by Joe Lstiburek

The four hurricanes that struck Florida last summer proved the effectiveness of the tough building codes passed by the state after Hurricane Andrew's strike in 1992. Hurricanes Charley, Frances, Ivan, and Jeanne killed dozens of people, but not one of those deaths was caused by failure of any structure built to the new codes.

Unfortunately, though, many stucco-clad homes in the state experienced a problem we would not be discussing if they had simply fallen down or blown away: They got wet.

After 20-plus inches of windblown rain soaked the whole region, there were hundreds of reports of water intrusion through stucco walls of otherwise undamaged homes. The Florida Home Builders Association hired my company, Building Science Corporation, to investigate the situation, identify causes, and propose solutions.

We applied a wide range of investigative techniques (see Figure 1): We inspected homes soaked by the storms as well as new homes built after the storms; we tested and experimented with new buildings and mocked-up assemblies; and we did bench-top testing of materials and components, including felt paper, plastic housewraps, and windows. We also reviewed relevant codes and standards, and interviewed builders, contractors, materials suppliers, manufacturers, and code officials.

Members of the author's investigative team examined stucco walls with infrared cameras (top left) and camera probes (top right). To measure water leakage, they applied hydrostatic head tests (bottom left) as well as simple water sprays and simulated wind-driven rain (bottom right).

In this article, I will explain what we learned. Anyone who wants to keep rain out of a building might benefit from the lessons of Florida's wet walls. After all, whether you build in Florida or somewhere else, water is water, wind is wind, and the laws of physics are the same.

Why Stucco Leaks

We were asked to focus on stucco-clad homes. Actually, there are two kinds of stucco used in central Florida: traditional "hard coat" or "three-coat" stucco, and the modern "cementitious decorative finishes," popularly called "thin-coat stucco." Both types of cladding let water into homes during the storms.

Our testing of Florida homes confirmed what we already knew about stucco: It always cracks, and the cracks always leak. In homes we tested, stucco that was cracked leaked at the cracks; and where the stucco was not cracked, it did not leak (Figure 2). But there's a little more to the story than that.

Figure 2.Using new garage block walls as test structures, the investigators compared the leak performance of bare masonry-block walls with that of various stucco claddings, with and without cracks. A water spray on a bare block wall (top left) lead to a characteristic leak pattern at mortar joints (top right). Water sprays on unpainted stucco (middle) did not cause leaks if the stucco was not cracked. However, when typical small stucco cracks (bottom left) received the water spray, water intrusion occurred at mortar joints and at the wall base (bottom right).

Traditional stucco and thin-coat stucco aren't fundamentally different. Both are surface treatments applied to a substrate, and they behave in a generally similar fashion. Most important, neither can be considered waterproof or leakproof. On the contrary, when you use either, you can be sure it'll leak. But with both decorative thin stucco and traditional three-coat stucco, there are ways to decrease the frequency of cracking.

Traditional stucco. This centuries-old coating is supposed to go on in three coats. The first is the scratch coat. It's applied about 3/8 to 1/2 inch thick and allowed to cure. Because it's cement, it shrinks as it cures, and as it shrinks, it cracks. Then, two or three weeks later, when it is done shrinking, you go back and apply the brown coat, which serves to fill in the shrinkage cracks in the scratch coat. After that cures, you go back yet again and apply the finish coat (the color coat), and you're done.

With each coat you apply, you change the mix of cement, lime, sand, and water slightly, so that each coat is a little weaker, more permeable, and more flexible than the one it covers. Thus the softer, outer coats have relatively more lime and sand — and less cement — than the hard inner coat.

But modern-day stucco applicators, in Florida as well as in other places, typically don't wait several weeks for the first coat to cure; they go back and apply the second coat the same day. That means, of course, that both coats shrink and crack at once — one reason the stucco in Florida leaked as much as it did.

Thin-coat stucco is applied in just one coat, so it's going to crack no matter what. However, with both thin-coat stucco and two- and three-coat applications that are not given time to cure between coats, it's possible to reduce the amount of cracking by using fiber mesh in the mix, and by adding polymers to make the coats more flexible.

So why not change codes to require either a mandatory curing period between coats or the use of additives in the mix? That wouldn't really be a practical solution. Neither technique will prevent shrinkage cracks altogether. Nothing will. More to the point, there's another cause of cracking that we also can't prevent: settlement (Figure 3). Virtually all buildings move and shift enough in the first few years after construction to cause some cracking of the stucco — and no stucco, whether traditional or polymer-modified, is immune to that.

