Hey all,

Been looking through the forums to see if I can find something similar to my issue and I can't quite. So I thought I would ask.

First, I will say that I am aware of what my subfloor SHOULD be, reading a number of threads on subfloor and the recommended thicknesses of it, etc (IE the target is 3/4" T&G plywood).

So my situation:
We recently bought a house built around 1980 that the upstairs just had some awful floors. The two bedrooms that are getting our attention had this awful brown shag carpet that likely hadn't been changed since the house was built. The other two bedrooms and the hallway have linoleum that is also very dated. Because the downstairs had solid hardwoods, we're going to do the same upstairs and give the floor a uniform look/feel.

We pulled out the carpet and pad from one of the rooms. Underneath was particle board. That had some water damage and needed to come out. So we pulled that out and found plywood. This also has suffered some water damage, so that was going to be patched ... and in cutting into it, it was only found to be 1/2" thick. The joists are spaced a normal 16", but the 1/2 plywood clearly sags underfoot in the mid-gaps. As I got to the room thresholds, it's clear that the whole floor, even under the linoleum, is this 1/2" plywood under 1/2" particle board.

We are only starting work in the bedrooms, but across the entire floor, we are going to be putting in hardwoods, so in the entire floor the particle board is being removed to get us to the plywood, but that brings us to my questions/concerns:

Clearly the subfloor needs to be brought up to 3/4" minimum thickness. That raises my questions:
1. Should I do this by adding another layer of plywood on top of this and getting my thickness to 3/4"? Or should I remove the 1/2" plywood and install new plywood, blocking the joists around the edge?

2. If doing a full replacement, which would eventually go out to the entire floor, are there any concerns I should have about doing that? I'm not worried about door clearance as they are on the docket to be replaced. But the walls are built on top of the plywood.

3. Are the weak/sagging spots a concern if adding a new layer of plywood?

Whatever the subfloor solution ends up being, I would put in an underlayment of quiet walk and then the solid hardwoods. I have installed these before, so I feel pretty confident about this, the subfloor is my main point of concern.

For completeness, these first rooms are around 300 ft^2. The hall is around 200 ft^2 and the other two bedrooms are around 500 ft^2 each.

The best solution is to add another layer of half inch plywood, or 5/8th inch thick plywood. Use sub-floor adhesive and 2 inch fasteners, minimum. Grind down high spots with 16 grit discs before adding a layer of roll roofing where you are an eighth inch low, before the plywood. You need a flat sub-floor to lay hardwood. A bead of glue between the joists adds strength.

you're right