Performance of Stucco
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Stucco and Moisture Resistance
Stucco over concrete masonry is popular, in part, because it is
very durable in high humidity environments. Although most prevalent
in Florida, stucco is estimated to be used on as much as 20% of
block installed in the U.S. Where this kind of finish is also subject
to high winds, concern has been raised about how well it resists
The following article summarizes new research about traditional
stucco over concrete masonry to determine the system’s effectiveness
in resisting moisture penetration. Testing was done in a laboratory,
using test periods of 28 hours and high winds (up to 180 mph) as
might be expected during hurricanes. (Figure 1).
|Phase I test walls show wall panels with water
chamber assemblies mounted on them.
Following the 2004 hurricanes that hit Central Florida, significant
moisture penetration and damage was noted on new block homes, residences
constructed after 2001. To help determine what happened, the Florida
Concrete & Products Association (FC&PA) contacted the National
Concrete Masonry Association (NCMA) and set up a research project
under the direction of Jim Gulde, a noted stucco expert.
Objectives of the Two-Phase Study
The objectives of the project were as follows:
- Generate data on water intrusion through stucco over concrete
- Evaluate performance based on stucco thickness and masonry workmanship
- If the water penetration performance was favorable, provide
information to assist marketing efforts for concrete masonry
Stucco was installed in accordance with industry practices, although
thinner applications (1/8 and ¼ in.) than allowed by code
were made for comparison with code- and standard-recommended thicknesses.
Some joints between units were also purposely left unfilled to evaluate
the effect of workmanship. (See table.)
The study was completed in two phases, the first at lower wind
speeds and the second at higher speeds as shown in the table. Using
4 ft x 6 ft panels subjected to the ASTM E 514 protocol (Standard
Test Method for Water Penetration and Leakage through Masonry),
the moisture penetration through the walls was observed and recorded
- area of wall surface affected and
- amount of water collected on through-wall flashing.
|Table: Wall Finishes and Wind
Speeds in Phases I and II
|Phase I: wind at 62 mph for 4
hours, then 110 mph for 24 hours
||Phase II: wind at 155 mph for
4 hours, then 180 mph for 24 hours
|Control wall, no stucco
|1/8 in. (3 mm)
(1 good + 1 flawed)*
(3 good + 1 flawed)*
|Skim coat of stucco
|¼ in. (6 mm)
|One-half the code-required thickness
|½ in. (13 mm)
(1 good + 1 flawed)*
(3 good + 1 flawed)*
|ASTM C 926-required thickness
|*An additional variable was full head and bed
joints, or “good,” vs. “flawed” mortar
joints, an indication of the quality of block construction.
Findings and Conclusions
Portland cement plaster over concrete masonry is an effective finish
for resisting water penetration into and through the walls. It should
be installed in accordance with industry practices—the proper
mix at the correct thickness, followed by curing—to achieve
its full potential. A complete description of industry practices
is available in ASTM C 926, Standard Specification for Application
of Portland Cement-Based Plaster.
At wind speeds of 110 mph (177 kph), all the coated walls resisted
water penetration, but the uncoated wall failed early in the testing
at lower wind speeds (62 mph or 100 kph).
|The control wall (uncoated) after 4 hours
exposure to 62 mph (100 kph) wind-driven rain.
- At wind speeds exceeding 155 mph (249 kph), traditional
½ in. (13 mm) stucco provided excellent resistance to moisture
- At 180 mph (290 kph), the amount of dampness on the leeward
(non-wind) side of the wall was similar for both stucco material
thicknesses (½ and ¼ in. or 6 and 13 mm). The thicker
coating allowed much less water penetration—measured by
water collected on the bottom of the cells on the flashing—than
the skim coat (1/8 in. or 3 mm) at extended time and pressure.
|The ½-in. (13-mm) coating allowed much
less water than the skim coat to pass through the wall over
the test duration.
The complete report, Water Penetration Testing of Stucco on
Concrete Masonry Construction, was issued and is available
free of charge from NCMA. Find
the full report under “Research Reports.”
This work was completed in May 2007, a joint program of the Florida
Concrete & Products Association and the National Concrete Masonry
Association Education and Research Foundation. All tests were conducted
at the NCMA Laboratory in Herndon, Virginia.