Figure 3.Houses shift after construction as supporting soils consolidate and materials change size with changing temperature and moisture content. This movement of substrates, though typical and expected, inevitably brings about some cracking in brittle surfaces like stucco, creating the need for ongoing maintenance and repair in the early years of a building's service life.

What, then, do we do? The answer is that you have to assume there will be cracks and that water will be able to get through them. Accordingly, you have to design wall systems that tolerate those leaks, and accept that some maintenance and crack repair by the occupants will be necessary over the years after the home is finished and occupied.

Two Kinds of Walls

The picture in Florida is complicated by the fact that builders there typically use two types of exterior walls on the same house. It's common to lay a concrete-block wall for the first story and then stick-frame second-story walls and gable ends; stucco is applied to both stories. Both types of walls leaked in the Florida storms, for somewhat different reasons.

Building paper and housewrap. Upper-story stick-frame walls, in principle, should function as "drained assemblies." The stucco is applied over a "weather-resistive barrier" (you can use housewrap, asphalt felt, or Grade D asphalt-saturated kraft paper). Any water that penetrates to the weather-resistive barrier is supposed to drain down it until encountering a flashing that directs the moisture out to the exterior.

One reason Florida walls leaked was that the housewraps or papers installed beneath the stucco did not function effectively as a drainage plane (Figure 4). Stucco tends to bond to housewrap and building paper, eliminating the air space in which water is supposed to drain. Also, the housewrap or building paper itself loses water repellency when stucco adheres to it, or when it comes in contact with surfactants (soaplike chemicals that reduce the surface tension of water). In Florida, water that reached the housewrap or building-paper layer in the walls often bled through into the frame assemblies.

Figure 4. Upper-story walls in central Florida are typically stick-framed, while lower-story walls are laid up with concrete block (top left). Both wall types leaked, but for different reasons. Designed as drained assemblies, frame walls were handicapped by the bonding of building paper or housewrap to the stucco coatings, which prevented free drainage (top right). Forced into contact with trapped water, most housewraps allow some water penetration, as shown in table-top testing at the author's shop: Water dripped onto a horizontal housewrap surface (middle left) soaked into the absorbent test material underneath (middle right). In the field, saturated stucco coatings over imperfectly performing housewrap allowed water into sheathing and stud bays (bottom). Reverse flashing. However, even if the housewrap or building paper had worked as intended, there still would have been problems caused by incorrect flashing. At the base of the frame walls, where the upper-story cladding meets the lower-story cladding, builders typically install a metal expansion-joint component over the building paper and run the building paper down into the top edge of the lower-story stucco coat (Figure 5). This detail allows water running down the upper-story drainage plane to flow into the lower-story mass wall, rather than move to the exterior. Builders can't be blamed for using this detail: The method is required by the code, as interpreted by local officials.

Figure 5.The joint detail typically required in Florida between upper-wall and lower-wall stucco sections (top) does not manage water well. When the control-joint accessory is installed over the top of a continuous sheet of building paper or housewrap (bottom left), water is not kicked to the exterior but is allowed to pass behind the joint into the wall below (bottom right).

Lower-story mass walls. Masonry-block walls with stucco cladding aren't intended to function as drained assemblies. They are "mass assemblies." Water that penetrates the stucco through cracks is supposed to be absorbed by the masonry mass, which it doesn't damage, and then dry slowly to both the exterior and the interior during periods of dry weather (Figure 6). Central Florida's masonry walls, for the most part, were able to manage moderate amounts of rain but were simply overwhelmed by the huge water onslaught from three consecutive hurricanes.

Figure 6.Masonry walls are intended to absorb and store minor amounts of penetrating water, and then allow the moisture to escape later during dry periods. But when their storage capacity is exceeded by continuing water intrusion, they can let water enter the home at the wall base or bleed through onto interior finishes.

Rather than suggest a modification of the stucco materials or installations, I've suggested a few small design changes that would enhance the ability of these systems to absorb and dissipate water without allowing it to enter living space or damage finishes. But even improved masonry mass walls shouldn't be expected to handle rainwater that leaks in from windows, service penetrations, or other holes in the wall assembly — and it turns out that leaks at windows and other openings were a major source of water intrusion during Florida's storms.

Windows and Other Penetrations

The industry standards and building-code rules for windows don't require them to be leakproof when facing the kind of wind and rain that central Florida saw in 2004. Windows installed in Florida homes are rated for water holdout at 15 percent of the design wind load, or no lower than 140 pascals of pressure (approximately the pressure of a 35-mph wind). Clearly, these limits were exceeded during August and September of 2004. In a hurricane, the codes expect windows to stay in the wall, but not necessarily to hold out all the rain.