Shower drain installation plywood subfloor

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by Emil Volk, May 29, 2011.

  1. Emil Volk

    Emil Volk New Member

    Messages:
    5
    Location:
    Maryland
    The problem I have relates to my discovery that I had a shower installed in my house that does not have a liner. Originally when the house was built ~ 25 years ago there was a large PVC tray throughout the shower with a drain in the center. The tray was cut out and only the drain in the center remained (with a small amount of the tray left). Tiling was then done---two layers of tile on top of the wood sub floor (it is on the second floor). I have removed all walls (green-board) and the tiling on shower floor to reveal the thin set on top of the plywood. I have cut out a section of this plywood sub floor (5/8 " inch) around the drain. I then removed the existing drain by cutting the rising pipe horizontally from the side. On doing measurements with respect to a new drain installation I found that I had to remove the P2 trap so I removed from the system by vertically cutting a rectangular section of the downstream pipe.

    I will cut a new section of plywood to fit it over the rectangular hole created between 2 adjacent joists. The new section of plywood will have a circular hole cut out to accommodate the new shower drain (see photo shower drain).

    Questions I have:

    How best to fit the new shower drain plus new plywood section over the hole created in the original plywood.

    First what I am thinking is to connect up the new P2 trap to the existing drain pipe downstream. The new rising pipe will be fitted into place into the P2 trap and finally up into the shower drain. { I am aware that dry fitting the pipe joints leads to the joint not being closed up completely, but when using adhesive the fitting goes all the way up into the union}.

    The new section of plywood will be fitted into the vacant plywood section and the shower drain base will be fitted into the hole cut in the plywood to accommodate the drain.
    Now the final plumbing step is using applied adhesive to push the shower drain over the rising pipe, twist and set horizontally with a level. You can see the line I marked for the dry fitting inside the shower drain. In order to twist the shower drain then it must be sitting over the plywood and you have to be able to rotate it while pushing down over the rising pipe leading to the P2 trap. In this case the rising pipe will not fit all the way up into the shower drain base joint, but will probably go up a little more than dry marked as per photo. Any comment on how much it will go up? As the rising pipe moves up into the shower drain base then the drain base will move down (do not want the drain base to go down too far!

    So how should the shower drain base be placed over the plywood? You can see in the photo the shape of the PVC accommodates 4 the bolt fittings for the shower liner, but I do not think I should use spade bits on the plywood to accommodate these 4 shapes on the hole , since then if placed as such into the plywood then I could not twist the shower drain when fitting it with adhesive into the rising pipe.

    Another issue is how to immobilize the rising pipe plus P2 trap while securing the shower drain over the rising pipe below the new section of the plywood. I will not be able to access it by hand, since the new plywood will be in place. Underneath the sub floor ( and below P2 trap) is a sheet of 3/8" plywood so the trap could be clamped to that.

    I am aware that the mud prepan over the plywood will support the shower drain base.

    Comments are welcome on this plan, strategy and the issues raised.

    emil

    Attached Files:

  2. Redwood

    Redwood Master Plumber

    Messages:
    7,450
    Location:
    Connecticut
    You need a liner or, a tray installed keep going with the demo work. Tile and grout is not waterproof.
  3. Emil Volk

    Emil Volk New Member

    Messages:
    5
    Location:
    Maryland
    Thanks for your comment: the demolition work is complete. I am aware of the need for a liner. My post deals with issues prior to including liner in shower drain

    EMIL
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,051
    Location:
    New England
    It's not obvious in your picture...

    Is the liner sloped to the drain? Is it designed for a mortar bed on top? That drain assembly doesn't look like it has weep holes, so that is a problem all in itself. There will be moisture that gets below the tile, and it needs a way to get to the drain. That requires the liner to be sloped and an alternate path down the drain (called weep holes).

    If you get a good coat of cement on both the riser and the drain, and twist a little, you'll distribute the cement enough to get a good seal. Ideally, you'd twist all the way to the bottom of the socket, but while desirable, not always necessary.

    My favorite shower construction makes use of a surface membrane. Lots of purveyors of those systems: Schluter (Kerdi), Hydroban, Wedi, Noble, etc. This makes the entire shower waterproof, not just the floor (the walls are typically water resistant in a conventional shower). You might check out www.johnbridge.com for help on tiling and shower construction.

    Ideally, a plywood subfloor patch spans two joist bays. But, if it can't, then install some blocking along the unsupported edges to help keep the deflection down.
  5. Emil Volk

    Emil Volk New Member

    Messages:
    5
    Location:
    Maryland
    More explanation is needed to outline exactly what I am doing.

    the sub floor plumbing has been removed; the only plumbing left is the downstream drainage pipe that leads to the drain (probably 4"). I have ordered all replacement plumping including P2 trap.
    I am trying to overview the project going forward in order to make a plan. The bottom part of the drain assembly (drain base picture) illustrates the mark inside the drain base that the 2" rising pipe will go to without adhesive. The weep holes are not visible since they are on the other side of this drain base. When installed the drain base points in the opposite direction to the picture. Once the shower drain base is installed then mud is added to create a prepan with a slope down to the level of the drain base. it is then the liner is installed into the shower drain base. The upper part of the shower drain is screwed into place clamping the shower liner into position. Then again mud is added to create the shower pan with a slope down to the top of the upper part of the drain assembly.
    Do you follow now? Please can you answer any of the issues

    EMIL

    Do you follow my plan?

    EMIL
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,051
    Location:
    New England
    That will work. Ideally, over plywood, the mortar preslope would be a little thicker at the drain than the thickness of that base. The liner should go at least a couple of inches above the curb height, and have no screws or other anchors in it except at the top edges and outside of the curb. To get the setting bed, you'd need some lath bent over the curb and some masons (sticky) mortar to cover it on the curb. You need to proper corners to seal around the curb cuts and neat folds in the corners. This usually requires notching the studs a little, and blocking between the joists to support the liner properly and provide attachment points. Check out that website I mentioned...lots of stuff dedicated to tiling and especially shower building. Peruse their library, and ask tiling questions (they have some plumbers, but mostly tilers there). Their site gets lots more hits and responses from tilers than here, so you'll get a quicker turn-around.
  7. Redwood

    Redwood Master Plumber

    Messages:
    7,450
    Location:
    Connecticut
  8. Emil Volk

    Emil Volk New Member

    Messages:
    5
    Location:
    Maryland
    I am checking out the site. My shower is 38" deep (front to back) and 48" wide. Does the Kerdi tray fit that size? I do no thing it does so does that rule it out.

    EMIL
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,051
    Location:
    New England
    Kerdi (and any sheet membrane) works just fine over a mortar base. The preformed foam ones can speed things up if they fit. If you used the 48" square one, and cut 5" off each of two opposite sides, it would fit, but those sides would then be about 1/8" lower than the uncut sides because of the constant slope. This would work, but you then wouldn't have the tile the same height all the way around on the bottom row. That's small enough to hide, but it will be there (less than 1/8" on those two sides).

    It's much less expensive to use mortar instead of the foam. The sheet goods attach the same way, and it works fine. Another nice thing, is you can use regular drywall for the shower walls since the membrane makes it totally waterproof. Much easier and less expensive than dealing with cbu, which is heavier, needs special screws, and tape.
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