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A Close Look at Stucco

A Close Look at Stucco

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    Flashing details frequently involve the interaction of two trades. Here, for example, the stucco crew members responsible for installing the building paper had to remove and reinsert a gable vent installed by the framers so that they could run building paper under the vent flange.

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    Integrating building papers with window flashings can be a challenge on site. In California, where this crew works, stucco contractors aren't typically licensed to install windows, so the paper crew must work with the existing flashing. Here, the window crew left the window perimeter protected by strips of flashing, with side strips lapped over the bottom strip and the head strip lapped over the side strips and the window flange. The paper installer lifts the bottom flap and runs paper under it.

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    The installer seals the flashing to the paper with adhesive membrane.

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    He lays paper under the side window flashings and staples the flashing down before applying membrane to this joint as well.

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    Paper-backed lath then butts directly to the window flange, but water that gets behind the stucco at the window sill and jamb will be handled by the membrane, the flashing, and the first layer of paper.

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    Less than ten years old, this building had been repaired twice previously with surface-sealing methods that did not work. The author's crew first stripped the stucco. At this window corner, badly detailed flashing and paper let water soak the sheathing and framing, leading to rot.

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    After pulling out the window, the author's crew repapered the wall with Tyvek StuccoWrap, then applied membrane and Tyvek FlexWrap to the rough sill to protect the wood structure even if the window leaks.

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    The crew then replaced the window.

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    The crew slid the top flange under the Tyvek and sealed the flange to the Tyvek all around the window.

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    The crew applied a second protective layer of Grade D building paper.

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    Roof-to-wall intersections create particular problems. This bad detail held up the work: The rafter placed tight to the building wall does not allow paper to slip behind it, and the flashing left by the roofers will direct water behind the stucco and into the wall if used as configured.

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    A water diverter (kick flashing) was installed by the author's crew in a retrofit job: This design will lead water out away from the wall and dump it off the roof's drip-edge rather than into the wall. Papers installed on the main wall can readily integrate with the membrane and flashing between roof and wall at the rafter tail.

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    A wall-and-roof intersection built without the correct flashing detail shows discoloration and staining after a few years in service; the author typically finds significant rot in buildings with this detail, even though the wall surfaces may show only minor traces of trouble.

In the neighborhoods of coastal California where I grew up, stucco was the most common exterior cladding. As a kid helping my father on projects, and then as a construction supervisor and contractor myself, I have been around stucco for most of my life. Nowadays, the kind of stucco that I grew up with is known as "conventional stucco," or "traditional stucco." When I was a kid, we just called it stucco.

Good old stucco is still around, but it's not alone. There are now at least 35 different modified, proprietary hard-coat stucco systems on the market and more than 30 varieties of exterior insulation finish systems (EIFS) that have the look of stucco. With products continually leaving and entering the market, and new hybridized systems coming along that combine characteristics of EIFS and hard-coat systems (or go on over some other base like fiber-cement board), it's hard to keep track, much less understand them all.

All these new systems are penetrating markets where traditional stucco is largely unknown, and where the details that make stucco work are not part of the local tradition. In place of the standard, generic water management details that governed traditional stucco, which you could look up in the body of the building code, these new systems are regulated through evaluation reports ("ER reports") and proprietary specifications that you have to locate and study individually.

Without the base of experience and knowledge, and without simple references for proper detailing, installers have made a lot of mistakes with hard-coat stucco in new markets. Over many years in California, in the course of remodeling or adding on, I've opened up hundreds of homes clad with traditional stucco without finding significant mold or rot. But in recent years in Utah and Texas, I've been called in to remediate hundreds of homes clad with newer modified systems, where the lack of proper water management details has caused major decay problems in homes that are practically new.

The good news is that any kind of stucco — traditional three-coat, proprietary one-coat systems, and even EIFS — can work well if you apply the flashing and drainage plane principles that have always been part of traditional stucco. But before we get into those details, let's look at the differences between traditional "three-coat" stucco, the new proprietary "one-coat" systems, and EIFS (the polystyrene-based exterior insulation finish system).

Three-Coat Stucco

Conventional or traditional stucco is called three-coat stucco because it has a 3/8-inch scratch coat, a second 3/8-inch brown coat, and a thin "color coat" on top, for a total system thickness of about an inch. All three coats are mixed from Portland cement, sand, water, and some lime for workability; the top coat has color powder and may include some polymer additives.

But the system starts with a drainage plane based on some type of building paper over the wood framing of the home. Building codes call for two layers of Grade D kraft paper, which is made with virgin wood fibers. The paper is there to drain water, so it has to be carefully tied into flashings around all windows and doors. Metal flashing systems are also installed to divert roof water away from the stucco system, and to protect any penetrations. The paper and flashings have to overlap each other in a way that creates a shingle effect.

Over the papers and flashings, a stucco netting or metal lath is fastened to the wall with staples. Stucco netting looks like chicken wire, but it is actually a heavier-gauge galvanized-steel wire. Expanded metal lath has the look of a heavier grating, but it serves essentially the same purpose.

Next comes the base coat, troweled into the lath mesh and tooled with grooves while wet, to provide keys for the second coat to lock into. The 3/8-inch-thick second coat is applied and tooled flat, and then both must cure for 7 days before the color coat gets troweled on. Like all cement, stucco will shrink and crack; many traditional contractors will wait 14 days to make sure the first coats have completely "cracked out," so new cracks won't telegraph through the top coat.

Three-coat stucco is designed to be porous. Rain soaks into it, then drains out when the storm ends. The papers and flashings are vital to protect the house — without them, water will soak the wood and create conditions for rot.