Designers want their dreams and buildings to look pretty, owners don’t want leaks and contractors want to do what they know works. This leads to confusion, arguments, finger pointing and since everyone is reluctant to change, and everyone wants to avoid litigation. The person with the most power or persuasive argument usually wins out. This article is an attempt to help in the decision process and assist those that decide cement stucco on the parapet is worth the effort.
Cement stucco buildings that have no overhangs can present real challenges. The overhang, as with any building, provides protection against moisture, in particular moisture that falls from the sky, namely rain. Most would agree that to have cement stucco parapets on your building is simply inviting trouble. Let’s face it, no overhang, are you crazy? But stucco lends itself to the look of no overhang, whether it is a modern-style building or classic southwestern hacienda, that stucco parapet looks nice, and why ruin it with a metal cap? And so the argument ensues.
I have been a contractor, code consultant, defect investigator and worked at an architectural firm. I have been from the dry southwest to the wet northwest and I have learned what does and does not work with cement stucco. Cement parapets are very problematic, unless you take special precautions and rigorous inspections to ensure they will not leak. Even in the deserts of southern California, it rains, and if done incorrectly, the stucco parapet will leak.
Here is the part that might surprise you: the cement stucco parapet can work, even in the wet climate. Again, with very careful planning, purposeful execution and inspection to verify, it can work. Most contractors prefer to not do stucco parapets. I can’t blame them—they require more time, more attention to detail and they can be risky. But they can be done, and as a designer or developer, you need to insure your contractor is on board and that you each support proper practices to make it work.
A metal cap is always the simple, easy and best guarantee to keep water out. The metal cap may not be as bad to look at as most designers claim but it is also not always a recipe for disaster as some contractors claim. If a metal cap is used, the overlap of the cap flashing leg should be at least 2 inches and may need to be extended up to eight inches depending on the walls exposure to wind-driven rains. It would seem simple to state that a building in Southern California only needs two inches and buildings in Seattle need eight, but it is not that simple. A designer should take into account the height of the building and location with respect to wind-driven rain exposure. This is very similar to the way windows are selected for buildings with Performance Grades and Design Pressures.
The flashing leg length is a critical design component, just as not putting screws through the top is not as preferred as using cleats to hold down flashing caps. If screws are used, it is recommended to use a self-healing membrane under the cap and a sealing washer on the fastener.
RULES TO FOLLOW
When it is determined that a stucco parapet is a must, there are some basic “best-practices” rules that should be followed.
Design: the following items are primarily for the designer/developer when planning to put cement plaster (stucco) over the parapet:
The width or top (horizontal) part of the parapet should not exceed 8 inches.
The slope of the parapet should be toward the roof, the more rainfall/snow accumulation, the greater the degree of slope. Half inch drop per lineal foot is minimum.
Specify a self-healing membrane, minimum of 40 mils, over the entire cap and that it should extend a minimum 6 inches onto the water resistive barrier.
Ensure there is a pre-installation meeting with contractors, supervisors and foreman to verify procedures are followed.
Implement an inspection protocol prior to application of any cement.
(OPTIONAL) An outer layer can be added for additional protection.
An elastomeric coating along top of parapet, needs re-application every few years.
A skim coat of enriched cement and mesh (lamina) prior to finish coat application provides ultimate protection.
Installation: the following is for the contractors and field personnel when cement plaster is to be put over a parapet:
Ensure the carpenter or others have placed a shaped cap to provide slope, do not rely on plaster to create the slope.
Apply water resistive barriers up to top of parapet; it acceptable to overlap the top of parapet with WRBs.
Apply a minimum 40 mil thick self-healing water proof membrane over the top of parapet, overlap and seal all horizontal (top) by a minimum of 6 inches. Place no fasteners along top. Overlap the WRBs on the sides by 6 inches minimum.
Apply lath in large enough sections to lap completely over the top of parapet. Attach lath only on vertical (wall) sections. Do not place fasteners along top of parapet.
Apply corners aids or beads with wire ties to attached lath. Do not place fasteners along top of parapet.
Inspect and verify the top of parapet is sloped with no penetrations.
Apply cement stucco as required
Apply optional outer protection (elastomeric or lamina) if specified.
The cement plaster parapet can and has been done. It is not to be done on the cheap. Simply applying one or two layers of water resistive barrier over the top and then punching holes in the secondary moisture barrier is a recipe for disaster.
The cement membrane is highly water resistant, but the likelihood cracks will develop and water will seep in on the horizontal section is pretty good, even in California—I know, I have seen it happen. The cement stucco parapet can work in wet climates, such as Seattle.
It is true that when designing and installing cement plaster over a parapet in Seattle will require greater attention to waterproofing than in Los Angeles, but good design and installation works anywhere. Do not be afraid to use cement stucco parapets, but be very diligent. W&C