Subfloor for bathtub only?

Discussion in 'Shower & Bathtub Forum & Blog' started by KJNicholls, Nov 24, 2007.

  1. KJNicholls

    KJNicholls New Member

    Nov 24, 2007
    Spokane, WA
    Great forum - learning allot.

    My unfinished basement project includes a bathroom with at tub/shower. I went to Home Depot, bought an acrylic tub and have properly framed for the model I purchased.

    The folks who built the house left me access to a sub-slab pipe. The hole in the basement slab measures ~14x14 and the center is RIGHT UNDER THE TUB DRAIN.

    I'm assuming that my acrylic tub is going to crack around the drain because of lack of support.

    My idea: Install a subfloor JUST UNDER THE TUB (I have some 1/2" OSB lying around).

    My questions:
    1) OSB ok in this application? Thickness recommendations? Or should I go get some plywood.

    2) Whatever thickness I use will raise the tub higher than the rest of the floor- do I just install the subfloor in the entire bathroom OR will normal tile ontop of the slab be able to 'hide' the tub subfloor layer?

    Thanks in advance,

  2. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona

    You are assuming wrong, but you should fill that hole with concrete anyway after the tub is set and the drain connected.
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  4. frenchie

    frenchie Jack of all trades

    Dec 30, 2006
    Brooklyn, NY and Fire Island, NY
    HJ - Really? I've never heard of filling the hole after. I usually work on raised floors, not slabs, but there's always a cut-out (although maybe 1/2 the size of this one). And the few slabs I've seen, it was never filled in. How's the next renovator going to change out the tub?

    KJ - The wood "subfloor" idea sounds very very iffy to me. Good way to create a lot of rot. Why not just fill in the excess space with some concrete, level with the rest of the floor?
  5. smellslike$tome

    smellslike$tome Plumbing Company Owner

    Nov 3, 2007
    plumbing company owner
    Birmingham, Alabama
    I'm sorry, but absolutely not! There is no reason to fill the hole! Frenchie is correct. What will you do if it ever becomes necessary to access that tub trap? What if you discover later that there is a leak that maybe doesn't show up right away or gets missed during testing? As for your tub not being supported at the drain, when was the last time you stood on your bathtub drain? Even if you did the tub should not crack assuming you bought a good quality tub. However, if you bought something say like "Sterling" then, in my opinion the tub is very likely to crack no matter what you do because, in my opinion, it is not a good product.

    I have installed hundreds of tubs, granted this was mostly several years ago when I was still doing new construction plumbing, and it's true that I didn't actually use any of them personally but to the best of my knowledge no quality tub will crack from standing in it. Now I have replaced some tubs that were cracked or split or otherwise damaged but these have always been products that were of inferior quality to begin with.

    If you dig a little you will probably find that the 14 x14 hole is full of gravel or dirt which has been placed inside what is called a tub box. The tub box is there for a reason, so that the plumber can access the terminal end of the waste line serving the tub in order to connect the drain to the tub waste and overflow assembly. Depending on what the previous plumber did the waste line will either terminate with a cap under the slab within the tub box or it may have the ptrap already installed with a riser coming up out of the tub box to some point above slab level. This is normal. It is not necessary to support the tub at this hole. If it absolutely drives you crazy you can fill the hole with sand up to the slab level after the drain has been connected but make sure you have thoroughly tested the tub for leaks (in the rough) before you cover it up. Whatever you do do not fill the hole with concrete.
    Good luck.
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