Homeguides – The subfloor is the decking installed on top of flooring joists; the finished floor is then installed on top of the subfloor. The thickness of plywood panels in a subfloor is regulated by the spacing of the joists. This number is known as a span rating. When installing a plywood subfloor on top of the slab at grade, the thickness is governed by how long the fasteners for the flooring will be.
Joists and Spacing
Floor joists are made from 2-by-8-inch or 2-by-10-inch lumber, or from engineered quiet beams, which are made from a thin layer of chipboard on edge between two lumber caps forming an “I” shape. Floor joist spacing has traditionally been 24 inches on center, or from center to center of the joists. Newer construction has floor joists spaced at 16 inches on center. You will need to determine the spacing before installing the joists, or measure the distance between joists for existing construction.
CDX plywood is traditional wood plywood made from thin layers of lumber, known as veneers. Glue is sprayed between each layer and the sheet is hot pressed. CDX plywood should be a minimum of 7/16 inch thick. The thicknesses are as follows: 7/16-, 15/32- and 1/2-inch-thick panels require joists spaced 16 inches on center; 19/32- and 5/8-inch-thick panels require joists spaced 20 inches on center. For retrofitting older construction, 23/32- and 3/4-inch-thick panels can be used over 24-inch on center joists. The heaviest rating is for 7/8-inch-thick panels, for which joists can be spaced 32 inches on center.
Oriented Strand Board
Oriented strand board, or OSB, is a wood structural panel that can also be used for flooring. It is made from shredded strands of wood pressed together with wax and a special adhesive, then hot pressed into a panel with up to 50 layers of strands. OSB can be used for subfloors, although it is not as common as CDX. For 19/32-inch-thick panels, the joists should be 20 inches on center. For 23/32-inch-thick panels, the joists should be at least 24 inches on center.
Sturd-I-Floor is a specialized engineered wood structural panel expressly designed for single-layer flooring installations on upper floors, where carpet will be laid directly over the subfloor. These panels are more rigid than either traditional plywood or OSB. For 19/32- and 5/8-inch panels, joists should be 16 or 20 inches on center. For 23/32- and 3/4-inch panels, joists should be 24 inches on center. For 7/8-inch panels, joists can be up to 32 inches on center, and for 1 3/32- and 1 1/8-inch-thick panels, the joists can be spaced 48 inches on center.