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Stucco window sill leaks (C) Daniel Friedman Thin-Coat Stucco: 1-Coat & 2-Coat Stucco Exterior Wall Specifications

  • POST a QUESTION or COMMENT about the installation & specifications for one-coat or two -coat thin stucco applications on exterior building walls.

Thin coat stucco methods:

This article discusses the specifications for thin-coat stucco building exterior wall installation.

This article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains.

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Thin-Coat Stucco Specifications

Thin coat stucco installation (C) Daniel Friedman

This article series includes excerpts or adaptations from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) , by Steven Bliss, courtesy of Wiley & Sons.

[Click to enlarge any image]

In an effort to speed up stucco application time and simplify the process, several manufacturers have introduced proprietary thin-coat stucco systems variously referred to as one-coat, two-coat, thin-coat, or fiberglass-reinforced stucco.

Our photo of a thin-coat stucco wall being constructed (left) was at a Barnes and Noble bookstore in Poughkeepsie, NY.

All these systems apply a single base coat and a top coat with a total thickness of 2/8 to 1/2-inch, compared to  7/8 to 1 inch for traditional three-coat stucco. The thinner finish weighs from 5 to 6 pounds per square foot, compared to 9 pounds for three-coat, and it is cost-competitive with traditional stucco.

Like traditional three-coat stucco, thin-coat is applied over wire mesh or expanded metal lath by hand or pump. It is backed up by a waterproof drainage plane consisting of Grade D building paper, integral flashings, and a weep screed along the top of the foundation to drain away any trapped water.

Some manufacturers, such as United States Gypsum, have introduced hybrid systems in which the stucco is applied to a cementitious board rather than to wire mesh. The advantage is that cement board is impervious to moisture. The drainage plane, and in some cases a layer of foam insulation, lies behind the cement board.

Thin Coat Stucco Application Procedure

Stucco wall damage from weed whacker (C) Daniel Friedman

The base coat in thin-coat systems has acrylic polymers and chopped fiberglass added to increase its strength and resistance to shrinkage cracking and to freeze-thaw cycles.

The base coat is premixed with only sand and water added at the job site. Most contractors using these systems apply an elastomeric color coat, similar to a thick acrylic paint with fine aggregate, and formulated to bridge small gaps less than 1/16-inch.

This produces a smoother finish that is more water- and stain-resistant and less prone to cracking than a traditional stucco. The top coat can also be a traditional cement stucco finish.

Most of these systems require a 24- to 48-hour moist cure and a total of six or seven days of curing before the top coat is applied. Some require a primer for acrylic finishes.

Our thin-coat stucco wall damage photo (above-left) shows the vulnerability of this system to damage by common events at or around a building: in this case the use of a weed-whacker to trim growth close to the building wall. This EIFS installation is also installed so close to the ground as to invite insect attack on the structure.

Also see Insects & Foam Insulation.

Pros and Cons of Alternative Stucco Systems

Stucco window sill leaks (C) Daniel Friedman

To their credit, properly applied onecoat systems are more waterproof and less prone to shrinkage cracking than traditional stucco. It is easier to obtain a uniform color and texture with the synthetic color coat than with a traditional cementitious finish coat.

Whether a customer prefers the uniform color of a synthetic finish or the more muted and variable color of cement stucco is a matter of taste.

Our photo of a leaky stucco window sill on a New York home (left) shows a damaged, leaky sill where plastic mesh was used as a modern substitute for expanded metal lath.

On the downside, one-coat systems are less impact resistant than traditional three-coat stucco. And with a thickness of only 3/8-inch, one-coat systems are less able to hide irregularities in the framing and are more likely to have thin spots that are prone to problems.

Also, one-coat stucco systems are not completely waterproof. Over time, water will find its way in at joints, penetrations, or cracks, and the synthetic stucco will be slower to dry out than the more permeable traditional stucco.

Finally, each system is proprietary and must be installed according to the manufacturer’s approved specs and details, which vary from system to system. Otherwise, warranties are voided and code approvals, which are based on building code evaluation reports, are invalid. For both reasons, contractors should avoid mixing and matching components from different thin-coat systems.

Exterior Insulation and Finish Systems Suppliers

- - Adapted with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) .

Stucco Finish System Articles


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