As previously mentioned in the January issue of Walls & Ceilings, wall and ceiling contractors know the frustration of hairline cracks that are inherent with a stucco system. The January 2007 article on stucco cracks dealt with myths, planning, caveats and suggested contract language when working with such systems. This article will focus on the investigation of cracks and some suggested repairs.

The set up

So, you have installed a cement or “hard coat” stucco system. You have followed the industry standards, used the January W&C article as a guide to minimize cracks, but the stucco job still cracked. Before you started, everyone involved, including the building owner, was cautioned that stucco cracks are a reality. At the end of the day, however, you got the phone call that the owner is not happy with the few hairline cracks that have developed.

This certainly could not happen, could it? After all, everyone was advised, informed and acknowledged that this was a possibility. You followed all the guidelines and the cracks are truly hairline and there are only a few of them at that. It does happen and it will happen again. I have been on site with homeowners who were upset about very fine hairline cracks, hardly visible from 6 feet away, that were emanating from the apex of a window corner. They are certain that the fault is the plastering contractor and/or the materials he used.

On more than a few occasions, I have walked projects with building owners to view cracks in stucco. They claim they understand that a hairline crack is possible and accept the idea of an occasional hairline crack in stucco, but when we walk their building, they scan the wall surface from a few inches away and find the common “reentrant crack” unacceptable.

What kind of contractor are you?

As an ex-plastering contractor, I get upset with the waste of time some of these building owners burden us with. Their unreasonable demands, perfectionist attitude and sometimes extreme personal wealth lead them to believe they are always right and will always get things their way. This is particularly true of owners who have used quality plastering contractors who are well-read, who educated themselves on their craft and who put their best foot forward on a project.

As an ex-technical consultant for the plastering industry, though, I also get upset with the lack of training and education so many plastering contractors seem to have about minimizing cracks or how to speak about cracks in stucco from an educated point of view. These contractors tend to rely on bravado and intimidation to make a case or defend poor practices.

You have to decide which category you fall into as a plastering contractor. Are you the type who wants to keep learning and who documents his work and training? If so, you should be on solid ground when defending yourself. If, on the other hand, you are the type of contractor who chooses not to educate yourself and instead opt to rely only on past knowledge and tough talk, I wish you luck should you ever end up in court.

Investigation of cracks

A crack in cement plaster stucco is stress relief. The tricky part is to identify the source of the stress when investigating a crack on a stucco project. Finding the source of the stress can be made easier if you place all stresses into one of two categories: stress within the plaster membrane and outside stress placed upon the plaster membrane. This may be a bit simplistic, but it can help with a daunting task.

All buildings are subjected to a variety of stresses in various forms and this stress is often transferred to the plaster stucco membrane. Stucco is not and never was intended to be a structural material. While Portland cement stucco is similar to concrete in that they both have similar ingredients, stucco cannot be compared and required to meet the ASTM testing standards of structural concrete or load bearing masonry. Cement stucco is a relatively thin, brittle shell over a framed structure and can crack if the stresses are transferred and overcome the strength of the plaster membrane. This statement is not meant to condemn stucco; stucco has so many outstanding qualities it is impossible to beat.

Shrinkage stress

The first source of stress from within the plaster itself is considered shrinkage stress. All Portland cement shrinks as it cures. Controlling and minimizing that shrinkage is critical to minimizing cracks in both categories of stress. Some common mistakes made that add to shrinkage stress are:

• Poor mix ratios. Cement shrinks and sand does not. Some plasterers like a cement-rich mix because it works easier. Unfortunately, it also shrinks more. Sand volumes must stay up to minimize shrinkage.

• Lack of curing. Cement needs to set and harden. Excessive or fast evaporation of water from the cement plaster will cause shrinkage cracks. This is particularly true in the first day or two of application and with warm windy conditions. This can also happen to cement stucco applied over absorbent masonry-type surfaces.

• Poor sand quality. Sand with excessive fine materials, clay or too much dirt can all lead to volume loss of material (shrinkage stress). The clay absorbs excessive moisture and in turns shrinks more. Sand should have good gradation (variety of sizes) and be angular in shape. Example, round marbles don’t grip as well as triangular shaped particles.

• Excessive thickness. Since all Portland cement shrinks, trying to add a full thickness in one pass will also tend to shrink more and lead to shrinkage cracking. The old-timers referred to this as “single-footing.” It should be noted that there are some proprietary products with special additives that have had success in applying stucco in thicker applications with minimal shrinkage. These products do work and come with a price tag to match.

