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Three-Coat Stucco, continued Note that lath must be installed with the long dimension perpendicular to the supports. Also, metal mesh must be installed with the right side up, or the stucco will slide off the building. At corners, make sure that the lath is not installed too tightly or else it will not be embedded sufficiently in the scratch coat and the normal expansion and contraction of the building due to temperature changes will pop the stucco off. To prevent this, we "pooch out" the lath at corners, pulling it away from the building so there will be room for the stucco behind it. Alternately, you can apply cornerite, an expanded metal lath designed for this purpose (Figure 3).

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Figure 3. This house is ready for stucco. Special care is taken to ensure the lath is not pulled too tightly at corners. If it is, the stucco will pop off over time. A lath accessory called cornerite (left) can also be used at corners to ensure proper embedment.

The Mix

Although it comprises only about 20% of the finished product, Portland cement is what makes stucco stucco. Most cements are of good quality these days, so it really doesn't matter which brand you use. Watch out for older bags of cement, because humidity can partially react with the cement over time even if the bags have never been opened. Some cement will have plasticizers added. If you use a plastic cement (as I do), do not add lime or other additives. Cement is typically packaged in 94-pound sacks, which equals one cubic foot. This is helpful when you are trying to figure out proportions.

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Figure 4. Clean sand, dry cement, and the correct amount of water are key to a good stucco mix.

Sand. Stucco is mostly sand, so you should use clean, sharp, good-quality sand (Figure 4). Good sand is the key to dense base coats, with either traditional three-coat or the one-coat method. Ask for and get verification that your subcontractor is using sand that conforms to ASTM 897 for gradation and has a minimum SE (sand equivalent) rating of 70. (SE is a designation for the amount of fines in the sand.) If you want to check the sand yourself, take a random sampling from various areas of the sand pile. A glass jar test will quickly tell you if the sand is clean. If it's dirty, you'll see silt or other contaminants; more than a very slight amount of contaminants is cause for concern. Soluble salts or excessive fines are also harmful to the stucco. Salts can result in efflorescence, while clay fines may expand when wet, absorbing extra water, and causing the stucco to be weaker when it dries. If you have any doubts about the quality of the sand the stucco contractor is using, I suggest setting aside a bag for future evidence, just in case there are problems. Water. Clean, potable water is the rule. It takes only 2.8 gallons of water to fully hydrate a sack of cement, but it takes about 6 gallons per sack to make a workable mix when using good-quality sand. Poor-quality sand may require more water, and you might use up to 10.5 gallons of water per sack if you have added lime to the mix.

Application

With proper substrate, preparation, and materials, you are more than halfway to a good stucco job. Now it's up to the applicators to mix the stucco and apply it correctly.

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Figure 5. The author's crews pump brown and scratch coats (left). Finish coats (right) are hand-troweled.

Almost all of our base coat stucco is applied with a plaster pump (Figure 5). The plastering pump is temperamental: The mix has to be blended properly -- otherwise it will not slide well through the hose -- and the equipment must be in good working order. Although pumped plaster is generally a slightly wetter mix, it still results in a perfectly acceptable job. Hand application with a trowel is simpler, but it actually requires more skill. Given the proper mix, an experienced plasterer with a trowel can provide a stronger, denser, more carefully applied job, but this is not always the case.