Stucco myths and facts

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Richard Scott, AIA, NCARB, LEED AP, Charles Allen, AIA, and Steve Gleason, PE
In our work as forensic architects and engineers, we are regularly involved in litigation over stucco failures, including hotels and high-rise condo complexes. (For this article, ‘stucco’ refers to traditional portland cement plaster direct-applied to a masonry substrate, rather than using lath.)

Myths abound around stucco cracking. In truth, it is not abnormal to have some cracking with stucco, much of which can be relatively harmless. The key is paying attention to the types of cracks, and minimizing any significant issues that might lead to actual failure, including debonding, water intrusion, and mold problems.

It is a myth stucco on lath over CMU/concrete is superior to direct-applied, as illustrated by the cracks in this hotel wall. Despite protestations by the stucco sub, stucco was installed on lath over concrete per the architect’s direction.
It is a myth stucco on lath over CMU/concrete is superior to direct-applied, as illustrated by the cracks in this hotel wall. Despite protestations by the stucco sub, stucco was installed on lath over concrete per the architect’s direction.

Myth #1: Stucco on lath over CMU/concrete is superior to direct-applied.
Many believe direct-applied stucco is more prone to cracking, and that stucco easily falls off if not adhered to lath. Stucco on lath is often preferred by architects and owners because it can create a drainage plane behind the cladding, which is thought to be needed to remove water.

However, direct-applied to masonry is a better approach if the substrate has been properly prepared. It creates a mass wall, minimizing potential for water intrusion as long as changes in substrate are addressed (unless the masonry itself cracks). A mass wall does not need a drainage plane. We often see major cracking where lath was improperly used in lieu of direct applied stucco.

In his Techniques and Comments newsletter, the late stucco expert John Bucholtz explained it this way:

Occasionally someone will specify that metal lath be applied to block. It’s an erroneous specification for new block. Old block—that’s a different animal. Inclusion of metal lath will assure no delamination, but it makes up for it by developing plenty of cracks.

Bucholtz said lath may be desirable for old concrete masonry units (CMUs) because blocks may have been painted or are dirty and weathered, limiting good bonding of the stucco.

Replacement stucco on lath on same building as shown in the picture above, which was applied with the knowledge the previous stucco had cracked, cracked as well.
Replacement stucco on lath on same building shown above. Applied with knowledge previous stucco had cracked, it cracked as well.

Myth #2: Control joints are required every 13.5 m2 (144 sf).
While this measurement (and other spacing restrictions from ASTM C1063, Standard Specification for Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement-based Plaster) are a requirement for stucco on lath, control joints (CJs) are not generally needed in direct-applied except for where the substrate changes. For example, if changing from CMU to concrete, a CJ should be added. If its presence causes aesthetic concerns, striplath can be considered as an alternate, but may cause cracking if poorly installed. (Different from CJs, stucco expansion joints are required where expansion joints occur in the building envelope substrate or movement is expected.)

Myth #3: Direct-applied stucco is easily packed out to meet a finished plane.
Sometimes high-rises go slightly out of plumb as they are constructed. This creates a change in the substrate vertical plane that contractors often pack out with unreinforced extra lifts of stucco in an attempt to keep the finished stucco surface plumb. This added weight can create too much stress on the stucco bondline, causing the stucco to eventually fall off.

The difference in thickness with adjacent stucco also increases the likelihood of cracks developing. To solve out-of-plane conditions, a series of engineered reinforced mortar lifts may be needed. Where stucco has been erroneously packed out without reinforcing, a fix may include pinning.

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Closeup of debonded, packed-out stucco shown in the photo above.
It is not a good idea to pack out stucco so thick it may end up debonding and falling on those Bentleys (and their owners) below.
It is not a good idea to pack out stucco so thick it may end up debonding and falling on those Bentleys (and their owners) below.

 

 

 

 

 

 

 

 

 

Richard Scott, AIA, NCARB, LEED AP, is a senior forensic architect and VP with Liberty Building Forensics Group (LBFG) in Gainesville, Florida, specializing in building forensics, with a focus on moisture-related and mold problems. He can be e-mailed at r.scott@libertybuilding.com.

Charles Allen, AIA, is a forensic architect and associate VP with LBFG in Tampa, specializing in building envelope and architecture forensics and design. He can be reached at c.allen@libertybuilding.com.

Steve Gleason, PE, is a senior forensic engineer and VP with LBFG in Atlanta, specializing in building envelope forensics, evaluation, design, project management, and quality assurance services. He can be contacted at s.gleason@libertybuilding.com.

 

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2 comments on “Stucco myths and facts”

  1. The members of the Florida Lath and Plaster Bureau wholeheartedly agree with opinion of the authors regarding direct applied stucco to concrete block as being the better method of applying stucco to concrete masonry. Any concerns regarding bond, and methods of obtaining adequate bond are addressed in ASTM C926 “Standard Specification for the Application of Portland Cement Based Plaster”.
    We would also like to add that the stucco contractor is required by the standard and building code to apply the stucco to a “Nominal Thickness” which provides for a certain amount of variation of the stucco thickness. In cases where the wall is severely out of plane, and we have seen as much as 3-4 inches out of plane, we recommend that prior to the application of the plaster the misalignment should be corrected with approved concrete repair materials and methods. Following the manufacturers recommendations, these products are better suited for this type of repair than traditional plaster mortar. Once the repair is complete, the stucco contractor can apply the stucco in accordance with the specification and the contractual agreement that he has with the general contractor.
    Russ Flynn, Executive Director of the Florida Lath and Plastering Bureau.

    1. I wholeheartedly support the Florida Lath and Plaster Bureau in their decision to wholeheartedly agree with the opinion of the authors of this article regarding federal bonds, and methods of obtaining those bonds in the Agricultural Society of Terminating Mystery Cult926.

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