Ceramic Tile Advice Forums - John Bridge Ceramic Tile

Welcome to John Bridge / Tile Your World, the friendliest DIY Forum on the Internet


Advertiser Directory
JohnBridge.com Home
Buy John Bridge's Books

Go Back   Ceramic Tile Advice Forums - John Bridge Ceramic Tile > Tile & Stone Forums > Tile Forum/Advice Board

Sponsors


Reply
Thread Tools Search this Thread
Unread 08-03-2022, 03:37 PM   #1
chess.piece
Registered User
 
Join Date: Aug 2022
Location: upstate NY
Posts: 2
Bathroom subfloor and shower pan questions

Hi, I've looked through a fair bit of this forum and cant quite find what I'm looking for. I'm in the process of remodeling a second floor bathroom, first time tiling a shower / bathroom floor, and am now second guessing some of my original thoughts.

The subfloor is 1 layer of 5/8" plywood (not t&g) over 2x8 nominal joists, 16 O.C.. The floor will be tiled using 7" x 20" ceramic tile.

My plan until now was to use 1/2" cement board over the plywood and then tile over that. Is 1/2" CB sufficient? is an extra layer of plywood required? If so, how thick?

The subfloor is the same underneath the shower, which is a 3x3' neo-angle. I used roof felt and diamond lath over subfloor. Small hexagon tile will be used for shower floor. I have a few questions about this part.

1. The drain (oatey) is sitting directly on the subfloor, which leaves about 1/4" at the drain, up to 3/4" at the wall for the pre-slope. Is 1/4" at the drain for pre-slope too thin? What kind of issues would I have if so?

2. At risk of sounding stupid, I believed until now that the drypack bed (about 1.5" thick total at drain) would provide enough structure for the tile in the shower pan. I have the pre-slope in place over the 5/8" subfloor only, do I need to rip this up and add another layer? If yes, would 1/4" or 1/2" cement board cut it? or 1/8" - 1/4" plywood?

Thanks in advanced!
__________________
Joe

Last edited by chess.piece; 08-03-2022 at 03:48 PM.
chess.piece is offline   Reply With Quote
Sponsored Links
Unread 08-03-2022, 06:11 PM   #2
jadnashua
Veteran DIYer- Schluterville Graduate

STAR Senior Contributor

 
Join Date: Jul 2004
Location: Nashua, NH
Posts: 14,989
While 5/8" ply is considered the absolute minimum tileable surface, that only applies if the layer is T&G, or all of the long seams are fully supported. Most pros won't tile over the bare minimum, and if it's a remodel, and may have a bunch of holes in it, probably never. IMHO, you need a second layer of ply on there. Industry says 3/8" is okay, but finding 3/8" that isn't shaped like a Pringle is tough, so 1/2" works better.

There is no structural strength in cbu, so unless you want extra height, there's no benefit for 1/2" cbu on a floor.

Over a wooden subfloor, industry standards call for a minimum thickness of any deck mud to be 1.5", but people fudge that to 3/4" at the drain, then the setting layer should be the full thickness (some might fudge that to 1.25"). If you want the pan height to be lower, consider something like Kerdi.

If you can't get underneath the current subfloor to add cleats or blocking to support those long edges, you really need a second layer of ply.
__________________
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
jadnashua is offline   Reply With Quote
Unread 08-03-2022, 07:56 PM   #3
cx
Moderator emeritus
 
cx's Avatar
 
Join Date: Jul 2001
Location: Boerne, Texas
Posts: 95,687
I'll hafta take issue a bit with Jim on those minimums, but not with the general tone of his recommendation.

The minimum thickness for a mortar bed over a wood framed subfloor is actually 3/4s of an inch over a cleavage membrane and expanded metal lath. But, that requires a minimum subfloor of nominal 3/4" T&G plywood over 16" joist centers, which you do not have.

Over a nominal 5/8ths" subfloor over 16" joist centers, the minimum mortar thickness is 1 1/4" with welded wire in the vertical center, but, that subfloor must also have T&G edges.

The subfloor you have doesn't qualify for a ceramic tile installation using any known industry recognized installation method.

I think your best option, if you cannot access the floor from below, is to remove the existing subfloor and start over with nominal 3/4" plywood or OSB with T&G edges or blocking under the joints if square edged.

Also, the minimum required thickness for the top mortar bed over the waterproof liner is 1 1/2" and that should be a uniform thickness following the slope of your waterproof liner, which will be a minimum of 1/4" per horizontal foot, measured from the drain to the farthest corner of the shower.

My opinion; worth price charged.
__________________
CX

Y'ALL NEW VISITORS READ THIS HERE!
cx is online now   Reply With Quote
Unread 08-12-2022, 10:10 PM   #4
chess.piece
Registered User
 
Join Date: Aug 2022
Location: upstate NY
Posts: 2
Thanks, I appreciate the quick responses.

I had planned on using the CB because of potential water damage due to being in a bathroom. If I replace the CB layer with 1/2 plywood how would I prevent water damage? Paint on membrane?

Thanks again.
__________________
Joe
chess.piece is offline   Reply With Quote
Unread 08-13-2022, 08:01 AM   #5
cx
Moderator emeritus
 
cx's Avatar
 
Join Date: Jul 2001
Location: Boerne, Texas
Posts: 95,687
Joe, we're not suggesting the plywood in lieu of the CBU, rather in addition to it. The plywood would be part of your structural subfloor; the CBU is your tile substrate. Different materials for different purposes.

And neither provides any waterproofing. If you want your floor waterproofed, you'd need to add a waterproofing membrane (ANSI A118.10), which is not normally indicated for bathroom floors. And to actually waterproof the floor, you'd need to continue the membrane up the walls a bit and put some sort of dam across the entry opening. Not commonly done, nor commonly necessary.

What you need to do is first provide a suitable subfloor, then a suitable tile substrate.

My opinion; worth price charged.
__________________
CX

Y'ALL NEW VISITORS READ THIS HERE!
cx is online now   Reply With Quote
Reply

Stonetooling.com   Tile-Assn.com   National Gypsum Permabase


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Basement bathroom remodel--Subfloor questions FullerNE Tile Forum/Advice Board 9 10-13-2017 05:20 PM
Matt's Bathroom Subfloor and Underlayment Questions mattsl Tile Forum/Advice Board 3 04-08-2014 07:09 AM
Bathroom subfloor/underlayment questions Aludski Tile Forum/Advice Board 18 12-29-2010 09:45 PM
Bathroom subfloor over 3/4x7x45deg questions BryanU Tile Forum/Advice Board 4 02-01-2008 09:52 AM
Bathroom subfloor questions metler Tile Forum/Advice Board 5 06-02-2004 07:36 AM


All times are GMT -6. The time now is 10:45 AM.


Sponsors

Powered by vBulletin® Version 3.7.4
Copyright ©2000 - 2022, Jelsoft Enterprises Ltd.
Copyright 2018 John Bridge & Associates, LLC