Should Subflooring Run Vertically or Horizontally With Floor Joists?

Subflooring Properties

• Subflooring comes in 4-by-8-foot sheets of plywood or oriented strand board. The sheets, which must specify that they are suitable for subflooring, feature tongue-and-groove assembly along their long edges. The shorter ends of the sheets are blunt. Typically, subflooring sheets are about 3/4-inch thick, although additional thicknesses are available for custom-floor needs.

Butt Joints

• By positioning the long edge of subflooring sheets at a 90-degree angle to the direction of the floor joists, the blunt ends of the sheets meet on the center of a joist. This is called “breaking on center.” An 8-foot-long subflooring sheet spans six floor joists. When the blunt ends of two panels meet on the center of a joist, they form a butt joint. The joist beneath supports the ends of the panels so they cannot sag.

Tongue-and-Groove Edges

• Since the blunt edges of the subflooring sheets meet on the joints, the long edges need a different type of support. That’s where the tongue-and-groove assembly comes in. Each sheet has a protruding lip that runs down one long edge and a deep groove that runs down the opposite edge. When installed correctly, the groove on one sheet fits snugly over the lip of an adjacent sheet, strengthening and supporting the long edge joints so they do not sag.

Installation

• Standard subfloor installation includes running a 1/2-inch-thick bead of subfloor adhesive, available in large tubes that fit in caulking guns, down the center of each joist before setting a panel. When the panel is in place, the builder inserts decking screws or 8D nails every 6 inches, 1/2-inch away from the edges of the butt joints and in every floor joist beneath the center of the sheet.