The facts presented in this blog lead to an interesting story. During the heyday of the residential construction boom in coastal South Carolina circa 2005, many General Contractors were forced to go out-of-state to find framers and other subcontractors due to the demand creating a local labor shortage. It turned out that the state of Texas had excess capacity and availability of framers and carpenters. Many Texas framing crews came to South Carolina to satisfy the shortage. It soon became apparent that the Texas crews, many of which were from inland locations, were accustomed to installing 4’x8’ exterior wall sheathing with the long dimension vertical, i.e., parallel to the studs. It is especially important and required that the long dimension be oriented perpendicular to the studs. Laboratory testing has shown that the shear walls, non-shear walls, and horizontal diaphragms are stronger with this configuration. Increased strength is achieved in both the longitudinal and transverse directions, improving the ability of the sheathed assemblies to resist lateral, shear, and orthogonal loading. When the local building officials performed the rough framing inspections of these structures, the exterior wall sheathing as installed by the Texas crews was written up as a deficiency. Engineering approval of the originally installed configuration was unlikely, due to the building locations being in a hurricane wind zone and a seismic zone. The only corrective action was to remove the sheathing and reinstall it properly. As one can imagine, this required major rework and trashing some of the new, expensive sheathing material. The removal of the nails alone was extremely time consuming and laborious.
Proper installation of structural sheathing is a critical aspect of the construction of wood-framed structures. Structural-grade plywood or oriented-strand board (OSB) is used to sheath the exterior walls, roofs, and floors of these buildings. Plywood or OSB classified as “Structural I Exposure I” by the American Plywood Association (APA) is typically specified. Structural I provides enhanced racking and cross-panel strength properties. Exposure I is used for applications where construction delays may be expected prior to the finished coverings being installed. Gypsum wallboard (GWB), i.e., drywall or sheetrock, is typically used to sheath the interior of the exterior walls, the interior walls, and the ceilings. Both types of sheathing are available in 4’x8’ sheets. The installation of this sheathing serves many purposes, including:
- Laterally and diagonally bracing the framing members
- Providing the frame walls with shear strength, uplift restraint, and wind pressure resistance
- Forming the floor, ceiling, and roof assemblies into diaphragms
- Protecting the framing members from the weather, impact, and wear-and-tear
Of primary importance is the fact that the sheathing creates horizontal diaphragms, or “platforms”, out of the floor, ceiling, and roof assemblies. These diaphragms effectively transfer shear forces imparted into the frame walls by lateral wind and/or seismic loading throughout the structure from the foundation to the roof ridge.
To effectively perform their purpose, the sheathing panels must be properly fastened to the framing members they protect. This means that:
- The exterior wall plywood/OSB sheathing must be properly attached to the exterior side of the exterior wall studs
- The exterior wall GWB sheathing must be properly attached to the interior side or the exterior wall studs
- The interior wall GWS sheathing must be properly attached to both sides of the interior wall studs
- The plywood/OSB floor sheathing must be properly attached to topside of the floor joists/trusses
- The GWB ceiling sheathing must be properly attached to the underside of the ceiling joists
- The plywood/OSB roof sheathing must be properly attached to the outside of the roof rafters/trusses
- The sheathing panels must be installed with the long (8’) dimension perpendicular to the framing members
Refer to the details below. In the first illustration, proper attachment of exterior wall sheathing is depicted. In wood-framed structures, the exterior walls are customarily relied upon to transfer shear loads. In addition, the sheathed frame walls must resist windward positive wind pressure and leeward negative wind pressure. The key difference between shear walls and non-shear walls is that the intermediate panel joints perpendicular to and between the studs are blocked. This means that the lengthwise, long (8’) edges of the sheathing panels can be nailed off to the blocking with a nail spacing much closer than the stud spacing. This blocking may be oriented as flat-wise “purlins”. In the second illustration, proper attachment of the floor, roof, ceiling, and interior wall sheathing is depicted. As shown, no blocking is required at the panel joints perpendicular to and between the framing members for these assemblies. The most common method of attaching plywood/OSB sheathing to framing is via common or ring-shank nailing. However, structural staples may also be used. GWB sheathing is attached to the framing via drywall screws.
As the reader can see, proper installation of structural sheathing is very important to the lateral load-resisting capacity of a wood-framed structure, and to its structural stability as a whole. It is a critical link in the chain of the construction of a safe, sound, and secure wood-framed building.
George Sanford, PE, holds a Bachelor of Science in Mechanical Engineering from North Carolina State University in Raleigh, North Carolina. George has more than 20 years of applied structural engineering experience specializing in residential, commercial, and industrial structures and foundations. Throughout his career, George has designed and analyzed structures, supervised engineers, prepared construction documents (drawings and specifications). He has an in-depth knowledge of many building codes, standards, rules, and regulations including the agencies that govern and provide guidance to building designers such as the International Code Council (ICC) American Society of Civil Engineers (ASCI), Steel Joist Institute (SJI) and the American Iron and Steel Institute (AISI).