Builder's Guide to Stucco Lath & Plaster

Contents

1. Stucco Basics, 1

What is Stucco?, 1
The "Foundation", 2
Base/ Subsurface, 2
Plaster Coats, 4
Stucco Application Checklist, 6

2. Base/Substrate Preparation and Flashing, 9
Importance of the Base, 9
Are You Ready?, 10
Masonry/Concrete Construction, 11
Wood/Metal Frame Construction, 18
Flashing, 22
"Accessories" Are Not Optional, 28
Galvanized vs. Zinc, 31
Metals vs. Plastics, 31
Seals and Caulking, 31
Control Joints, 32
Beads, 36
Grounds / Base Screeds / Weep Screeds, 36
Integration of Accessories, Lath and Barrier, 37
The Weather-Resistant Barrier, 43
Felt, Kraft Paper or House Wrap?, 43
Installing the Barrier, 46
Lath, 49
Metal Lath to Wood Framing, 49
Metal Lath to Metal Framing (Using Screws), 50
Metal Lath to Concrete and Concrete Masonry, 50
Exterior Insulation and Finish Systems (EIFS), 55
One-Coat Stucco Systems, 60

3. Plaster Mixes, 63
Different Coats = Different Mixes, 63
Portland Cement, 63
Sand, 64
Water, 64
Precautions While Working with Cement, 65
Mixing Scratch & Brown Coats, 66
Mixing the Finish Coat, 68

4. Applying Plaster, 71
How Many Coats?, 71
Is the Weather Right?, 72
Are You Ready?, 73
Hand or Machine Application?, 74
Machine Placement and Set-Up, 75
Application Patterns, 80
Application Techniques, 81
The First (Scratch) Coat, 81
The Second (Brown) Coat, 83
The Final (Finish/Texture) Coat, 86
How Thick?, 86

5. Plaster Finishes, 89
The Final (Finish/Texture/Color) Coat, 89
Preparing the Surface, 90
Texture Finish Choices, 90
Achieving Finish Textures, 96
Built- In Color, 98
To Paint or Not to Paint?, 98

6. Decorative Stucco, 101
Shapes attached to stucco (plant-Ons or Implants), 102

7. Properties of Plaster, 105
Structural Strength, 105
Plaster in the Northridge Earthquake, 109
Thermal Properties, 112
Insulating Qualities, 112
Fire-Resistive Properties, 113
Water-Resistance, 121
Acoustic Properties, 122
The Acoustics of Typical Walls, 123

8. Cost Estimating, 125

9. Stucco Maintenance and Repair, 135
Identifying Damage, 135
Paper and Lath Repair, 137
Cleaning Plaster, 141
Refinishing or Changing the Color, 143

10. Glossary, 145

11. Associations and Major Manufacturers, 157

12. References / Bibliography, 165

APPENDIXES

A. History of Stucco, 169

B. Cement Manufacture, 179

C. Governing Codes and Specifications, 185

D. Scaffolding and Safety, 231

E. Stucco and Related Details, 247

INDEX, 279

CHAPTER 1
STUCCO BASICS

WHAT IS STUCCO?
Stucco-also called Portland cement plaster-is an attractive and durable cement-based coating that can be applied over masonry, properly prepared metal or wood framing and/or sheathing, or special insulation board surfaces.

Stucco can be applied to both interior and exterior wall surfaces; this book, however, is mainly concerned with the exterior use of stucco. Keep in mind that the same techniques and materials can be used in interior spaces where moisture resistance, a relatively rough/rustic look and a low maintenance finish is desired.

Stucco makes an excellent exterior surface. As mentioned before, it is durable and resists weather. It provides some level of soundproofing, and it can withstand moderate earthquakes with minor cracking. And stucco provides good wind protection. For these reasons and more-even in severe climates-you'll find many older stucco buildings still in great condition.

When it comes to aesthetics, depending on the finish texture you use, stucco can create great interest with highlights and shadows both in daylight and at night with property lighting. And when it comes to color, stucco offers the choice of long-lasting, "built-in" color, or it can be painted.

A stucco finish is really two or three thin coats of a mortar-also called "plaster" -that is generally one part Portland cement to three parts sand, with a small amount of lime and water added.

Although stucco at first seems like a very simple technique, inadequate preparation, incorrect or inappropriate mixtures, or improper application can result in major problems causing the plaster to bulge, separate, crack, or worse: allowing the entry of water, causing the wooden subsurfaces and frames to rot or develop mold or mildew.

