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STUCCO CRACKINGQuestion: What about cracking? Answer: By its very nature stucco is susceptible to cracking. Stucco shrinks slightly as it dries, and it doesn’t take much movement to make a tiny crack at the surface. Over time, especially in houses built on sloped lots, stress movements can occur. These stresses appear as cracks in the coat. This type of cracking is evident, for example, with cracks at the corners of window frames, door jambs, and other openings. There are several reasons that cracks can occur in stucco. Most cracks in the finish coat are aesthetic in nature and do not damage the integrity of the building structure. Some cracks occur due to the settling of the property. This is especially true of new construction. Shrinkage cracks are a type of crack that is more aesthetic in nature unless the cracks are more than 1/32” wide. Shrinkage cracks are caused by different thickness of stucco on the wall, improper stucco curing, application during hot, dry windy weather, or insufficient control joints called ‘expansion joints’. Stud cracks are straight-line vertical fractures, which appear over or near framing studs. These can be caused by warping or twisting of studs, shrinking studs, vibrations from heavy traffic, vibrations from machinery, and severe blows from swinging doors. Cracking can also result from using green or wet lumber for framing. Lumber can twist and bow as it dries, cracking the stucco in the process. Cracks can also be caused by inadequate spaces between plywood sheets to allow for expansion. Hairline cracking almost always occurs in stucco, but this is normal. Please read the article below, it will help you better understand stucco. The nature of
stucco The nature of stuccoIt is the nature of stucco to crack, though it would be nice if everyone who bought a house that has a stucco exterior finish never had to worry about cracks that happen to show up from time to time. But since stucco is a cement product, it is subject to cracking. Cracks can be anything from minor hairline cracks to fairly large cracks. Fortunately, the hairline cracks are the ones that happen most. When your home was built, it had a structural frame that was probably 2x4” wood studs. These studs are covered with paperbacked lath which is a combination of building paper that serves as the weather resistive barrier and a base for plaster. The lath is usually woven wire stucco netting (sometimes erroneously called “chicken wire”). The lath and building paper are fastened to framing members every six inches along the studs and the top and bottom plates. Often sheathing is applied over the framing and then the paperbacked lath is applied. But whether your home is “open frame” with paperbacked lath or sheathed, the next step after lath is installed is application of two or three coats of portland cement plaster. The first coat of portland cement plaster (which most people call stucco) is the scratch coat. It goes on to cover most of the stucco netting. It is scored horizontally and kept moist for a couple of days, unless weather is humid and cloudy, in which case it doesn’t have to be “wet down” as required in warm, dry weather. The scratch coat is applied 3/8-inch thick. After the scratch coat comes the brown coat, which is also 3/8-inch thick. Finally the finish coat is applied approximately 1/8-inch thick. It is usually an integrally colored stucco, or, in some code jurisdictions, a breather type of paint is permitted providing the thickness of the plaster is 7/8-inch. Most plastering contractors strive to install the lath and stucco work so that it is entirely satisfactory and will perform its job of protecting the building without problems. They don’t want to make call-backs and so take their work seriously. Causes of cracking in stuccoWhen cracks occur, it isn’t necessarily the stucco that’s responsible. What takes place in the house – or in any building – is that stresses are generated form many sources like windloading on walls, impact, vibrations, seismic activity, and from framing as it dries and twists, bends, warps, or bows. Stresses – which are always present – are like vibrations radiating through the plaster membrane. These stresses constantly travel through the stucco. When they find a place in the stucco that is not resistant to the force of the stress energy – weakened planes – they cause a break or fissure as they dissipate. These weakened planes, like stresses, always exist to some degree at corners of doors and windows, around penetrations of the stucco, and on “open frame” construction at faces of studs. When cracks appear they don’t look attractive, but this does not mean the stucco will not perform its function or that it is going “sour.” Cracks are objectionable for two reasons: they affect appearance of the house, and, if large enough, can allow water to get through the stucco finish. However, if water gets through cracks and reaches the back plane of stucco, there’s still protection from leaks because the water-resistive barrier (asphalt-saturated building paper) keeps water away from framing by directing it downward to escape to the outside by way of the “foundation weep screed.” A weep screed is a metal device installed before building paper and stucco are applied. Building codes require the weep screed at all exterior stud walls, installed at the foundation plate line, usually four inches above the ground. This is not critical though, since, as long as the water can escape from the wall, the weep screed will function well. But when water can’t escape, it can dam up and reach the top of the first course of building paper, and possibly enter stud cavities to appear inside the building as a leak. Sometimes windows are not flashed properly and this can be a source of water intrusion. When water appears at window jambs and sills it isn’t necessarily the fault of the windows. Often, when flashing above is faulty or missing, wind-driven rain enters and travels downward being halted at the window head where it flows over and around window flashing to create the appearance of water entering at windows. The most common crack in stuccoThe most common crack in stucco is the one that forms at corners of doors and windows. It is virtually impossible to prevent this kind