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  1. #1
    Join Date
    Dec 2013
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    Toronto
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    Default Replacing plank subfloor with plywood

    We just bought a 100+ year old house and am updating throughout. The floors were in terrible condition and were stripped down to the wooden plank subfloors which are in good condition. For most of the house, we plan to put 1/2" plywood directly over the plank subfloor and then new hardwood. For the hallway and kitchen, we will be replacing the hallway radiator with radiant floor heating (pipes) under the hallway and kitchen tile. We are concerned that the hallway and kitchen floors will be substantially higher than the new wooden floors and our contractor suggested removing the existing plank floors and placing the plywood directly onto the joists for the area under the tile to save some height rather than placing the plywood on the existing subfloor.

    As plywood is not as sturdy as solid planks, we are wondering if this will have any effect on the structure of the house? We would appreciate any thoughts or feedback.

    Also, we can't increase the height of the floor too much throughout the house as the electrical outlets are in the baseboards.

  2. #2
    Join Date
    Oct 2013
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    Ontario, CA
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    Thumbs up Re: Replacing plank subfloor with plywood

    Your problem seems to be something really serious one. My neighbor faced a water damage problem and had to replace all his floor wood after mold treatment. They asked for a contractor to fix the whole issue and I still remember the fabulous job done by them for her flooring. So I would suggest you to look for contractor in your area.

  3. #3
    Join Date
    Aug 2007
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    6,617

    Default Re: Replacing plank subfloor with plywood

    Quote Originally Posted by jennyp View Post
    As plywood is not as sturdy as solid planks, we are wondering if this will have any effect on the structure of the house? We would appreciate any thoughts or feedback.

    Also, we can't increase the height of the floor too much throughout the house as the electrical outlets are in the baseboards.
    Solid planks can span further distances than plywood, particularly if they are tongue and groove, so it is likely that the subfloor framing will have to be beefed up to accommodate using plywood in place of the existing planks. Generally, this isn't too big a deal, but it must be addressed, especially if you plan to install tile.

    As for the height issues, this will depend upon the thickness of the existing planks vs. the thickness of the plywood being used plus whatever radiant heating/finished floor you're going to install over it. I would recommend you go over these calculations with your contractor to determine if changing the planks is going to provide the space you need. Worst case scenario, you'll have to move the outlets out of the baseboards or at least raise them enough to accommodate the new finished floor height.
    I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!

  4. #4
    Join Date
    Jul 2010
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    SoCal
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    Default Re: Replacing plank subfloor with plywood

    Your contractor has the right idea, but 1/2" plywood may not be thick enough for what you are planning on doing.

    You need to know your floor deflection to see if your new floor can be supported safely. Go to johnbridgetileforum and use their on line formula to determine that.

  5. #5
    Join Date
    Dec 2007
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    Tennessee
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    Default Re: Replacing plank subfloor with plywood

    If the planks are in good shape, I'd leave them alone. You could use a thinner plywood over them such as 3/8" or even a 1/4" for the tile base as long as the planks provide enough support. Skip the cement board and apply the thinset directly to the plywood if you need to save height, or use 1/8" cement board if you have the space.

    If you take up the planks, plywood made for floors is usually stiffer than regular general purpose plywood. You would use a 3/4" plywood for floors that is tongue and groove. There is also an OSB version but I am not sold on that idea yet. Both the plywood and the OSB are stiff enough at 3/4" to support tile above then as long as the rest of the structure is OK, i.e the floor joists. With regular plywood, you need a 5/8" under and a 1/2" on top for a total thickness of 1 1/8". Going this route, you would top the plywood or OSB with 1/8" cement board.

    Unless you are using electric radiant heat, you can put your radiant heating under the planks and put insulation under the radiant heating elements to keep from heating the basement or crawl space.
    Last edited by keith3267; 12-25-2013 at 07:03 PM.

  6. #6
    Join Date
    Jan 2008
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    Default Re: Replacing plank subfloor with plywood

    Since you're doing the whole area, do what is needed for the tiled areas, then use furring strips to raise the rest of the subflooring to an appropriate level to match the tile when all is finished. On the tiled areas I'd strip out the old planking down to the joists- you're likely going to want some joist reinforcing work there anyway. My favorite subflooring is Advantek or at least a generic copy of it and I would not use anything under 5/8" for subflooring anywhere unless it was fully supported underneath by a solid and substantial floor where the new subfloor is more a spacer than structural.

    I would worry about a contractor that wanted to use 1/2" for subflooring under any tile installation- that's begging for trouble, not just asking for it, and a really good contractor would know better than to try that

    Phil

  7. #7
    Join Date
    Dec 2010
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    Houston Texas
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    Default Re: Replacing plank subfloor with plywood

    If you are planning on tiling, the bare minimum plywood over 16" OC joists is 5/8" CC or better T&G, but most pro's will use 3/4" BC T&G as it is easier to find.

    If you are planning on tiling, the bare minimum plywood of planks in good condition is 1/2" CC or better, screwed to the planks but NOT to the joists. The planks need to be properly screwed down as well.

    The John Bridge Tile Forum has a handy dandy Deflecto-meter to see if your floor joists can handle tile. There you can start your own thread, post pictures, read the extensive library and get speedy accurate answers

  8. #8
    Join Date
    Feb 2013
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    Boston area
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    Default Re: Replacing plank subfloor with plywood

    I'm confused by several things in this thread... I would have assumed the plywood would be much stronger and more stable than any plank. And the planks are up to 12" wide, meaning you have joint that can flex under your tile at least every foot. And those old boards probably flex and squeak. Plywood that was glued and screwed to the joists would prevent squeaks. Also, can I assume the old planks are a full 1" or 7/8" thick? That should leave room for thicker plywood and/or cement board for extra stiffness.

    I'm not that experienced in this department, can someone straighten me out if I'm way off base?

  9. #9
    Join Date
    Dec 2010
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    Houston Texas
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    Default Re: Replacing plank subfloor with plywood

    We are not talking about the floor flexing, that is controlled by the joists alone. The planks or plywood have nothing to do with joist deflection. What they do have the role in is between joist deflection. (Nothing to do with a bouncy floor) The planks may be fine for carpeting or hardwoods, but are not a suitable subsurface for tile. For tile or laminate, another layer needs to be added.

    The advice is dependant on the final wear surface.

    Plywood can be and is often times stronger than the planks. This depends too on the quality, thickness and installation of said plywood and conversely the planks.

  10. #10
    Join Date
    Jun 2014
    Posts
    3

    Default Re: Replacing plank subfloor with plywood

    This is exactly what I am dealing with this week. Our house has diagonal, 7" wide, 5/8" thick planks on top of joists that span between cinderblock walls. I had to pull a few of them up and of course they spintered to pieces being 60 years old. I am not aware of anyone selling 5/8" boards anymore... It's a 5x8' bathroom and my plan is to cut the floor out from wall to wall. I will install a 2x4 ledger on the edges of the room sistered to the joists to support the edges of a new floor, since the planks run underneath the wall and a new subfloor will stop at the wall and won't reach the joist. I will then install 5/8" OSB with 3/8" plywood over that to even out the floor, then I will put 1/4" backer and tile over that for a full 1 1/4" under the tiles.

    Am I running into trouble with that plan? Any pitfalls you can foresee?

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