Cracks caused from any of these conditions are typically one-time events of stress. Shrinkage cracking tends to have a map-like cracking pattern. The bottom of a dried-out mud pond baked by the sun is an extreme example of this type of cracking. The fast dehydration and volume loss in material overcomes the strength of the mud paste. In cement plaster, the strength is gained over the first few days after application. Extreme hydration loss can result in random map shrinkage and may even have the curled edges of the mud pond. The result is a fractured wall membrane. These are the cracks caused from internal shrinking stress. I have seen this on stucco jobs and have had the contractor try to tell me it is normal and expected and that, “the finish coat will hide it.” The problem with a finish coat over this type of base coat is that the cracks can re-appear because the membrane is too weak with no continuity of strength. Please note that a few hairline shrinkage cracks are normal and should always be expected; they tend to show up about one to two weeks after the brown coat is applied. Excessive shrinkage cracking is not acceptable and hiding it with a finish coat only delays the inevitable.

Outside sources

The second type of stress is from an outside source that overcomes the plaster membrane. Structural loads, lateral force racking, thermal changes, wind, lumber shrinkage and seismic events are all examples of outside sources of stress. These types of stress relief cracks tend to be longer and reach from one architectural break to another. Pinpointing the source of the outside stress is the most difficult of tasks.

More often than not you are making a best guess when asked to identify the outside stress source for the crack. This is often frustrating to building owners who expect a clear-cut answer and who want someone to blame. Even with an experienced team of investigators, the answers may not satisfy the owner. On a recent investigation, a homeowner hired the best engineers money could buy to investigate the cause of a few hairline cracks in his stucco. Since he is very wealthy, money was not an object, and he was not pleased when the team could only speculate as to the cause of the stucco cracks.

The point of the above story is that it is OK for engineers to make their best guess and announce it as such, “a best guess.” Too often, the investigation becomes something like a 1930s homicide investigation and someone, guilty or not, has to go to jail. The owner wants someone to blame and some investigators are willing to tell building owners, the same building owners who pay them, whatever they want to hear. This can lead to unfair accusations and inequitable penalties for some contractors, designers and/or material suppliers.

Binocular vision during the investigation

There are no criteria or requirements for compressive strength in a Portland cement stucco membrane. Unlike concrete, there are no structural requirements for a stucco system. I am amazed at how many investigations use what I term “binocular vision.” The first and only priority is to get a sample of the stucco sent to the lab for tests. The test results may show some inconclusive evidence about the full hydration, that the aggregate may or may not have other impurities and the conclusion is set, the mix was bad. Based on this questionable test data, all kinds of assumptions are made and no one bothers to look at other possible and more likely problems. One good example was a stucco job I investigated where the team of forensic experts could not get the sample to the lab quick enough. I showed up and noticed the staples used to attach the lath were 7/8 inches long, which would not be a problem if there was not a layer of 5/8-inch gypsum sheathing on the wall. The problem is that only 1/4-inch holding power into the wood stud is not sufficient for stucco weighing up to 13 pounds per square foot.

The stucco membrane should be hard and dense, not soft and weak. The best test is a pocketknife or screwdriver. Is it hard and dense or is it soft and friable? If it is soft, I am concerned; if it is hard and tends to dull the knife, I tend to focus my attention elsewhere.

The other common binocular investigation mistake is the thickness of the stucco membrane. The measured thickness is also often questioned and almost always misinterpreted. The investigator notices the stucco is not uniform in thickness and is actually wavy on the backside with shallow channels. They measure the thinnest point and claim it does not meet code minimums and believe they have found the problem. Stucco over a building paper is supposed to have waves formed by the paper wrinkling when the plaster was applied. These shallow waves are the intended “weep plane” for the system. No plasterer can control the paper wrinkles or even how much it wrinkles. What he can control is the intended thickness by applying the proper grounds to the stucco and “rodding” or “screeding” the wall to a reasonably flat plane. The codes confirm this fact by using the term “nominal” thickness of plaster.

Try seeing the big picture

A good technique for crack investigation on stucco is to think big picture rather than binocular vision. Stand across the street and visualize the stresses being imposed upon the building. This approach, combined with the understanding that the underlying structural framing, often can lead to an epiphany as to why the stucco is cracked. Visualize the building in high wind or a seismic condition. On one occasion, I noticed some very large machines on the roof of a health lab. I asked when the equipment was set on the roof. It turned out it was set on the roof about two days before they started noticing the cracks in the stucco.