In the next few pages, let's briefly review the steps to creating an exterior stucco finish. In the chapters that follow, we'll cover each of these topics in greater detail.

THE "FOUNDATION"
Just as with any other aspect of construction, your finished product is only as good as your base or foundation. In the case of stucco, the base/subsurface is the foundation.

The proper application of stucco requires some care and attention to preparation of the underlying structure and base/subsurface. The entire process involves several distinct steps, none of which should be overlooked.

The building frame/structure should be stable and solid; any shifting, settling or twisting of the finished stucco surfaces can result in cracks or even complete failure.

Stucco-finished walls (like most shear walls) can add limited strength to a building's structure, but they most certainly cannot make up for shoddy construction.

BASE/SUBSURFACE
Different substrates dictate different approaches to creating a suitable base or subsurface for the coats of cement plaster that are to follow.

METAL AND WOOD FRAME BUILDINGS
In most one-story buildings, the subsurface materials (water resistant barrier and lath) can be attached to the frame structure without plywood or OSB sheathing, unless seismic or wind resistance values are desired. In any event, the inclusion of any rigid sheathing typically improves the quality of the plaster installation.

In multi-story buildings, sheathing is usually required on the first floor exterior wall surface before the subsurface materials are attached. (Attach the lath to the frame-not the sheathing.) Better quality construction will use the sheathing regardless.

CONCRETE/MASONRY BUILDINGS
In the case of masonry or concrete walls, you can apply stucco directly to the base (without a weather-resistant barrier or lath), using a bonding agent.

In some cases (e.g., when the concrete/masonry surface is too rough or when it has been treated with sealants or other finishes that prevent a good bond), you will need to attach self-furred metal lath to the concrete/masonry base.

EXTERIOR INSULATION AND FINISH SYSTEMS (EIFS)
As we will see in Chapter 2, each EIFS system is different. Generally, we can say that successful EIFS systems require close adherence to the manufacturer's instructions.

ONE-COAT BASE COAT SYSTEM
As we will also see in Chapter 2, One-Coat Stucco Systems are a recently developed, low cost, imitation stucco system that provide greater insulation and speedier installation. As systems, they must be evaluated by code authorities and building officials before being accepted in lieu of conventional plaster or EIFS.

The weather-resistant barrier can be provided by a housewrap, building paper or building fel!, although felt is seldom used for several reasons.

WEATHER-RESISTANT BARRIER & LATH
The number of layers to the weather-resistant barrier (one or two) and the types of materials you may use will depend to a great extent to local building codes.

The weather-resistant barrier must be installed in a shingle-like fashion so that any moisture that enters behind the stucco finish will be guided down and safely away from the interior of the building.

Lath "accessories" -casing beads, weep screeds, corners, etc.-should be installed with caulking for optimum moisture resistance.

The weather-resistant barrier, lath accessories and the buildings flashing should be viewed and installed as an integrated system that works together to protect the structure and its interior from water intrusion.

The lath, fastened securely to the frame of the underlying structure, provides the link between the structure and the Portland cement plaster coat( s).

Properly installed lath laterally reinforces the plaster basecoat and can minimize cracking.

The lath should be consistently "furred," meaning that the lath should be held away from the plane of the weather-resistant barrier to allow the encasement of the lath within the cement plaster. To achieve this, furring nails can be used to attach the lath, or self-furring lath can be used.

Individual sheets of lath should be adequately overlapped and wired together to create a continuous layer of reinforcement.

The accessories should be wire tied to lath wherever they meet.

PLASTER COATS
Once the base/subsurface is prepared, it's on to mixing, applying and curing each of the plaster coats. Stucco can be applied by hand or by machine.

Portland cement plaster is mixed on the job by combining plastering sand, Portland cement, a plasticizing agent, and enough water to produce a consistency suitable for the application method you'll be using. Consideration must be given to selecting a plaster mix that can accommodate the maximum allowance quantity of sand.

Timing is everything when it comes to applying each stucco coat, but relative humidity can be just as important. Low humidity and hot, dry winds increase the likelihood of too-rapid evaporation of water from the fresh plaster.

If at all possible, choose an overcast day to apply any of the stucco coats to walls that have southern exposure.

The ideal temperature for installing stucco is between 50 and 80 degrees Fahrenheit, regardless of exposure.

Excessive heat can accelerate the drying process and cause shrinking and cracking.

