How to Build a Subfloor
Complete step-by-step instructions, including how to install carpet as the finishing touch
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When renovating a garage or a room, you need to begin planning from the floor up. If it’s necessary to put in a subfloor, here’s how to do it:
Materials and Tools:
6-millimeter plastic sheeting
pressure-treated 2-by-4-inch boards
pressure-treated 2-by-6-inch boards
4-by-8-foot sheets of plywood flooring
1. Use a builder’s transit to check the floor for levelness. If it’s level, remove the baseboards remaining along the wall and then sweep off the floor to remove any debris. If it’s not, you can pour self-leveling concrete to bring it to level.
2. Lay down a 6-millimeter sheet of plastic on top of the slab to act as a vapor barrier. Use a utility knife to cut the plastic to size, and secure it to the bottom of the wall with staples.
3. Attach pressure-treated 2-by-4’s to the floor, using an actuated nail gun to drive the nails through the 2-by-4s and into the concrete. Be sure to wear safety glasses and ear protection, as nail guns are quite loud.
4. Use a stud finder to locate the studs, and then drive 16-penny nails through the 2-by-6 band and into the studs. Cut joists to length and attach to the outer band by toenailing them into the 2-by-6’s with 16-penny nails. Continue working along the length of the room until all the joists are installed and the floor framing is complete.
5. Place insulation between the joists, keeping the paper side up and being sure not to compress the insulation. Cut the insulation with a utility knife. Wear long sleeves, a dust mask and eye protection whenever you work with insulation.
6. Measure out 4 feet from a corner and snap a chalk line across the joints to ensure that you keep plywood straight during installation. Apply the subfloor adhesive to the joists with a caulk gun. Put the first 4-by-8 sheet of plywood in place, line it up with the chalk line, and use a T-square to make a straight line along the top of the plywood to mark the location of the joists.
7. Use a screw gun to secure the plywood to the joists with 2½-inch screws. Continue the same process with the rest of the plywood, applying glue to all the joists and securing the plywood with screws. Be sure to stagger your joints. Use a sledgehammer and a scrap piece of 2-by-6 to help push the pieces of plywood together and ensure that you have a snug fit. Cut plywood to fit with a circular saw.
8. Install baseboards around the room and secure them to the wall with finishing nails. Use a pneumatic nailer to speed up the process. Use a scrap piece of baseboard to leave an even ¾-inch gap between the baseboards and subfloor so that you can tuck the carpet in around the edges. Use a stud finder to locate studs and drive nails through the baseboard and into the stud.
9. Nail tack strips with the teeth inclined toward the wall to the plywood subfloor. Start in a corner. Be sure to leave about a ½-inch gap between the strip and the wall.
10. To install carpeting, first roll out the padding and cut it to width with the carpet knife. Leave the padding an even distance from the tack strips. Attach the pad to the floor with a hammer stapler.
11. Roll the carpet out, overlapping the walls by about 2 inches around the perimeter of the room. Use a carpet stretcher to hook the carpet over the tack strips.
12. Make sure the carpet is well seated on the tack strips, and then stretch and tuck it under the baseboards. Cut away excess carpet along the wall, and continue tucking the carpet under baseboards with a stair tool.
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