Exterior Plaster and Stucco
Summary
Exterior plaster, or stucco, is a portland cement-based coating applied in scratch, brown, and finish coats over lath or a masonry base. Its water control depends on the weather barrier, lath, and weep screed behind it, so these hidden parts matter more than the finish coat.
Cols
Terminology
- Scratch coat: first coat over lath, scored to key the next coat
- Brown coat: second coat that levels the surface
- Finish coat: final coat that sets color and texture
- Lath: metal mesh, often self-furring, that holds the plaster
- Weep screed: metal or plastic accessory at the base of the wall that lets water drain out
- Weather barrier: water-resistive layer behind the lath
- Control joint: break that relieves cracking from shrinkage
Standards
Systems
- Common specification: three-coat system about 7/8 in. thick nominal, with scratch and brown coats about 3/8 in. each plus a finish coat, over self-furring metal lath and a secondary weather barrier
- Field-mixed three-coat stucco is generally applied 3/8 to 3/4 in. thick, with thickness and mixes based on ASTM C926; the two figures differ, so check the project specification
Test and application standards
Materials
- Portland cement, lime, and sand: the plaster mix
- Metal lath: self-furring or paper-backed expanded and woven mesh
- Weather barrier: building paper or other approved water-resistive barrier
- Finish coat: textured final coat
Detailing
- Install the weather barrier and lath, then the weep screed with a 3-1/2 in. attachment flange, set 4 in. above earth or 2 in. above paved areas
- Lap the barrier over the weep screed flange and end the lath on the flange
- Provide control joints at panel limits and openings
- Cure the coats as the specification requires and keep them from drying too fast
- In wet climates, use a drainage layer under the plaster as current code guidance recommends
- Flash windows, doors, and penetrations to the barrier
Uses
Specific uses
- Houses and low-rise commercial buildings
- Over frame, masonry, or concrete walls
- Decorative textures and colors
- Soffits and parapet walls
Pros and cons
Pros
- Hard, continuous exterior surface
Cons
- Cracks if control joints and curing are neglected
- Water that gets behind the stucco needs a drainage path
Manufacturing and cost
Construction
- Mixed on site or supplied as preblended bags, applied in coats over lath, floated, and finished with the chosen texture
Main manufacturers and suppliers
Sources
- Journal of Light Construction: top quality three-coat stucco, https://www.jlconline.com/how-to/insulation/top-quality-three-coat-stucco_o
- Quikrete: FRS guide specification, https://quikrete.com/pdfs/FRS-GuideSpecification.pdf
- Specmix: 09 24 00 portland cement stucco, https://www.specmix.com/wp-content/uploads/09-24-00-Portland-Cement-Stuccco.pdf
- IIBEC: stucco codes and standards, https://iibec.org/wp-content/uploads/2025/09/2025-08-stucco-codes-and-standards.pdf
- ASTM International: C926 application of portland cement-based plaster, https://cdn.standards.iteh.ai/samples/77391/8285d4db6a0c4d68a3c01a35666400e0/ASTM-C926-11.pdf