I am not aware of any official standard or guide to determine the acceptable lineal feet of crack per square feet of wall surface for stucco. I would first question this method as appropriate for the site. For example, a large wall surface with no cracks except for one short wide crack at a critical location would be unacceptable to me and should be repaired. Certainly a 3-foot crack on 500 square feet would meet even the toughest standards. On the other hand, if I had a series of small windows along a ribbon of stucco, these reentrant hairline cracks are what I would term as normal and expected. Certainly, the 50 feet of lineal crack on 200 square feet of wall space is beyond the most liberal of standards. The point is, every wall or ceiling and every crack has to be evaluated and investigated on its own merits.

How harmful is a hairline crack?

First and foremost, building owners should understand that a hairline crack does not automatically mean the stucco is faulty, failing or will not function for the service life of the building. A hairline crack in stucco also does not mean the stucco system will automatically leak. In fact, independent testing on several stucco cement walls has demonstrated that tight “hairline” cracks rarely leak enough to cause a problem to the system.

As unbelievable as it sounds, Portland cement has a self-healing quality called “autogenous healing.” This is somewhat common knowledge with people who build concrete tunnels, water tanks and dams. Autogenous healing also explains why stucco over masonry has proven to be an excellent cladding for centuries in Europe.

It should put homeowners and designers at ease that a hairline crack is not necessarily a potential water leak. In addition, consider that there is a secondary weather-resistive barrier, hopefully two layers, behind the stucco on framed and sheathed construction. With both facts, there is little reason one should be overly concerned about a hairline crack in stucco and water intrusion. It has to be noted that the autogenous healing process will not seal wide cracks. Although not a scientific method, a crack as wide or wider than a standard business card is probably too wide to count on autogenous healing.

Making repairs

Once the decision is made to make repairs, there are several options. The aesthetic results can range as widely as the repair. Since most hairline cracks are not harmful to the stucco system, the decision to make repairs should not be made lightly. Crack repair in stucco can be simple or costly. Basically, you tend to get what you pay for. You should make some decisions about the cracks prior to repair. Are they shrinkage or structurally related? Are they excessive? Are they a danger of water intrusion? Are they truly objectionable? Is the finish coat a cement or an acrylic? Are they dynamic or static? Dynamic cracks expand and contract, static cracks remain the same size. There are simple crack movement indicators that can be used to determine if they are dynamic.

The following options range in order from least to most expensive. These methods are not intended for wide structural cracks. Severe structural cracks wide enough to place a quarter into the crack and that are dynamic in nature, should have an expansion joint cut into the stucco to allow for stress relief.

Option 1: Treat the joint with the same finish material. This is typically done with a semi-stiff bristle brush and dabbing it over the crack and trying to blend it to match the existing finish. This method is the least expensive and may look worse than the crack itself. Also, the crack will often return.

Option 2: Paint the wall or ceiling after treating the crack with an acrylic paint or paint designed for stucco. A caveat is that the patched crack may flash through the paint. Even when the patch material is the same as the original and the patch is very well done, the cracks can flash through during harsh side lighting.

Option 3: An elastomeric paint coating. These coatings stretch and flex with the crack and have proven to work well in covering cracks. Elastomeric coatings will change the surface texture of the wall and tend to have a matted look when applied to a mil thickness that will perform as intended. Vapor permeability is another concern. Elastomeric coatings can bubble and blister if moisture gets trapped under the coating. The final concern is future recoating. Most elastomerics have limited options for recoating.

Option 4: Recoat the wall from architectural break to architectural break with the same finish material. This method looks very good and works well. The caveat is that if the crack is dynamic, the crack will return.

Option 5: A new basecoat and mesh over the wall area. This method requires troweling a fiberglass mesh into a polymer-enriched basecoat that is manufactured by one of the EIFS or stucco manufacturers. This provides a strong, flexible lamina over the surface of the cracked plaster. A crack rarely comes back through this system. The new lamina is vapor permeable and can be finished with an acrylic finish coat. Check with the basecoat manufacturer as to the appropriate basecoat to use over the existing surface. This method is excellent for highly visible areas like entryways, but it is not inexpensive. While no method can guarantee a crack will not return, this method has proven successful on all but the most severe structural cracks.

A final word of advice

In dealing with unhappy building owners and a stucco job that may have experienced a little more than what might be considered “typical” cracking, I try to get owners to accept the cracks on less visible walls and have the contractor basecoat and mesh only the most visible walls. This requires a little give and take from each side – the owner accepting a few hairline cracks and the contractor understanding that no one likes to open the door to welcome guests looking eye level at conspicuous cracks.

I have suggested that architects specify the entry walls by the front doors on expensive homes receive the basecoat and mesh over the brown coat prior to finishing to ensure no cracking. This tends to be the area owners notice first.