A Closer Look at Stucco (Journal of Light Construction)

Three-Coat Stucco

Conventional or traditional stucco is called three-coat stucco because it has a 3/8-inch scratch coat, a second 3/8-inch brown coat, and a thin “color coat” on top, for a total system thickness of about an inch. All three coats are mixed from Portland cement, sand water, and some lime for workability; the top coat has color powder and may include some polymer additives.

But the system starts with a drainage plane based on some type of building paper over the wood framing of the home. Building codes call for two layers of Grade D kraft paper, which is made with virgin wood fibers. The paper is there to drain water, so it has to be carefully tied into flashings around all windows and doors. Metal flashing systems are also installed to divert roof water away from the stucco system, and to protect any penetrations. The paper and flashings have to overlap each other in a way that creates a shingle effect.

Over the papers and flashings, a stucco netting or metal lath is fastened to the wall with staples. Stucco netting looks like chicken wire, but it is actually a heavier-gauge galvanized steel wire. Expanded metal lath has the look of a heavier grating, but it serves essentially the same purpose.

Next comes the base coat, troweled into the lath mesh and tooled with grooves while wet, to provide keys for the second coat to lock into. The 3/8-inch-thick second coat is applied and tooled flat, and then both must cure for 7 days before the color coat gets troweled on. Like all cement, stucco will shrink and crack; many traditional contractors will wait 14 days to make sure the first coats have completely “cracked out,” so new cracks won’t telegraph through the top coat.

Three-coat stucco is designed to be porous. Rain soaks into it, then drains out when the storm ends. The papers and flashings are vital to protect the house — without them, water will soak the wood and create conditions for rot.

One-Coat Stucco

Since the mid-1980s, a handful of manufacturers have introduced thin coat stucco products that collectively are called “one-coat” (or sometimes “two-coat”) stucco. One-coat is nearly identical to conventional stucco in concept and design, except that the base coat is applied in one layer instead of the original two-step scratch- and brown-coat process. The base coat is mostly sand and Portland cement, as in conventional stucco, but it also includes synthetic polymers and fiberglass reinforcing strands that increase both the tensile and the compressive strength. The required total thickness is just 3/8 inch, instead of the standard 3/4-inch total for the three-coat base.

The idea behind one-coat systems was to save labor and time in the schedule. With the added components, base coat could be put on in just one layer, with no second plastering and no wait in between.

In practice, I’m not sure one-coat is all that economical. The proprietary mix ingredients add cost, and finding and following the special instructions for the proprietary systems add complexity. One experienced stucco contractor, a friend whose work I respect, told me that he gave up working with one-coat because it was too complicated. His crews rebelled against the required special detailing, and he also found that with only 3/8 inch of thickness, it was harder to achieve a nice, uniform finish over the usual irregularities in a house frame. (A common defect I see in one-coat installations is a base coat much thinner than the required 3/8 inch, at least in spots.)

The other big selling point for thincoat systems is that the fiberglass and polymer additives help the stucco withstand the winter freeze-thaw cycle.

The thinner base coat is still applied over wire lath or expanded metal, and over a system of papers and flashing the same as we need for conventional stucco. The same screeds and expansion joints are also part of this system, although at different thicknesses. But unlike three-coat stucco, one-coat systems require a 48-hour moist cure. The applicator is responsible for keeping the base coat moist for the first 48 hours after application. The color finish is also required to go on within 72 hours of the base coat application. Proper curing is more critical with one-coat than with traditional stucco, because the acrylics tend to isolate cement particles from water in the mix. If the coat isn’t kept moist, it may dry out before the cement has a chance to react with water (hydrate), which it must do to form the strong cement compounds that give the cladding its strength. Without the correct moist cure, the base coat is likely to be weak and crumbly.

One-coat stucco usually receives one of the new acrylic color finishes, instead of traditional stucco’s purely cementitious, textured color coat. It has a smoother and less porous look, because acrylics instead of cement bind the aggregates together — it’s like sand mixed with latex paint. Many people perceive this acrylic top coat as the defining characteristic of one-coat stucco, but synthetic finishes are not really an essential component of a one-coat system — they just happened to be developed about the same time that one-coat was widely marketed. One-coat base-coat systems got code approval in ER reports without mention of any particular color finish. As long as the base coat is applied at least 3/8 inch thick, you can paint it or apply either a conventional cement color finish or a synthetic acrylic color finish over it.

An acrylic coating’s higher plasticity gives more resistance to cracking and creates a more closed, water- and stain-resistant surface. But one-coat stucco finishes are still porous enough to let rain enter the system — the perception that one-coat systems reliably repel water at the surface is incorrect. And even if the coatings were waterproof, one-coat systems do crack, and they can leak at all the joints and penetrations, so water is sure to get behind them. At the same time, they are less breathable and slower to dry out than conventional stucco. So they are less forgiving of any defect in the proper placement of building papers, flashing, and lathing staples — if water reaches the wood structure of the house, it is less able to escape by evaporation.

I’ve seen many failed stucco systems that someone has tried to repair by applying a thick polymer coat over the existing stucco, and by surfacecaulking window and other joints. This is worse than useless — it actually accelerates the damage. Water will still enter the system somewhere, and then it’s trapped next to the house. My company’s educational video, available from our website at www.rambuilders.com, shows an example of a home just four years old, whose framing and sheathing is completely gone because of that kind of attempted “repair.” Damage that might have taken 10 or 20 years to develop under normal, breathable stucco happened in 1 or 2 years after the sealer was applied.

To learn more about stucco, you can consult the full version of this document at the source: Dennis McCoy