Before fixing the subfloor under the carpet you ll need to figure out why the subfloor became damaged in the first place.
How to fix a hole in sub floor.
Repeat on the other parallel exposed joist.
Measure and cut the plywood use 5 8 cdx plywood to replace the original subflooring.
Hammer or screw the birch plywood every four inches along each seam.
Cover the entire area where the vinyl.
These boards will now support the new plywood that you re going to cut to replace the damage.
Get your wood glue and apply a generous line around the edges of the opening you routed and then a bead along all for edges of your patch and press it into the opening.
Measure and cut plywood as needed to cover the damaged area.
Birch plywood is extremely smooth with no surface voids visible grain or high sections.
Remove the board and insert a new sheet in.
Remove the screws when replacing a full sheet of subfloor.
Usually water is the culprit in which case the source of the water leak must be identified and fixed or you could end up with the same problem down the road.
I wanted to use the largest pieces of wood i could fit between the joists under the subfloor to disperse over the largest area possible any weight that will eventually be put on this patch.
This exterior grade plywood is a good choice for damp or occasionally wet areas because it can withstand a moderate amount of moisture without deteriorating.
The first step is to put some dead wood underneath the hole so you have a way to secure the patch.
Sister a support board for the new plywood take a 2 x 4 or 2 x 6 2 x 8 and screw it into the exposed joist being careful that the top edge aligns with the floor joist.
Cover the patch with a sheet of cling film saran wrap or use a plastic grocery bag.