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Why is my stucco cracking?
Monday, July 9, 2018
There are three types of stucco finish walls on Florida homes: stucco on concrete block, Exterior Insulated Finishing System or EIFS (also called synthetic stucco), and stucco on wood frame construction. Stucco over concrete block has limited problems compared to the other two, and EIFS already has well-known and documented moisture intrusion problems, along with lawsuits dating back to the mid-1990s (see What is the difference between EIFS and stucco? for more about EIFS). So let’s look at the defects found in the third type: stucco that is applied over a paper-backed metal lath on wood frame wall construction, which was especially popular in Florida during the building boom of 2004 to 2008.
When buckling, ripples and stains appear in stucco, like in the photo above, homeowners get worried. But the trouble begins with small cracks like the ones shown below, barely visible, that let water into the wall.
The inherent problem with stucco on a wood frame structure is that wood moves around—expanding, shrinking, and sometimes twisting—with changes in humidity. Wood is also somewhat flexible. Stucco, on the other hand, is comparatively stable and stiff, but it expands and contracts with changes in temperature more than wood. When you apply a stucco surface to a wood wall, there must be built-in details to keep the differing movement of the two materials from cracking the less-flexible stucco.
The Florida Building Code uses the ASTM C-926-06 specifications for the application of stucco, which refers to it by the more technically correct name of “Portland Cement-Based Plaster.” The specs are based on these five time-tested standards:
1) THICKNESS - Stucco should be at applied in three coats, at least 7/8” thick (not including any texture) to resist cracking.
2) FLEXIBLE CONTROL JOINTS - Placed at regular intervals along the wall, they absorb the expansion and contraction of the stucco due to temperature changes.
3) WEEP SCREED - A weep opening at the bottom of the wall lets any water that penetrates the stucco drain out behind it, instead of getting trapped and rotting the wall framing. When a wood-frame second floor is built on a concrete block first-floor structure, the weep screed will be a strip located at the bottom of the second floor level.
4) CASING BEADS - These wrap around anything that penetrates the stucco surface—such as windows, doors, and soffit returns—to provide a gap that can be caulked and prevents hairline cracks that will admit water into the wall.
5) DRIPS - At any change of plane from a vertical to a horizontal under-surface of the stucco, a drip edge lets water fall off at the corner and not migrate sideways due to surface tension.
All of this differs dramatically in complexity from the installation of regular siding, which depends on simple down-lapping of smaller pieces of building material for waterproofing, and movement is absorbed by the numerous overlapping joints, plus caulk around doors and windows.
If any of the five anti-cracking measures are ignored, you will have a stucco problem. Maybe not immediately, because it takes a few years for the initial small cracks to let in some water, which rusts the steel lath, and opens the cracks further, letting in even more water...and so forth. But it will happen.
Here’s a listing of how each one of the five can be done wrong:
1) THICKNESS - When the total of the three coats of stucco dips below 7/8-inch thick, those areas are more prone to cracking. Sometimes only two coats are applied, with not enough curing time between coats. Also, if the backing paper and lath is sloppily installed, it can create pockets of thin coverage.
2) FLEXIBLE CONTROL JOINTS - The total area of stucco between control joints should not exceed 144 square feet, with the additional restrictions that the joints not be more than 18 feet apart along the wall and a length-to-height ratio that does not exceed 2.5 to 1. The control joints should be tied to the metal lath only, not attached to the wall sheathing underneath, so that the joints can move independently from the wall structure. Metal lath that is continuous behind the expansion, connecting both panels, defeats the joint. Control joints that are placed too far apart or attached directly to the wall sheathing will also not do their job. A crack along the side of a control joint is an indication that it was likely not installed properly.
3) WEEP SCREED - No matter how carefully stucco is installed, some small cracks will appear over time. Trapped water wets the wood structure and starts rot when there is no opening at the bottom of the wall or the opening is obstructed. Some weep screeds have protective tape over the drain holes that should be removed after installation and it gets forgotten.
4) CASING BEADS - Sometimes they are simply not installed. Without a groove to apply flexible caulk, cracked stucco along the side of a window frame is a common place for the stream of rainwater that runs down the side of a window to enter the wall.
It begins as in the photo above, but buckling stucco and staining follow over time. The photo below shows typical damage in the wall framing from this defect.
Here is a diagram of how one brand of stucco window casing (E-Z Bead) is installed, along a closeup of the product.
5) DRIPS - They are not as aesthetically pleasing as a simple corner bead where vertical surfaces return back horizontally, but ugly water intrusion damage ensues if a 1/4” minimum drip edge is not installed. The photo below shows the rotted wood sheathing found under the stucco at the corner of an open porch with this defect.
Other defects that can cause stucco cracking include: not enough fasteners securing the metal lath, undersize fasteners that do not penetrate deep enough into the wall sheathing, improper lapping of the building wrap to the weep screed, and not leaving the required 1/8” gap between sheathing panels. Because many of the defects that allow water entry are concealed by the stucco itself, they cannot be verified without digging into the wall. But their symptoms bloom and spread on the wall surface over time.
It usually takes five to seven years or more from time of construction to see clear signs of distress in stucco walls that are the result of defective stucco installation. But every stucco finish will develop a few hairline cracks, so we recommend checking for them at least once a year, and sealing the cracks with a masonry caulk.
Repainting the walls and touching up the caulking every 7 to 10 years is also a good idea, since paint and caulk are your first layer of protection from water intrusion. Because home builders occasionally claim that inadequate maintenance of the wall finish is a contributing factor in stucco failure claims from their customers, your diligent maintenance may have the added benefit of helping you secure your claim for damage to your home if it is due to defective stucco installation.
When the cracks multiply and get worse, in spite of your maintenance and repairs, we suggest calling a professional inspector for further evaluation. The one you choose should be familiar with the installation standards and have some experience in diagnosing stucco problems, plus carry a couple of moisture sensing tools, such as an infrared camera and an electronic moisture meter, in their tool bag.
If you are wondering why older homes with stucco walls don’t have the same severe cracking problems as outlined above, it is because most of them are stucco over concrete block. The block has a similar rate of expansion and contraction as stucco, and concrete block is more forgiving of a little moisture intrusion. It can absorb and dissipate through evaporation any small amounts of water that penetrate the stucco.
The thickness of stucco, along with sufficient curing time between coats, also makes a stronger surface, and older homes that are of similar stucco-over-metal-lath are more likely to have been done correctly.
And one last note. Just because you have a concrete block house does not mean you are exempt from the problems of stucco-over-wood-frame construction. Many CBS homes have entry porticos that are built at the upper part with wood framing. Two examples are shown below. The first is 25 years old and cracking where a drip was not installed, and the second is a home under construction that shows the typical transition to wood frame above the entry—that’s only visible before the stucoo is applied.
Also see our blog posts How do I pick the right contractor to fix my stucco? and How can I tell if a crack in a stucco wall is a structural problem and what is causing it? and Is the stucco on a wood frame house allowed to extend down into the ground? and What are the pros and cons of concrete block versus wood frame construction?
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To learn more about exterior walls and structures, see these other blog posts:
- Some photos above provided by Mark Cramer.
- Illustrations by CodeCheck
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