of cracking since stresses at corners are far more significant than at other locations and stucco responds to those stresses by exhibiting cracks where stresses find relief and dissipate. Cracks of this kind are not an indication of substandard plaster work. They can be repaired but may recur. It would be an arduous task to list all the things that can cause stucco cracks. No matter how good the stucco installation, plaster is always susceptible to cracking the same as any portland cement based product. Sidewalks and driveways, for example, are portland cement based products, and they develop cracks, often severe. These components are much thicker than a stucco membrane, yet they exhibit the nature of portland cement plaster. It is like the story of the scorpion that offered to carry a frog across a river on its back. The frog said it was afraid the scorpion would bite it. The scorpion assured the frog it would not be bitten. Half way across the river, the scorpion bit the frog. The frog, astonished at the behavior of the scorpion, asked why the scorpion had bit it when the scorpion had promised not to. The scorpion replied: “Because I am a scorpion.” This is the way with stucco. Its very nature promotes development of cracks, although many stucco applications have few, if any cracks, while others have what can be termed excessive cracks. Just what is “portland cement plaster (stucco)”?Stucco is a blend of portland cement, sand, and water. Industry standards and building codes specify quantities of each ingredient to be used for each of the three coats of plaster (stucco). Have you ever wondered where the words “portland cement” originated? Many people have an idea that “portland” refers to some company that makes “portland cement.” Or that “Portland” refers to some city where the first cement plant started. Actually, portland cement is more than 150 years old. It was invented in Leeds, England, by a young bricklayer named Joseph Aspdin. He worked with huge stones on buildings constructed in England, and wondered why some kind of exterior surfacing couldn’t be used to match the stone in use, and save money at the same time. Experimenting with different things, one day he discovered that by combining limestone with argillaceous clay, he had an hydraulic cement – that is, a cement which was resistant to deterioration from exposure to water. And, it looked like the stone that was so popular for construction then. Aspdin got a patent for his new product. Since the stone he had worked with so much was the inspiration for his discovery, he named the new product in honor of the stone. That stone, so popular then, was called “portland stone.” Thus, the name “portland cement” originated. Every cement manufacturer produces portland cement. The name “portland” identifies it as an hydraulic cement; a cement that is absolutely not affected by exposure to water. So when a new house or building has a stucco finish, it has a finish that is waterproof – that is, it is not deteriorated by any volume of water to which it may be exposed. As a matter of fact, the more water to which it is exposed, the more durable it becomes. After many years service, the stucco is stronger than ever and performs for the life of the building. Little maintenance is required with stucco. When portland cement and water are mixed, a chemical process called “hydration” begins. This process, which continues for many years, is what hardens the stucco and develops its ultimate strength. When first applied, stucco is not as strong as it will be later. At the same time, however, stresses generated in a new building are greater during the same early period. So it is often a case of strong stresses overcoming the lower strength of the new plaster membrane. That’s why most cracking occurs during the first few months after stucco is applied. Later, after it has hydrated and developed greater strength, cracking is not as frequent since the stucco can now better accommodate the stresses that are generated because stress generation also subsides as framing components “find themselves.” Contrary to common belief and assertion, properly mixed, properly applied, moist-cured stucco will not permit passage of water. There is no such thing as “saturated stucco” when it is mixed and applied properly. Our experience with stucco crackingAfter being in the stucco business for over twenty years we have experienced many different cracking situations. In doing tract homes we noticed similar cracking on the same walls of numerous homes. We had to ask the question “why does one wall consistently crack on a certain model of home while all other walls on the house have relatively few cracks?” This told us that there had to be a structural situation. The answer is that these walls are under more stress than other walls. We have found less cracking when houses are framed with 2x6” studs instead of 2x4” studs. We have found that houses that have been sitting framed for a long period of time before stucco is applied have fewer cracks. We have found that most houses will settle over time, creating stresses on the concrete slab, the framing and the stucco. Many times we see cracks showing up two years later after a very wet winter. When inspecting the job site, we notice a lot of cracks in the sidewalks, driveways, etc. When we did an internet search on stucco cracks, the search engine came up with over 20,000 sites regarding stucco cracks. This problem has existed since the creation on portland cement plaster (stucco) and will continue. To date nobody has found an infallible cure for this situation, but it is my belief that it all starts from the foundation up. Good soil compaction, steel reinforced foundations, quality framing with dry lumber, proper sheathing spacing, better engineering, etc. will result in less structural cracking. Also, the use of expansion and control joints in the stucco is necessary. Many people do not want to pay extra for them, or do not like the looks of them, and therefore they are not installed. It is my opinion that they should be installed on every stucco job. Here are some links to websites having to do with stucco cracks: DeHaven Construction Company Stucco Manufacturers Association Stucco News El Rey Building Analysts |
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