Case of the Month: Falling Stucco
The homeowner reported the collapse of the stucco that was on the soffit (ceiling) of their attached patio. GEI was assigned to determine the possible causes.
Our expert visited the home and observed a portion of the roof/ceiling structure and soffit stucco of the patio structure at the side of the house.
The construction of the house was completed in 1994, and the homeowners were the original owners.
The covered patio structure was approximately fourteen feet wide by twenty-two feet long, and the soffit in the patio area formerly had a stucco finish. The cantilever portion of the roof (beyond the soffit area) was approximately two feet wide and did not have any stucco finish. At the time of his inspection, nearly all of the soffit stucco had fallen, with only a portion of the west side of the patio still partially attached to the ceiling structure.
There were potentially a half dozen possible causes of the stucco failure, working either alone or in concert.
The first possible cause was water. Sometimes seasonal rains or roof runoff can infiltrate into the soffit area and cause moisture in the wood members. Moisture compromises the holding power of the mechanical connectors and causes distress in the soffit stucco. Rain penetration comes from wicking action or wind-driven rain passing around the unsealed joints in the building. The homeowner did not report any recent roof water leaks in the patio area of the house, and our expert did not detect excessive fresh water stains in the roof/ceiling wood members of the patio or in the remaining portion soffit stucco. Some of the roof wood members had old water stains, but the homeowner stated that it had not rained since the soffit stucco had collapsed, so that was not a factor.
The second potential cause was vibration. Excessive vibration loads from construction activities in the neighborhood could have damaged the house structure, but the homeowner did not report any recent construction activity in the vicinity of this project nor was there any evidence of vibration-caused damage in other areas of the home.
The third possible cause was soil movement. Generally, excessive soil movement under the foundation of a structure of this nature can cause a movement in the roof/ceiling structure which could have lead to the structural failure of the soffit stucco. The building was located on a hillside slope, and there were some low spots in the lot close to the building. The foundation of the building was not fully visible, but there were old cracks in the concrete-slab floor of the patio. One of the cracks in the concrete-slab floor of the patio was approximately 0.030 inch wide by thirty-two inches long and was caused by soil movement under the slab over the years. Our expert also observed cracks in the exterior wall stucco siding of the house, and one of the cracks in the wall stucco siding at the west side of the patio was approximately 0.020 inch wide by thirteen inches long. It also was caused by soil movement under the foundation of the house over the years. There were small dents or chips in the concrete-slab patio floor, and the homeowner stated that the collapse of the soffit stucco had caused the dents or chips in the slab. The failure mechanism of the soffit stucco did not indicate a possible failure in the foundation of the building. Based on the field evidence, it was the expert’s opinion that the overall foundation of the building had experienced a soil movement over several years. It was also his opinion that the foundation movement was not the prime cause of the failure of the soffit stucco. The soil movement was also confirmed by the cracks in the exterior wall stucco siding in the other areas of the house and relatively away from the patio area.
A fourth possibility was wind. The homeowner was concerned that recent wind loads were a contributing factor to the soffit stucco failure. Sometimes, high speed wind loads can contribute to a soffit stucco distress problem of this nature. The Uniform Building Code states that every building shall be designed to resist the effects of wind of not less than 70 miles per hour. The weather data for the month indicated twelve miles per hour sustained winds, with a high gust of twenty miles per hour. Therefore, the wind loads were below design loads and not the prime cause of the soffit stucco distress. Our expert also observed a portion of the roof from the roof eave line and did not observe any fresh physical damage to the roofing material in the patio area due to a wind/storm related incident or any object hitting the roof structure. He also went to the attic space of the house and did not observe any fresh structural cracks in the visible portion of the roof rafters of the patio area.
A fifth possibility was earthquake damage. The homeowner was also concerned that past earthquake loads were a contributing factor to the problem. Sometimes an earthquake force can cause damage to soffit stucco. A search of the data available from the Northern California Earthquake Data Center for earthquake events measuring a magnitude of 4.5 and greater, from January 1994 to the date of loss, and approximately within the radius of one hundred kilometers of the project found nine seismic events during that period. The closest earthquake to the home had occurred in 1995, had a magnitude of 4.80, and its epicenter was located approximately ninety-one kilometers from the home. The most recent earthquake had occurred on December 12, 1996 and had a magnitude of 4.60. Generally this type of earthquake is felt but rarely causes any structural damage to a building, as long as the building is designed/constructed properly. These were low values, and for comparison, the Loma Prieta earthquake of October 1989, had a magnitude of 7.1. Because of the time of the reported damage to the soffit stucco and nature of distress in the soffit stucco, it was the expert’s opinion that the past earthquake loads were not the prime cause of the distress in the soffit stucco. This was also concluded from the fact that, generally, damages caused by an earthquake are visible immediately after the earthquake event and also are shared with other walls and ceilings of the building.
The sixth failure possibility was inadequate mechanical fastening of the stucco. Generally, in a soffit construction of this nature, stucco is connected to the ceiling joists with mechanical connectors (i.e., nails, screws, wire mesh, staples, etc.). The building contractor used metal staples to connect the stucco finish to the ceiling structure. Some of the staples connecting the stucco to the soffit structure were spaced at approximately five inches on center and penetrated approximately 5/8 inch into the wood. The weight of the soffit stucco caused the staples to progressively pull out of the ceiling wood members. This type of distress in the soffit stucco originated with the construction of the building and was aggravated with time due to normal wear-and-tear of the building material and wood shrinkage.
In conclusion, the failure of the stucco was caused by an original inadequate fastening design of the stucco to the house structure to support the weight of the stucco ceiling. This was a construction caused problem, and the progressive failure of the soffit stucco started from the first day the stucco was applied.