Roof and Floor Decking / Sheathing
Roof and floor decking cover the roof/floor structural members (trusses, joists and rafters), and may act as a structural diaphragm. Roof sheathing accepts the roofing (membrane, shingles, tiles, metal), and floor decking accepts the finish flooring (carpet, tile, vinyl).
The terms ‘decking’ and ‘sheathing’ are often used interchangeably. However, ‘sheathing’ generally refers to plywood or oriented strand board. These are the most common materials used for roof, floor, and wall sheathing. Alternatively, older structures may have floor decks with 2x sawn lumber planks, and roof decks with 1x sawn lumber.
PLYWOOD:
Plywood is composed of plies, or layers, of wood sometimes referred to as veneers. The veneers are shaved from a log in a continuous sheet. The individual plies are placed cross grain to each other and glued together. The perpendicular wood grains increase strength and dimensional stability. The quality of the wood and the type of glue used in the manufacturing process vary with the grade and intended use of the plywood.
Plywood is commonly available in 4’x8′ panels with thicknesses varying between 1/4 to 1 inch. A 1/2 or 5/8 inch thickness is often used for roof sheathing, and a 3/4 or 1 inch thickness is common for floor sheathing. The thicker plywood used for flooring reduces deflection felt by occupants. The spacing of the support joists or trusses, the expected loads, and local building code determine the thickness required. The thickness of plywood is actually 1/32 inch less than the designated fraction of 1/2 inch, 3/4 inch, etc. For example, 5/8 inch plywood is 19/32 inch in actual thickness.
Pressure treated plywood is used in locations where moisture or other adverse conditions are present. Modern pressure treated wood is commonly treated with a copper solution, often alkaline copper quaternary (ACQ). The copper reacts with steel, accelerating the corrosion of unprotected fasteners. For this reason, nails and other fasteners used in pressure treated wood should be coated or stainless steel.
Pressure treated wood prior to 2004 often used chromated copper arsenate (CCA) which contains arsenic. This worked better at protecting from rot, and did not contribute as much to corrosion. Pressure treated plywood is usually heavy and wet with solution when purchased. Therefore, it shrinks and is more prone to warping as it dries. Sufficient time should be allowed for its moisture content to stabilize with its surrounding environment prior to coating or applying a brittle finish such as stucco or tile.
Plywood is prone to layer delamination when exposed to moisture, although this delamination tends to return to near its original state when dried.
Oriented Strand Board (OSB):
OSB is an engineered wood product made of wood strands (chips) pressed and adhered together with heat. Because it is manufactured from smaller “pieces” of wood, it does not require the use of mature trees. The structural characteristics of OSB are similar to plywood, and like plywood it is widely used throughout the United States as roof, floor and wall sheathing.
Unlike plywood, which may contain voids due to missing knots or other natural deformities in the wood veneers, OSB is solid throughout its cross section, and is typically more uniform and less expensive than plywood. However, it is heavier than plywood (~15% to 20%), and more prone to permanent moisture swelling due to the release of compaction stresses induced during manufacturing. This is particularly true at the panel edges. OSB is typically less resistant to impact, and is not as stiff as plywood. Thus, it is more prone to immediate and long-term deflection. There is also additional concern with fastener holding problems and dimensional stability when using OSB for roof decking.
SAWN LUMBER PLANKS:
Sawn wood lumber boards, or planks, 1-1/2 inch thick are also commonly used for floor decking. The width of the individual floor boards is significant with regards to point loads. When loads are applied over an area, 2×4 decking is as capable as 2×6 or 2×8 decking. However, if a point load happens to be applied to only one board, then a wider board is obviously stronger than a narrow board.
Sometimes floor boards are interlocked with each other by a tongue and groove mechanism. This has several advantages: point loads are transferred to adjacent boards, and movement is restrained across the boards, resulting in a quieter, tighter and smoother floor.
CONCLUSION:
The nailing pattern of roof decking must be sufficient to resist expected uplift and shear forces. Often, replacing the roofing material requires that the roof decking be re-fastened to meet current building codes. Roof and floor decking edges should be properly staggered. A gap of approximately 1/8 inch should be maintained between roof sheathing panels to allow for any panel expansion. Floor decking is often glued to the support joists or trusses in addition to the use of mechanical fastening. This helps to reduce squeaks and vertical movement which may affect finish flooring.