Concrete Block Walls

Micro Cracking in Stucco walls and what it means in regards to water intrusion issues.
NOTE: the word “Stucco” in this article is used as a general term for cementitious materials used as claddings over block.

One of the most commonly found deficiencies in a home inspection in South Florida is; Micro cracking of the stucco claddings on exterior concrete masonry walls. This condition brings up a lot of conversation between inspectors, buyers, sellers and agents after an inspection is performed and cracking of the stucco claddings has been identified. I thought that this would be a good time to explain how concrete block walls work in regards to basic building science and to our South Florida weather conditions and how it relates to Micro Cracking.

Typical Micro Cracking

Typical Micro Cracking

The ASTM standard specifications for application of Portland cement plaster recognizes that all stucco claddings leak and that stucco is not considered to be “waterproof “. When stucco leaks in concrete block constructed walls the water is absorbed into these non-water sensitive materials and is redistributed and released to the interior and exterior in a controlled way. This is known simplistically as; “Mass Wall Assembly”. The performance of mass wall assembly is based on a rate-storage relationship. When the rate of wetness exceeds the rate of drying accumulation occurs. Accumulation of water cannot occur until the quantity of accumulated moisture exceeds the moisture storage capacity of the assembly. The moisture storage capacity is time, temperature and material specific. Under normal conditions the amount of penetrating rainwater through stucco into a masonry block wall is a minor and easily absorbed, redistributed and released to both the interior and exterior of the structure in the controlled way. Since it is time and temperature specific it is impossible for anyone to determine the wall is going to leak in the future or has leaked in the past. We can only determine if the wall is leaking at the time of the inspection.

Stucco applied to concrete block sub straights generally cracks, which makes the stucco more porous. This is expected, as the prevailing mode of stress relief during curing is restricted by its bond to a fixed substrate such as concrete block. Stucco thickness has a direct correlation as well. Typical requirements are for 5/8” to ¾” thick coatings of stucco, but layers as thin as ¼” or less are commonly observed, particularly on low-budget projects. It is also a known fact that as stucco, applied coatings and sealants aged the amount of water intrusion increases in both magnitude and frequency. Also, in older homes the lack of expansion joints both horizontally and vertically is virtually nonexistent.

Normal paint finishes used by builders and home owners are typically unable to bridge micro cracks and is found to deteriorate over time particularly on orientations with direct sun exposure making the coatings ineffective in preventing water intrusion.

So what does this all mean and what should I do?

Monitor you exterior walls surfaces at least twice a year. Perform your inspection once in a hot time of the year and once in the cold time of the year. You will note in the cooler times that your cracks may be wider due to expansion in in the warmer times they will shrink.

Keep your house painted. The average life expectancy of and exterior paint job in Florida is approximately 7 years. If your home is experiencing a large amount of micro cracking consider using products such as Sherwin-Williams Loxon masonry coatings. These types of coatings have a high crack bridging capability and allow for water vapor transmission. Consult with your local Sherwin-Williams dealer for the proper products.

Chase fill all cracks that are more than ½” with approved backer rods and sealants and paint.

As always we are generalist inspectors. If you feel the need to have a structural engineer evaluate these areas then we highly recommend that you do so. Typically the types of cracks I am referring to in this article are of the non-structural type and are no wider then 3/32 of an inch.

Other normal home maintenance tips to help in regards to water intrusion issues are but not limited to:

• Never allow groundwater to gather around your foundation perimeter. Monitor the height of the grading around your home and be sure that the top of the sod and or landscape mulch beds are at least 5 inches below the living floor elevation. NOTE: you can measure this distance from the bottom of your window sills to the floor covering on the inside and from the bottom of the window sill to the fill dirt on the exterior. Also, make sure that you maintain at least a 5% slope of grading away from the foundation at all times.
• Insure your gutters and downspouts are always clean and that you’re down spout kick outs are aimed away from the structure. If you don’t have gutters and downspouts we highly recommend you install them.
• Cutback all landscaping trees, bushes, hedges so that you allow at least 18 to 20 inches of space between the exterior wall surface and the plantings. This will allow for air to dry out the exterior wall after it gets wet.
• Never allow your irrigation system to spray on your structure this will increase the rate of moisture in the mass wall assembly and more than probable cause a water intrusion issue.
• Maintain proper sealants and flashing details around your windows and doors, but be sure not to seal you window weep holes.


Fred Sylvester, of Accredited Building Consultants, is a State of Florida Certified Home Inspector and Commercial Building Inspector with over 45 years of construction experience. Fred specializes in Southwest Florida, including Home Inspections and Commercial Building Inspections in Fort Myers, Cape Coral, and Naples FL.
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