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Why Stucco Cracks

Unfortunately many of today’s consumers are receiving inaccurate information regarding cracks in stucco. As a result, the mere sight of a hairline crack causes great concern. It is the purpose of this article to define “normal” cracking as opposed to “excessive” cracking.

Stucco, like all cementitious products is susceptible to cracking. However, conventional stucco is a hard and durable system, and in most cases will outlast the structure itself.

Stucco cracking is a natural phenomenon; there is no such thing as a totally crack-free stucco system, although you may have noticed that some of the pre- 1950’s homes you’ve inspected have had fewer cracks. When the older homes were constructed, the studs used were 2X4-inches (not the reduced size now used), and they were kiln-dried. The lumber used came from much larger trees than those that are harvested today. The degree of twisting, bending, bowing, and warping was minimal compared to the distortions occurring in the lumber used today. In addition, the sheathing used for shear value was gypsum rather than expansive plywood.

When stucco cracks it does not imply that the function of the stucco assembly has been degraded. Only when cracks are larger and excessive is there a potential problem.

In 1982 the California Contractors Licenses Board published a set of workmanship guidelines that addressed cracks in stucco finishes. The publication stated: “hairline cracks if not excessively numerous are acceptable. If cracks exceed 3/32” it is unacceptable and should be repaired”.

Portland cement stucco cracks develop for many reasons. Plywood installation is probably the leading cause of cracks in stucco. Cracks caused by plywood are distinguished by the directions they take. They appear much like a large spider crack. They do not always occur at joints in plywood, simply because stresses radiate through the stucco membrane and find relief by forming cracks at inherent weak planes that may be located away from the edges and ends of plywood.

Contrary to long-standing American Plywood Association (APA) recommendations, most plywood sheathing is installed today with ends and edges butted together, when there should be a minimum of 1/8” space between panels at ends and edges. The adhesive used in plywood depends upon a hydrogen bond for its effectiveness. The slightest amount of moisture can adversely affect the hydrogen bond. Since the only adhesive exposure is at the ends and edges of plywood, moisture exposure caused the plywood to delaminate. Without a space between panels to accommodate the swelling, the deflection is outward. It is for this very reason that two layers of building paper over wood based sheathing became a code requirement. The installation of two layers of building paper helps to keep the plywood dry while wet stucco is on the wall.

Another cause of cracking is the absence of adequate moist-curing. Moist-curing is essential. Hydration cannot occur to an optimum level, developing good stucco strength, without sufficient moisture. Without moist-curing, the strength of the stucco suffers.

The thickness of the stucco also causes cracks. Code requires 7/8” minimum thickness. Many applications are ¾ inch or even 5/8 inch. Tests show that an extra 1/8’ thickness requires twice the impact to cause cracking. Lath laps can also cause cracking when lath and paper is placed over lath and paper of the preceding courses.

Lath laps create planes of weakness where stresses find relief by forming cracks or breaks at the lap locations. In addition, expansive soil when wet or dry can move a structure enough to cause cracks. Usually these cracks appear at the corners of doors and windows, which are the weak areas in a structure. These types of cracks are considered normal. Expansion and contraction due to changes of temperature can also lead to cracks, because wood frame walls expand and contract at a different rate than stucco.

An easily identifiable crack is a seismic-induced crack, a result of a major earthquake, as opposed to ordinary low reading disturbances. The crack will be clean and uniform as a result of a jolt to the framing to respond, even if only momentarily.

Every stucco crack has its own pattern, and the nature of cracking discloses the probably reason for its development. Examining cracks in stucco requires an intimate knowledge of stucco in order to correctly evaluate the reasons for the cracking, and to provide your client with a fair market cost for repairs.

Ed is Forensic Stucco Consultant with a Ed Daher Plastering and CREIA Affiliate Member in Vista, California . Ed can be reached at 760-727-3390 or eddaher@cox.net.

8 Responses so far.

  1. Nellie says:

    What about water intrusion at window perimeters, mullion, alarm plugs, mitre, flashing, etc. When water wets the substrate it swells up and since the Stucco system is ridgit, it causes cracking in Stucco – Stucco giving into the push from inside by swolen susbstrate.

  2. Barry Neal says:

    I’ve been trying to determine if “normal” cracking will continue with time or will cracking tend to slow down post construction phase? Let’s assume the home experiences a slowing or no further settling.
    Thank you.

  3. Bob says:

    We have hairline cracking on a new 3 coat system, We waited several weeks between coats for curing, Seems to be getting worse but cracks are less than 1/32″. Question is what caused it, will it slow down and stop, too many to score and patch, can we use a heavy coat of polymer primer to fill in cracks then wait and do a final coat of paint? The house is new construction in moderate climate in SF area. Stucco contractor did not hydrate at all between coats. Was done by a licensed stucco co. could this be because framer did not get enough expansion gaps between sheathing edges?

    • john macchiavello says:

      absolutely bob, that licensed stucco contractor should have seen and or recognized the sheathing expansion. As a licensed stucco guy I look at everything prior to application. I also hydrate the hard coat system as the job progresses to help cure the brown coat and cement. These are the steps to prevent cracking of final acrylic base stucco. you have to respect all flashing, expansion joints, and caulking around all surrounds. With all this said and done you may get a few hairlines here and there but that’s the nature of the beast.

    • Fred Herschbach says:

      I have a three year old home. They patched it up a year and a half ago. Didn’t fix the problem. They tell me that it’s normal because the styrofoam they put on the house expands and contracts and therefore it created cracks that are normal.

  4. Ron Helmick says:

    Hi Ed-

    I read with interest your stucco crack article. Im doing an art project and want to encourage interesting cracking on probably 4′ x 8′ panels.

    I made a list of the causes you mention. Do you have a quick thought of what to do to encourage interesting crack patterns etc.

    Cheers !


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  6. Thanks for letting me know that only an experienced stucco repairman can tell me what exactly is the cause of the cracks since they have the experience and knowledge to tell at a glance. My sister just bought a house but failed to get an inspection beforehand and only now noticed that there were a lot of cracks around the building. I’ll make sure to tell her to call a stucco repair service to get her house checked out as they can give her a proper diagnosis and solution to her house’s problems.

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