Replacing subfloors, lots of questions

Discussion in 'Remodel Forum & Blog' started by 1320ms, Oct 20, 2009.

  1. 1320ms

    1320ms New Member

    I'm in the middle of remodeling a house. After ripping up carpets, hardwood, and laminate floors throughout the house, i see quite a few areas where the subfloor needs repair (I expected this)..

    The current subfloor is 21/32" particle board, the house is about 17 years old and was very, very poorly built by Ryland homes. In the family room there is a sliding glass door that was apparently leaking, most of the water damage is to the left of the door, all the way into the corner with the adjacent exterior wall, this is right up against the framing.


    Where the water damaged areas are up against the exterior walls, and the framing sits on top of the subflooring, what is the best way to go about replacing the flooring? Obviously I'm not going to jack up the bottom plate to remove the old subfloor and slide in a new piece, so do I just cut it at the bottom plate and lay new subfloor down? Does this in any way alter the structural integrity of the wall if I'm cutting the subfloor at the bottom plate?

    When this is all done I'll be laying down hardwood flooring in most rooms with ceramic tile in the kitchen and bathrooms, should I step up to 3/4" plywood and replace all of the subfloor throughout the entire house? Should I just replace the rotting particle board with more 21/32" particle board? Or should I replace the rotten sections with 21/32 plywood leaving a mix of particle board and plywood as the subflooring?

    Where the interior walls sit ontop of the existing subfloor, do I again just cut at the sole plate of the wall? If I replace all of the subfloor, following this method, all of the walls would be sitting on a piece of 21/32 particle board that is cut at the sole plates on all sides, again, would this alter structural integrity of the house?

    Thanks for the help
    Last edited: Oct 20, 2009
  2. LiamM

    LiamM New Member

    hi 1320ms - this is my two cents

    my first choice would be to rent a toe-kick saw to cut nearly flush with the bottom plate of the exterior wall. second choice would be to carefully use a sawzall with a long blade. my third choice would be to use a regular circular saw to cut within 2"-3" of the bottom plate.

    yes, butt the new subfloor up to the bottom plate, but leave a gap of about 1/8" - 1/4" for expansion.

    you'll have to put blocking under the edges of all your new subfloor, and nail (or preferably glue and screw) the new subfloor into it.

    where the walls are perpendicular to your floor joists, you can install full-height blocking (same height as your floor joists) or 2x4's laid flat to fit tightly in between the floor joists.

    where the walls are parallel to the floor joists, i'd install full-height blocking instead of the 2x4's laid flat. install them every 16". make sure the edges of the new subfloor panels fall onto the blocking.

    i don't think it's necessary for the blocking to provide support to the bottom plate of the existing walls. obviously, when you install blocking when the walls are parallel to the joists, it should fit the entire width of the joist bay - so it will support the bottom plate. but when walls are perpendicular, i wouldn't really worry about trying to slide half of the blocking under the existing bottom plate/subfloor.

    i also don't think it's necessary to attach any of the blocking to the existing walls...i.e. you don't need to screw upwards through the blocking into the
    bottom plate (or through the bottom plate into the blocking). but if you have access, it might be a good idea.

    no, not that i'm aware of.

    if time and budget allows, yes. or 3/4" OSB. i'd rather go with 3/4" OSB than 1/2" plywood, if money's a factor. if plywood, use B-C rating. either case, glue and screw it down.

    no, i wouldn't feel comfortable with particle board as the only subfloor.

    my opinion, no. replace it all.


    follow same blocking spacing and recommendations as above.

    if the interior wall is perpendicular to the floor joists, it's already well supported. just make sure to block in the joist bay where the bottom plate ends, if it doesn't end right on a joist.

    if it's parallel to the joists, when you install blocking every 16", i'd try to screw up/down through the blocking into the bottom plate.

    hope this helps
    Last edited by a moderator: Oct 22, 2009
  3. 1320ms

    1320ms New Member

    Finally getting around to doing this job.. I was a little tired when I wrote my previous post, I said particle board, I meant to say OSB, 23/32nd OSB is what was used throughout the house, 24/32nd is obviously 3/4 inch, this is just 1/32 under that..

    I'm using a rotozip w/wood cutting wheel to cut flush with the drywall on all the walls, and a sawzall/chizel to get into the tight corners. I decided to just replace everything with the same 23/32nd OSB, so that if i get halfway through and decide that I dont want to rip every single piece out, I wont have any un-evenness

    I'm building out nailing blocks where need be for the flooring to sit on



    Next questions I have:
    I've got one area where the floor joists change direction, appears to do this to support a bay window that hangs off the house. The problem here is that where they change direction, they sit a little lower than the rest of the joists, I'd assume I need to shim this to make it sit even?

    Where the old glue is on the joists, I've tried a few things to get this stuff off, sanding, scraping, etc.. whats the best way to get it off? Or should I just leave it alone and apply new glue on top of it?
  4. frenchie

    frenchie Jack of all trades

    electric plane.
  5. GabeS

    GabeS Remodel Contractor

    Brooklyn NY
    I would only replace the rotting sections. No need to replace the entire sublfloor if most of it is in good shape. I don't see the logic in that.

    If the subfloor under you 2x4 exteriors walls is rotting that could raise some issues since that wall is resting on the subfloor. If it's only a foot or two I wouldn't worry, but if it's more you might have to do something. I never actually did that kind of job, but if I had to I would remove the rotting wood under the sole plate a little at a time(let's say two to three feet) and then hammer in a new piece of subfloor.

    Where you are going to tile you need to pay more attention to your strategy since the subfloor requirements and procedures are more important than in the areas you are installing hardwood.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    One hassle with not replacing all of it is in the end thickness...once upon a time, 3/4" plywood was actually 3/4" thick. Hasn't been that way for awhile now. Unless there's only a small area that is impacted, best to do-over.
  7. GabeS

    GabeS Remodel Contractor

    Brooklyn NY
    Not that difficult to level it off with the rest. Just add some luan or rosin paper under the hardwood finish flooring.

    I mean if more than 2/3's of the floor is bad (can't imagine that being the case) then by all means rip up the extra leftover, otherwise patch it up.

    Also, another option to putting blocking at the ends of the plywood, is just simply to use a tongue and groove subflooring which doesn't need blocking.
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