What is Stucco?
Stucco. That’s a good name for a building material with attitude. Of course, there’s more to stucco than attitude. There’s plenty of substance, too.
As with any building system, though, failures happen. Often those failures occur when the system is improperly installed.
Because stucco has so many variables, there’s plenty of opportunity for mistakes to occur. Siding, by contrast, is fairly straight forward: Cut the board and nail it in place. That will last for a while.
Stucco is more complicated. If you mix it or apply it wrong, or use the wrong ingredients, it will crack and fall off, depriving the structure underneath of protection.
That’s why remodelers who hire stucco technicians should be familiar with how stucco works and what techniques can be used to improve its performance — especially under harsh conditions.
The Basics
It isn’t easy being stucco. Stucco needs to resist the pressures of weather — particularly extremes like freezing and thawing — and withstand the dreaded “sulfate attacks.”
These occur in areas where soil has a high sulfate content. The sulfate gets absorbed into the plaster and causes expansion, which results in cracking.
Not only must stucco prevent these problems, it also must maintain certain properties during application. It has to adhere to the substrate. It must hold together — called cohesion — and be smooth and workable.
These are all tall orders, but proper mixing of the materials that make up stucco will allow it to meet all of these challenges.
Stucco is composed primarily of cement, water and sand. It’s often referred to as plaster because that, basically, is what it is.
But there are many different kinds of cement and a whole batch of additives that can affect stucco’s performance.
Let’s start with cement-the main ingredient in stucco. There is Portland cement, white Portland cement, blended cement, masonry cement and plastic cement.
The difference between normal Portland cement and white Portland cement is that the latter makes — surprise — white finish-coat plaster. These two cements are further broken into Type I, IA, II, IIA, III, IIIA and V. Whew! It’s simpler than it sounds, though.
Type I is basic everyday Portland cement. All-purpose stuff.
Type II is blended so that it resists sulfate attack.
Type III sets faster so it works better in cold weather.
And Type V is the most resistant of all types to sulfate attack.
The “A” designation indicates that the cement has an air-entraining agent. This means small air bubbles will blend into the plaster, making it easier to work with and frost-resistant.
Think of Portland cement as the single-malt scotch of the cement industry. Blended cements are like blended scotches with more than one kind of malt; they contain more than one kind of cement.
Masonry cement is a mixture of Portland or blended cement and plasticizers. Plasticizers improve workability; the most common plasticizer is hydrated lime.
There are a lot of other additives, too, that can improve stucco’s performance. Air-entraining agents, mentioned earlier, are a good example.
There are also accelerators, such as calcium chloride, which help speed curing, and antifreeze compounds.
(The Portland Cement Association recommends using Type III cement and heated water rather than antifreeze compounds. If you choose this method, mix and apply the stucco in an enclosed area.
The PCA also recommends that you avoid water-repellent mixtures. The association asserts that properly mixed and applied stucco is already water repellent.)
The final two ingredients — water and sand — need little explanation. The water should be clean. The sand should be fine and clean.
Mixing It Up
Getting all these elements into one cohesive, adherent whole can be difficult. The proper procedure should follow these steps.
Place most of the water in the mixer.
Put in about half the sand.
Stir in the cement.
Put in the rest of the sand.
After it’s all mixed evenly, add the rest of the water until the plaster is the right texture. It should be smooth enough to spread but stiff enough to stay on the wall.
The exact proportions depend on the coat. Scratch-coat (the first coat) stucco should have 21/2 to 4 parts sand to 1 part cement.
Brown-coat (the second coat) stucco should contain between 3 and 5 parts sand to 1 part cement. If you have added lime as a plasticizer to improve workability, add no more than 3/4 part lime to 1 part cement. The sand should be in proportion to the total volume of cement and lime.
For finish-coat stucco, add less sand. The proper amount is between 11/2 and 3 parts sand to 1 part cement. Finish coats also require more lime, if added. The best mixes have between 3/4 and 11/2 parts lime to 1 part cement.
The Substrate
Stucco works well on almost any wall surface, from glazed tile to concrete block. It also works on framed walls.
The more solid the base, the fewer coats you need. Concrete block walls, for example, only require two coats-a scratch coat and a finish coat.
Apply the scratch coat directly to the concrete wall, making sure the mortar joints have been struck smooth.
Frame walls need three coats: scratch, brown and finish. But that’s not all.
They also need two layers of building paper over the top of the sheathing, with wire lath nailed on top of that. The building paper should be Grade D water-vapor-permeable paper. That lets the vapor through, but won’t allow water to penetrate.
The wire lath is available in three styles: expanded metal (which is usually diamond shaped), woven wire and welded wire.
Wire lath should be used on all frame walls and anywhere where the surface is unsound, such as crumbling brick. The lath provides keys for the plaster.
application
Stucco plaster can be applied by hand or sprayed on. No matter which method you use, make sure the stucco completely covers any wire lath. That will provide a good bond and keep the wire lath from corroding.
The two base coats should be applied as quickly as possible. Still, you have to wait at least a day after applying the scratch coat before you spread on the brown coat. If the scratch coat isn’t rigid enough, it will fail under the weight of the new coat.
On large walls, make sure to include control joints to relieve stress on the wall. Cracks develop for a lot of reasons, including building movement, drying shrinkage and weakened sections of wall.
Place the control joints near areas where building structures penetrate the stucco, such as at wall intersections, where beams rest or where vents protrude.
One of the most common spots for cracks to form is where two different types of base meet. Always put a control joint where a concrete wall meets a framed wall.
For flat, straight, plain walls, the recommended maximum area without a control joint is 144 square feet, or 12 feet by 12 feet.
After the stucco has been applied, it needs to cure. To make sure it achieves its full strength, keep the stucco moist for the first few days. This is usually done only with the base coat, not with the finish coat.
Stucco walls in hot, dry environments should be covered with plastic to hold in the moisture. As with any cementitious application, drying too fast can reduce the material’s strength.
You can texture the finish coat in any number of ways, many of them quite artistic. That’s part of what gives stucco its attitude.