Too-cold temperatures, on the other hand, can make the stucco mix too stiff for proper troweling.

Just prior to applying the first coat over a concrete/masonry base, saturate the wall thoroughly and allow the surface to dry (this is called "Saturated/Surface Dry" or "SSD") to prevent the base from drawing out the cement plaster's moisture too quickly.

If you are applying a liquid bonding agent, read and follow the manufacturer's recommendations.

Depending on the base/substrate and other factors, a stucco finish may consist of one, two or three coats. Each coat serves a different purpose.
In a three-coat system, for example, the three coats in the order of their application are:

Scratch Coat (or Base Coat): The first coat is applied directly to lath over the substrate (in frame construction, the weather resistant barrier on sheathing or over open stud framing). A properly applied scratch coat places the plaster behind and around the lath/mesh. Once the plaster has initially set but before it has dried, it is scored horizontally with a scratching tool to produce lines approximately 1/8" deep, to provide "keys" for the next coat and to retain moisture. This first coat must be kept continuously damp for at least 48 hours unless the second (brown) coat is applied as soon as the scratch coat is rigid enough to accept it.

Brown Coat: The second coat is applied over a damp scratch coat to level the surface and bring the total thickness to grounds, leaving flat and true to plane with no deviation more than 1/4" when measured under a 5'-0" straight edge.

Finish/Texture Coat: The texture and color is established with the final coat. It may be a factory-blended or site-mixed blend of white or gray cement, blended with lime, oxide pigments and aggregate to produce a 1/8" finish coat that can be troweled, floated, brushed, sprayed-or a combination of these.

Following each coat, a curing process takes place.

Mist the stucco coat lightly one or more times over the cure period, to allow for a slow, smooth drying process. Alternatively, you can loosely wrap the building with plastic sheeting to retain moisture. In summer heat, you may need to do both.

Lightly mist the stucco scratch coat just prior to applying the brown coat, and mist the brown coat just prior to applying the finish coat.

As an alternative, the brown coat may be applied as soon as the scratch coat is rigid enough to receive the brown coat. When using this method, calcium aluminate cement (up to 15 percent of the weight of the Portland cement) is added to the mix (see 2508A. 6 "Alternate Method of Application" on page 187.)

IMPORTANT
This book presents general guidelines and suggestions for a successful stucco finish, based on common knowledge at the time of this book's publication.
No book can address all of the unique specifics of your project or the combination of materials that you choose to use.

Information in up-to-date manufacturers' documentation takes precedence over anything you read here. That's why it's important to obtain, read, understand and follow all available documentation for each product that your stucco project will use.

Local codes and ordinances may prohibit some of the techniques or materials discussed in this book. Before you commit time or resources to a particular approach, you must check to make sure that your project design and your chosen materials and techniques will satisfy local requirements.

Builder's Guide to
STUCCO, LATH & PLASTER

Covers every aspect of design, construction and repair...

Plaster base (lath) and flashing
Mixing and application basics
Plaster finishes and how to achieve them
Decorative plaster work: implants, molds, etc.
Structural, thermal, water-resistant and acoustic properties of cement plaster
Scaffolding and safety
Cost estimating
Typical causes of plaster failure, and remedies
Repair and maintenance of stucco

Companion CD-ROM Included!
The disc brings you the complete book, for quick and easy research and on-site reference. Requires the FREE Adobe® Reader® available for download at www.adobe.com. (Printing from the CD-ROM has been disabled.)

Max Schwartz
Since serving in the U.S. Army Corps of Engineers, principal author Max Schwartz has worked as a consulting civil and mechanical engineer. Licensed in several states, he's worked jobs from small residential developments to major industrial facilities, from the California suburbs to the Central American rainforest.

A recognized authority with extensive experience, Mr. Schwartz has lectured and taught civil engineering at UCLA Extension, and is the author of several standard references on engineering and concrete construction.

Walter F. Pruter
A recognized expert in the industry, Walter F. Pruter currently serves as president of the International Institute for Lath & Plaster ( a national effort to provide accurate information on lath and plaster construction), as well as a Senior Consultant to the Western Wall and Ceiling Contractors, Technical Services & Information Bureau, and is a Co-Director of the Lath, Plaster & Drywall Information Bureau.

Mr. Pruter authored the Plaster and Drywall Systems Manual, has edited a number of coursebooks on plaster and stucco and has contributed noteworthy articles to several magazines and journals.