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The house I recently purchased, built in 1973, has particle board subfloors. To clarify, this is not OSB and this is not just a particle board overlay over plywood -- the entire subfloor is 3/4" particle board (looks like oatmeal).

There are several issues with this subfloor: the floors are creaky and warped in places and I've been strongly advised not to install hardwood floors over particle board, as they cannot hold nails and will deteriorate if water is spilled. I purchased hardwood floors (pre-finished 3/4"). Now I need to either 1. remove the particle board or 2. nail plywood over the particle board. I'm opting to remove it because with option "2" I'll end up with undesirable height differences in places.

The problem with removing it is the existing subfloor runs under the interior walls, so once I cut it out and add new, there won't be any support where it overlaps the joists. Additionally, if the interior wall is not sitting on a joist, it could drop. This problem also occurs for doors and closets. To solve this I could install blocking, but then this could get very labor intensive. Another option is to cut along the joist and leave a margin of particle board along the walls. I could leave particle board in the closets since there is no load there.

Another component to this problem is in the kitchen. The previous seller put in new cabinetry and granite counter tops. I don't really want to tear out the cabinets due to the expense. I could cut the subfloor out around the cabinet but then have the problem of what is supporting the cabinets (parallel with joists).

I'd appreciate to hear from anyone that has ideas on the best way to proceed.

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up vote 4 down vote accepted

If possible, try framing out the edges. I would remove all of the existing sub floor (since it is particle board) and then add addition framing along the edges as needed for the new plywood. In cases where you can't meet a joist, go perpindicular and create a nice foundation for the floor to attach too.

I would make sure the floor has a firm foundation. It might be extra work to do it, but I would take those extra steps if possible.

enter image description here

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Great suggestion. This assumes the wall is not sitting on a joist. If the wall is sitting on a joist do you think I still need the framing? Would you suggest 2x8s for the framing/blocking? I don't have a good feel for how much subfloor would flex (cantilever) but one thought is that 3/4" hardwood, installed perpendicular to joists, would add substantial rigidity. Thanks and nice diagram! – Andrew Jan 31 '12 at 17:00
If your have a 2x8 near or under the the wall, I think you can just add a sister joist with blocking so that the plywood will have something to attach to. – Jon Raynor Jan 31 '12 at 17:22

Well, there really isn't a best answer, until you get into the job.

1-good tools make a good job. Buy or rent a compressor and a framing gun. To purchase, go to the pawn shops for good deals. Buying the right tools will greatly decrease the amount of labor.

2-if walls are installed on top of the particle voard, you have a code issue anyway. No inspector would allow it today, especially on that crap. It needs to be fixed. Buy a few jacks and some 4x4's. You can ballance the wall for a bit with these if you have to.

3-yes, adding plywood on top will give you issues with the heights of the floors. And doorways, closet doors, etc. It's a pain, since many doors are hollow-core, so you can't just cut them off..

So... Anything worth doing is worth doing right. It will cost you more right now but it will pay off in the long run. Good luck.

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I have only worked on one house like this. I talked to a house inspector in the area who has inspected almost 4000 houses, and he has seen this three times. In any case, I have come across similar situations with rotted subfloors, etc... Below is one solution - not ideal, but functional and relatively long lasting. You could try:

  1. Pull up the existing flooring and baseboards in the room you are working in.
  2. Use a skillsaw to cut through the particle board ~1-1/2" from the walls (use a sawzall or something similar to finish cuts). Try not to cut into the joists or blocking, so set your skillsaw depth carefully to 7/8".
  3. Remove the subfloor in the room leaving the 1.5" strip on the edges.
  4. Block the edges of the cut subfloor.
  5. Install 3/4" T&G sanded subfloor plywood where the subfloor has been removed. Use glue and screws. If you cannot get the T&G in, you can block the edges of the plywood, and use regular plywood instead.
  6. The walls should be sitting either on joists or blocking, and nails should penetrate into the joist or blocking. If not, you can use longer screws or lag bolts from underneath, two every sixteen inches to attach the wall. Watch out for wires, pipes, etc...
  7. Use a sander if the Particle board is slightly thicker than the new subfloor to level it out. If it is thinner, you will need to use some sort of floor leveling compound.
  8. Note that this is not really changing the structural rating of the existing floor - but it is a working solution that will allow you to get your work done!

Good luck.

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