Things You'll Need
Drill with screw set
Any weight placed on your floors must be supported by a subfloor, whether the weight is the result of the floor tiles or floorboards laid over the subfloor, or whether it's a heavy cast-iron bathtub or floor safe. If the subfloor isn't strong enough, this can cause a constant squeaking in the affected weak section or sagging of the subfloor panels and joists. Reinforcing the subfloor with blocking adds strength to the surface, simply by stiffening the floor joists. If your subfloor is accessible from underneath, the process of reinforcing it is a quick one. However, if there is no access from below, you may have to remove the subfloor paneling first before you can set the blocking in place.
Blocking with Access from Below
Gain access to the subfloor from underneath for easier placement of the blocking. For first floor levels, this can be from a basement; for the second floor, access can be from beneath an exposed ceiling or a suspended ceiling with the ceiling panels removed.
Measure the width of the floor with a tape measure. The width is the line of the floor that runs perpendicular to the run of the floor joists. Divide the width by half to determine the location of the center point of the width of the room. Place a chalk mark on both ends of the floor at the center point. Run a chalk line between the two points, then pull it away from the floor. Release the line to snap a line right down the center of the room, crossing all of the floor joists.
Measure the spacing between the joists with the tape measure. The spaces should be a uniform width, except for the last two joists on the end of the room, which may be closer together due to space limitations when the joists were installed initially.
Cut 2-by-4-inch planks into blocks that are the same width as the gap between the joists, using the circular saw.
Place a ladder beneath the chalk line so that you can reach the joists easily. Place wood glue on both ends of one of the blocks and along the top edge. Climb the ladder, then place the block between two of the floor joists, with the edge of the block aligned with the chalk line running along the center of the floor. The block should fit snugly between the two floor joists on either side of the gap. Make certain the block is flush with the surface of the subfloor above by pressing up on the block to make contact.
Use the drill with screw set to drive two screws through each of the joists at either end of the block and into the block ends, holding the block in place. Place two more screws through the block and into the subfloor above. Make certain you do not use screws that are so long that they run all the way through the subfloor and pierce the subfloor's surface.
Repeat the process with the gap between the next floor joists, except align the edge of the board on the opposite side of the chalk line to stagger its placement. You want the boards to stagger on both sides of the line all the way across the floor to avoid creating a straight line of blocking. Continue to place the blocks until you have reinforced the entire subfloor width.
Blocking without Access from Below
Remove the subfloor panels running the width of the room. If you don't know which direction you should place the blocks, run a stud finder on the floor to determine the location of the floor joists. Remove the panels down a line in the center of the room running perpendicular to the direction the joists run. Use a pry bar to pull up the subflooring panels from the edges. Pry a short distance along the edge of the panels at a time, to slowly wedge the panels up. This will help you avoid breaking the panels as you remove them.
Snap a chalk line across the floor joists, then proceed to install the blocking between the floor joists. Glue the two ends of the blocking in place. Place a carpenter's level over the blocking to make certain that it's flush with the tops of the floor joists. Drive screws through the joists into the ends of the panels to secure them in place.
Replace the panels on the floor, nailing them back to the floor joists. Locate the new blocking with the stud finder, then drive screws through the paneling into the blocks to secure the boards to the planks.
Wear safety goggles and a face mask when cutting the wood to avoid getting wood dust in your eyes and lungs.