I have looked for the answer to this question and not found it, so here goes. Having just purchased a new to us house, the wife would like hardwood upstairs. Looked under the existing carpet and the flooring consists of the following:

Subfloor-1x12 solid boards (pine?) laid perpendicular to the joists, which are 16" o.c.
Underlayment-3/8" particle board nailed to the pine boards

Now I know that I cannot put solid oak on the particle board, so I may have to rip that up. It appears that the PB goes under all the door jambs and walls, so what is the best method of removal? Using a circular saw to cut through the PB would work, but it would leave some material all around the perimeter. Is there a tool to cut flush to the vertical surface? Then I could remove the PB and possibly beef up with plywood. Is that necessary? What thickness.

Thanks for answering

You can run an undercut saw next to the walls. Some have a depth adjustment, some don't.

Another option is to install a floating floor OR a glue down floor over your particle board underlayment. Sounds as if the p. board underlayment was installed prior to framing up the walls. To install a nailed down floor, the p. board should go. That 1x12 solid subfloor is stout enough but I'm concerned about it's stability. So I'd consider adding another layer of plywood. 3/8" p. board breaks pretty easily. You may find you can "chip" it out along the edges, although sawing it out would be easier.

Gary,

Thanks for the reply. The wife has an aversion to a floating floor from a previous home, so if there is a way to get the solid 3/4" oak, then that's what the job calls for. I expected to have to remove the PB. What I was curious about was the pine and it's suitability as a substrate. I will price the installation of 1/2" ply to replace the PB. I assume the standard approach of stggered seams will do, but should they line up on the joists? My experience with tile floors says that the underlayment should NOT be nailed into the joists, only the subfloor. That allows for the whole surface to move together. Does that make sense? And do you gap the plywood at all? Appreciate the info.

It's always wise and recommended to gap your underlayment. 1/8" will do but if it's bigger it's no a big deal. Just avoid butting it up tight. If I were you, I'd re-fasten those pine planks subfloor boards with screws or nails. Then staple the 1/2" CDX plywood using 16 gauge staples with a 7/16" crown and a 1&1/4" leg. Staple it well. 4" on the perimeter and 6" in the field. You can glue and screw if you prefer. Going into the joists is irrelevant. Stagger plywood seams as well as possible. You could probably get by with a 3/8" plywood, which will be close to what your 3/8" p. board was.