How to Build a Subfloor of OSB or Plywood

The subfloor rests on the floor joists, and supports the finish floor. Most homes built since the 1960s have a subfloor of plywood or oriented strand board (OSB). With either OSB or plywood, the long axis should have a tongue on one side and a matching groove on the other. The long axis of the plywood or OSB is laid perpendicular to the direction of the floor joists. The subfloor is held in place with a combination of nails and glue.

Things You'll Need

• Measuring tape
• Circular saw
• Sledge hammer
• Subfloor adhesive
• 8d nails
• Framing hammer
• Chalk line

Instructions

1. Snap a line 4 feet in from the end of the building, perpendicular to the joists, beginning at a corner of the building and using a chalk line. Apply a ½-inch bead of subfloor adhesive to a section of the floor joists 8 feet wide and 4 feet long. Set the OSB or plywood in the adhesive with the tongue on the rim joist, and the groove on the line you’ve snapped. Align the sheet so that its ends break in the middle of a floor joist. Nail the sheet in place; nails should be spaced every 6 inches on the edges, and every 12 inches in the field.

2. Add the second piece of plywood or OSB, butting the short axis against the short end of the first sheet, and again aligning the groove with your snapped line. Continue to add sheets until you reach the end of the building. If necessary, trim the final piece to fit.

3. Insert the tongue on a half sheet (4 by 4 feet) into the groove on the previous piece to begin the second row. Drive the tongue in place with the sledgehammer, cushioning the groove with a scrap of wood to prevent deformation. Continue laying full sheets until you reach the end of the row, and then begin the next row with a full sheet.

4. Trim any overhang from the final row. Snap a line from one end to the other, and cut off the overhang with a saw.

Tips & Warnings

• To accommodate seasonal expansion and contraction, leave a 1/8-inch gap between all panels.

• Make sure each panel spans no fewer than three floor joists. This will occasionally necessitate cutting not only the last panel in a row, but also the penultimate panel.

• Keeping your nails 2 to 4 inches back from the groove will make it easier to install the next sheet.

• Always wear safety glasses or goggles when using power tools.

References

• "The Very Efficient Carpenter;" Larry Haun; 1998
• International Residential Code; R602.3(1)
• Home Addition Plus: Laying Tongue and Groove Plywood Sub Floor