InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
Recipes for mixing stucco wall coatings:
This article discusses recipes or mixes to prepare traditional stucco and includes details of how stucco is applied to a building surface.
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.
Our page top photo shows exposed metal lath in the stucco exterior of a poorly-finished home in New York. It looks as if the top coat of stucco may have not been applied at all.
Stucco is a mixture of Portland cement, sand,
and water, with a little lime or a plasticizer added for workability.
A proper mixture has good tensile strength and
weather resistance and the ability to bond well to the mesh
or substrate. It is also easy to trowel on and resists sagging.
In cold climates, it must also have freeze-thaw durability,
usually obtained by using air-entrained plaster.
The cement base can be masonry cement, plastic cement,
or Portland cement, which may have air-entraining
additives. Do not add lime or a plasticizer to masonry cement
or plastic cement since these already contain plasticizers.
While approximate proportions are well established,
the right mix for a job depends on the weather
exposure of the wall and weather conditions during application
(see Table 1-9).
[Click to enlarge any image]
Other than the right proportions, the keys to a good
stucco mix are clean, good quality sand and clean potable
water. Since sand makes up about 97% of the stucco
mixture by volume, it is critical to use good sand.
The sand used to prepare stucco
should be free of vegetable matter, loam, clay, silt, and
soluble salts and should conform to ASTM C897, which
designates the distribution of particle sizes (gradation).
Impurities in the sand or water can affect the strength of
the mix, and poor grading of the sand will hurt its workability.
Salts can cause staining on the finished surface from
Stucco Wall Application Procedure
Stucco can either be hand troweled or
blown with a machine. Some stucco contractors use a
pump for the base coats but apply the finish coat by hand.
Although the mixes are slightly different for the two approaches,
both can produce a high-quality finish. In threecoat
stucco, the first and second coats are 3/8-inch thick, and
the finish coat is
1/8-inch for a total thickness of
7/8 to 1 inch.
Applying the Stucco First or Scratch Coat
The first, or “scratch,” coat, which forms
the base for the next two coats, should completely encase
While still wet, the plaster is scored
horizontally with a special metal rake or trowel to create a
good mechanical bond with the second coat (vertical
scratching promotes cracking at studs).
For proper curing,
the scratch coat needs to be kept moist by misting or fogging
with water for 48 hours.
Except in very moist
weather, misting should start as soon as the freshly applied
stucco lightens in color and be repeated at the start and end
of each day until the second coat goes on.
Applying the Stucco Wall Brown Coat
The second, or “brown,” coat should go
on as soon as the first coat is hard enough to accept the
second coat without cracking, but at least 48 hours later,
according to the IRC.
The second coat fills any cracks in
the scratch coat, and the additional sand in the brown coat
helps prevent new shrinkage cracks. Whether it is hand troweled
or machine-applied, it must be leveled with a
straightedge (“rodded”) and floated to produce an even
surface for the final coat.
A short delay between the first and second coat helps
to create a good bond between the two and strengthens the
scratch coat by rewetting it for a more complete cure.
cracks larger than 1 1/16-inch in the brown coat should be
patched before the top coat goes on. In the Southwest,
where adobe is popular, the brown coat is often steel troweled
for an adobe look and serves as the final coat.
Applying the Stucco Wall Finish Coat
After the second coat is allowed to cure for
a minimum of 7 days (14 will allow a more complete cure),
the top coat is applied to provide the finish color and texture.
Many contractors now use premixed color coats,
some with acrylic additives to increase water resistance
Creating a uniform color and texture requires
a skilled applicator, uniform mixing, favorable
weather (avoid direct sun), and a uniform substrate without
variations in texture or water absorption. Problems in
the substrate will tend to show through the thin finish coat.
It is best to do an entire side of the building in one batch
with no cold joints. A modest amount of color variation is
considered part of the character of traditional stucco, but
too much is a sign of substandard work.
A certain amount of shrinkage cracking is also inevitable
in stucco exteriors. Application over wood-frame
construction results in more cracking than over concrete
block or other more stable substrates.
Coarse textures in
the finish will tend to hide the cracks better than smooth
finishes. Even under the best of conditions, small shrinkage
cracks of less than 1/16-inch will occur in the finished
stucco and are to be expected. Generally these do not leak
or indicate substandard work.
Expanded Metal Lath Supporting Exterior Stucco: Details
At below left we show a cut-cross-section of a steel-lath supported exterior wall on a Minneapolis, MN. home. Our photo at below right shows the marks left by metal lath on felt paper that had been used as a building sheathing wrap on the same home. These details were exposed during construction work for an addition.
Details about interior stucco or plaster and additional photos and information about the use of expanded metal lath are
at PLASTER TYPE IDENTIFICATION
ASTM C 842
ASTM C 841 Metal Lath or gypsum lath installation
Lath & Plaster Systems, [PDF] 092300/NGC, National Gypsum Lath and Plaster Systems, National Gypsum Corporation, 800-628-4662 describing National Gypsum's Kal-Kore brand plaster base
Gypsum Construction Guide, National Gypsum Corporation
Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Ask a Question or Search InspectApedia
Questions & answers or comments about how to mix wall stucco: recipes & application methods.
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: firstname.lastname@example.org
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: email@example.com
John Rudy, Advantage Home Inspections, Flemington N.J. 08822 home inspector, 908-806- 6364, Home, Radon & Termite Inspections, Central & Parts of North New Jersey, email: firstname.lastname@example.org
Malco® Products siding tools are available from that company, including the SideSwiper II SRT2 discussed at Malco's website. Websearch 09/07/2010 http://malcoproducts.com/product/roofing-siding-gutter/siding-vinyl/siding-tools-vinyl/sideswiper-ii. Malco also produces other vinyl siding repair tools such as aprons, awls, hole punches, saw blades, and tools for for fiber cement products including power-assisted cutters
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones