The homes I studied in Wilson were all built between 2004 and 2009. This home was almost completely destroyed; shown here is an end wall, which was braced at the corners with OSB and had nonstructural extruded polystyrene as infill sheathing. The home's bottom plate - shown with bits of the foam sheathing still attached - was intact; structural wall sheathing would have greatly strengthened this connection between the wall studs and the bottom plate.
A nearby home used plywood corner bracing and foam infill; a closeup view reveals that interior drywall - which would not have been present on the attic gable above - probably helped hold the wall together. The opposite gable showed similar damage.
Though this home's roof remained largely intact, several of the exterior walls were blown out - a testament to both the tornado's lack of strength and the inability of poorly executed framing connections to maintain integrity of the building envelope. The back corner is still supported by the OSB corner bracing panel. A braced corner in front was pulled off when its connection to the bottom plate failed. On the opposite side of the house, the foam sheathing at the gable truss is almost completely stripped from the framing. Below, the foam-sheathed portion of the wall was lost, though the OSB corner bracing is still in place.
This home in western Alabama was completely blown off its foundation by a tornado that came through the area around 5:30 a.m. on April 27. Remarkably, the family inside the home was swept away from the building, though no one was seriously injured. A length of sill plate was the only piece of the exterior walls that remained on the slab; the sills had been attached with rectangular cut nails.
A nearby home lost all its exterior walls; its sill plates had been fastened with spiral shank nails 24 to 48 inches on-center.
Another home in the same neighborhood lost its garage door and had its front garage wall pushed in - apparently by wind pressure, as there was no sign of impact. Gable-end failures occurred on the front and right side of the house. The home's masonry safe room, at the front center, was undamaged, and may have helped strengthen the home against the tornado's forces.
Seen here is the back right corner, where the gable roof over the children's bedrooms was lost. Like most of the homes in the neighborhood, this one was fully sheathed with OSB; pink housewrap and vinyl siding covered the sheathing. A closeup shows a typical truss-to-top-plate connection; the four toenails, placed in opposing pairs, caused the bottom chord to split.
The gable end was stripped from this home. The bottom chord of the gable truss had been attached to the plate below with 16-penny toenails more than 2 feet on-center; the roof sheathing was secured with nails every 48 inches and staples every 12 inches between the nails.
Though heavily damaged, this house may have fared better than nearby homes because it has a hip roof, which is better than a gable roof at resisting high winds. The wall seen here was knocked in by an impact that occurred between the two windows in the middle of the photo. Though the wall was sheathed with OSB - which helped hold the framing together - the connection at the top plate failed.
Another view of the same home shows missing sheathing on one section of the roof, probably related to loss of the lightweight vinyl soffit directly below.
These prescriptive recommendations are based on the APA technical report "Building for High Wind Resistance in Light-Frame Wood Construction," available at apawood.org/tornados.