Fixing a Damaged Section of Subfloor
During the course of any project, you will run across unexpected obstacles. In preparation for a new floor (tile, wood or
sheet), there may be a section of the subfloor which is too "bouncy" or soft. There are many variables that will contribute to
this situation. Support, quality of the material, adequate thickness or history but the fact is; this section will have to be replaced.
Items you will need:
Replacement Subfloor material
In order to get over this hurdle successfully you will need to determine where the joists are and plan your cuts
accurately. Popping a chalkline or using a straightedge and pencil, mark the area that you want to remove. Two sides should land on
the centerline of the floor joists (floor joists are usually 12" apart, Fig. 1. You can find the floor joists by locating the screws or
nails that hold the subfloor down.
Drill a hole through the middle of the area with a bit large enough to accommodate your measuring tape (usually 1") (Fig.
2). Measure the thickness of the subfloor through the hole (most subfloors are 5/8" or 3/4" plywood).
Set the depth of cut (deck) on your circular saw to the same measurement you obtained above (Fig. 3). This will ensure
that you do not damage the floor joists while cutting the subfloor.
Slide the blade guard out of the way. Start the saw and, while firmly holding the saw, ease the blade down into the
subfloor. Avoiding nails and screws, cut until the middle of the blade reaches the end of the line. It may be necessary to go slightly
past your marks to free the damaged section, remember that your blade is round not straight. In this case I used a hand held
angle grinder with a wood cutting blade to finish the cuts (Fig. 4). To clean the edges and corners of anything else, use a sharp
Repeat this for all four sides.
Using the hole you drilled earlier, pull the damaged section out (Fig. 5). If the circular saw didn't get all the way into
the corners, use a handsaw or an angle grinder with a wood cutting blade. When you have the section out, inspect the floor
joists. If they appear to be rotten or damaged in any way, stop and call a professional.
(This step is optional but recommended, especially if you are going to install tile.) Using an appropriate material (2x4's are
ok but use 2x6's if you can) frame a small box that will fit into the hole. This will support the two sides of the patch that do
not land on the joists. Run a bead of construction adhesive on the floor joists and screw the support frame in between the joists.
Using the piece you cut out as a pattern, cut a replacement. This replacement should be of the same material that you
removed. At least the same thickness material as you removed.
Run a bead of construction adhesive (Fig. 6) around the perimeter and then screw the patch into place (Fig. 7).