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Thread: New tub installation: is 1/2" plywood OK?

  1. #1
    DIY Junior Member deltasmith's Avatar
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    Question New tub installation: is 1/2" plywood OK?

    I'm replacing a fiberglass tub that has stress cracks in the floor which occasionally leak. I'm hoping to install a Sterling Ensemble (if I can get my hands on one - just returned two of them to HD in the last couple of weeks because of defects and shipping damages).

    The existing tub is sitting on 1/2" plywood, which is supported by joists at 16" oc. The existing tub does not have a mortar bed but I am planning to use mortar when installing the new tub.

    It looks like the bathroom floor was tiled after the tub was set, but I doubt that there's much in the way of underlayment under the tile. My guess is that the tile is sitting directly on the same 1/2" plywood that I can see under the old tub.

    My question is: once I've removed the old tub, should I replace the 1/2" plywood with something stronger (like 5/8 or 3/4), or should I add a second layer of 1/2" on top of what is already there, or should I consider the existing 1/2" plywood good enough to support the new tub? I'm thinking that the tile gives me 1/8 or 3/16 to play with, but if I raise the new tub any higher than that, there will probably be a gap between the apron and the top of the tile.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    1/2" ply never met the building codes, so hopefully, it is a nominal 5/8" (slightly under that after sanding). If it truly is 1/2", yes, I'd either cut it out or add some on top to beef things up. Same thing is true for the tile, 1/2" was never an accepted subfloor for tile, and if there is no cbu or membrane, all of the industry standards call for two layers of ply, the first a minimum of 5/8" T&G, and a second of at least 3/8". If you use a membrane, you can get by with a single layer of ply, but it must be at least nominal 5/8" and T&G, or have blocking installed on all unsupported edges (so, it's usually cheaper and easier to use T&G).
    Jim DeBruycker
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    Moderator & Master Plumber hj's Avatar
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    The real question is whether the joists are parallel or perpendicular to the tub.
    Licensed residential and commercial plumber

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    DIY scratch-pad engineer leejosepho's Avatar
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    If you have room for mortar under your new tub, you likely have room to first add a piece of heavy plywood cut to fit inside the skirt so you do not complicate things with your tile. And if the existing floor is damaged or soft from being wet, that should be replaced also.
    "Great minds discuss ideas, average minds discuss events and small minds discuss people." --Eleanor Roosevelt

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    DIY Junior Member deltasmith's Avatar
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    Thanks for the replies.

    Quote Originally Posted by hj View Post
    The real question is whether the joists are parallel or perpendicular to the tub.
    The joists are perpendicular to the tub.

    Hard to get my head in there and get a precise measurement, but it looks like the tape measure is saying the plywood measures 7/16. If I decide to add another layer of 1/2" on top, can I just lay it on top and screw down through both layers into the joists? Or do I need to put felt paper or something between the two layers?

    If I decide to cut it out and replace it, is 5/8 sufficient or should I use 3/4"? Any particular grade of plywood that I should look for? Also, it sounds like jadnashua is suggesting that I should put blocking under the plywood joints. Given that I will only have access to the one side of the blocks I would need to install (the side where I'm cutting out the plywood that sits under the tub), that could be tricky to do. Any tips on how to do that? Joist hangers instead of toenails perhaps? 16" centers doesn't leave a lot of room to swing a hammer.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    I think I'd screw a second layer of ply onto the existing one. Ideally, you'd want some ply with exposure I or exterior rated glue (essentially the same thing, but labeled differently) with no 'D' faces...D faces are voids. Like to avoid voids. Screws, 6" on the edges, and 8" in the field should bond them together fine. NOte, if the first layer really is that thin, it's really hard to get the screws to hold without tearing out - you may need to predrill and be careful with the torque when installing. Should you choose to glue it, do not use something like a caulking adhesive...you need a wood glue you can spread and cover the entire surface. The screws would then function as clamps until the glue sets. It's nearly impossible to not have voids with thick stuff from a tube.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member deltasmith's Avatar
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    Quote Originally Posted by jadnashua View Post
    Screws, 6" on the edges, and 8" in the field should bond them together fine. NOte, if the first layer really is that thin, it's really hard to get the screws to hold without tearing out -
    Thanks for the info and suggestions! But I think I'm missing something. Yeah - if I screw into 1/2" plywood with a void under it, it would be really hard to get the screws to hold. But why not just screw down into the joists? Are you thinking that the screws need to be closer than the 16" oc offered by the joists?

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    If it were to be tiled, you'd want to avoid the joists to provide some decoupling, but since it is just for interjoist strength, you can screw it into the joists. You do want to essentially laminate the two sheets, so you will gain some strength if you use screws in the field rather than just into the joists.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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