Download PDF version (346.1k) Log In or Register to view the full article as a PDF document.

Launch Slideshow

A Close Look at Stucco

A Close Look at Stucco

  • http://www.jlconline.com/Images/1661500533_0309_A_Close_Look_00a_tcm96-1248347.jpg

    true

    600

  • http://www.jlconline.com/Images/390018669_0309_A_Close_Look_01a_tcm96-1248348.jpg

    true

    600

    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.

  • http://www.jlconline.com/Images/1296283002_0309_A_Close_Look_02a_tcm96-1248349.jpg

    true

    600

    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.

  • http://www.jlconline.com/Images/2015275581_0309_A_Close_Look_02b_tcm96-1248350.jpg

    true

    600

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

  • http://www.jlconline.com/Images/1963740232_0309_A_Close_Look_02c_tcm96-1248351.jpg

    true

    600

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

  • http://www.jlconline.com/Images/1118505338_0309_A_Close_Look_02d_tcm96-1248352.jpg

    true

    600

    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.

  • http://www.jlconline.com/Images/1718010309_0309_A_Close_Look_03a_tcm96-1248353.jpg

    true

    600

  • http://www.jlconline.com/Images/282428308_0309_A_Close_Look_04a_tcm96-1248354.jpg

    true

    600

    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.

  • http://www.jlconline.com/Images/1771948815_0309_A_Close_Look_04b_tcm96-1248355.jpg

    true

    600

    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.

  • http://www.jlconline.com/Images/343457747_0309_A_Close_Look_04c_tcm96-1248356.jpg

    true

    600

    The crew then replaced the window.

  • http://www.jlconline.com/Images/2020250008_0309_A_Close_Look_04d_tcm96-1248357.jpg

    true

    600

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

  • http://www.jlconline.com/Images/172602903_0309_A_Close_Look_04e_tcm96-1248358.jpg

    true

    600

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

  • http://www.jlconline.com/Images/1011083090_0309_A_Close_Look_05a_tcm96-1248359.jpg

    true

    600

    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.

  • http://www.jlconline.com/Images/1146819495_0309_A_Close_Look_05b_tcm96-1248360.jpg

    true

    600

    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.

  • http://www.jlconline.com/Images/256972072_0309_A_Close_Look_05c_tcm96-1248361.jpg

    true

    600

    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.

One-Coat Stucco

Since the mid-1980s, a handful of manufacturers have introduced thin-coat stucco products that collectively are called "one-coat" (or sometimes "two-coat") stucco. One-coat is nearly identical to conventional stucco in concept and design, except that the base coat is applied in one layer instead of the original two-step scratch- and brown-coat process. The base coat is mostly sand and Portland cement, as in conventional stucco, but it also includes synthetic polymers and fiberglass reinforcing strands that increase both the tensile and the compressive strength. The required total thickness is just 3/8 inch, instead of the standard 3/4-inch total for the three-coat base.

The idea behind one-coat systems was to save labor and time in the schedule. With the added components, base coat could be put on in just one layer, with no second plastering and no wait in between.

In practice, I'm not sure one-coat is all that economical. The proprietary mix ingredients add cost, and finding and following the special instructions for the proprietary systems add complexity. One experienced stucco contractor, a friend whose work I respect, told me that he gave up working with one-coat because it was too complicated. His crews rebelled against the required special detailing, and he also found that with only 3/8 inch of thickness, it was harder to achieve a nice, uniform finish over the usual irregularities in a house frame. (A common defect I see in one-coat installations is a base coat much thinner than the required 3/8 inch, at least in spots.)

The other big selling point for thin-coat systems is that the fiberglass and polymer additives help the stucco withstand the winter freeze-thaw cycle.

The thinner base coat is still applied over wire lath or expanded metal, and over a system of papers and flashing the same as we need for conventional stucco. The same screeds and expansion joints are also part of this system, although at different thicknesses. But unlike three-coat stucco, one-coat systems require a 48-hour moist cure. The applicator is responsible for keeping the base coat moist for the first 48 hours after application. The color finish is also required to go on within 72 hours of the base coat application. Proper curing is more critical with one-coat than with traditional stucco, because the acrylics tend to isolate cement particles from water in the mix. If the coat isn't kept moist, it may dry out before the cement has a chance to react with water (hydrate), which it must do to form the strong cement compounds that give the cladding its strength. Without the correct moist cure, the base coat is likely to be weak and crumbly.

One-coat stucco usually receives one of the new acrylic color finishes, instead of traditional stucco's purely cementitious, textured color coat. It has a smoother and less porous look, because acrylics instead of cement bind the aggregates together — it's like sand mixed with latex paint. Many people perceive this acrylic top coat as the defining characteristic of one-coat stucco, but synthetic finishes are not really an essential component of a one-coat system — they just happened to be developed about the same time that one-coat was widely marketed. One-coat base-coat systems got code approval in ER reports without mention of any particular color finish. As long as the base coat is applied at least 3/8 inch thick, you can paint it or apply either a conventional cement color finish or a synthetic acrylic color finish over it.

An acrylic coating's higher plasticity gives more resistance to cracking and creates a more closed, water- and stain-resistant surface. But one-coat stucco finishes are still porous enough to let rain enter the system — the perception that one-coat systems reliably repel water at the surface is incorrect. And even if the coatings were waterproof, one-coat systems do crack, and they can leak at all the joints and penetrations, so water is sure to get behind them. At the same time, they are less breathable and slower to dry out than conventional stucco. So they are less forgiving of any defect in the proper placement of building papers, flashing, and lathing staples — if water reaches the wood structure of the house, it is less able to escape by evaporation.

I've seen many failed stucco systems that someone has tried to repair by applying a thick polymer coat over the existing stucco, and by surface- caulking window and other joints. This is worse than useless — it actually accelerates the damage. Water will still enter the system somewhere, and then it's trapped next to the house. My company's educational video, available from our website at www.ram builders.com, shows an example of a home just four years old, whose framing and sheathing is completely gone because of that kind of attempted "repair." Damage that might have taken 10 or 20 years to develop under normal, breathable stucco happened in 1 or 2 years after the sealer was applied.

EIFS

Exterior insulation and finish systems use just a thin (1/8-inch to 3/16-inch) synthetic top coat over a substrate of expanded polystyrene foam. Originally designed as a barrier system with no water management behind the foam, EIFS in the residential market now has to have reliable paper and flashing assemblies behind it to allow water to drain. However, EIFS still requires surface caulking and sealing at joints (caulking is not part of a hard-coat stucco system).

The details for EIF systems are all specified by manufacturers in their specs and ER reports. In practice, I've found that EIFS applicators still mix and match in the field, using whatever components are cheap or easy to find, and assembling the system however they feel like doing it. With constant pressure on budgets and schedules, it's not surprising that we still find defective EIFS applications all over the market.

Even the new water-managed EIF systems use sealing top coats, so they lack the easy path for moisture escape that traditional stucco has. With EIFS, any water in the system has to make its way to weep exits — it can't readily bleed or evaporate out the face of the wall.

Drainage Detailing for Critical Spots