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.