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LeviDIY 04-16-2009 12:47 PM

5/8" Fireboard (Paint) meets 1/2" Cement Board (Tile)
Hey everyone, long time listener, first time caller.

(Not so) Quick and dirty: I'm in a condo. First home, bought 8 months ago. Getting into DIY, finding its a big passion, but I don't have the money and time to get stuff done quickly or buy expensive tools/products :(. Installed flooring, repainted, minor electrical work, got my DIY feet wet with all that. Now onto the bathroom...

Originally wanted to just clean the tile, strip off wallpaper, switch out vanity top, refinish vanity and change light fixtures... basic stuff. Ripping out the wallpaper, uncovered small mold/mildew problem near the tub.. figured everthing has to go (Built in 1979, so no cement board behind tile, not even greenboard). Ripped out tub/shower surround tile and walls to the metal studs (the board the tile was on was soooo decrepid). Plenty of good Firecode Drywall (required I guess in the condo) on the walls away from the tub, so cut the Firecord drywall at the next stud over, halfway across the stud.

Put up 1/2" Hardibacker around the tub and after using construction adhesive and screwing into the studs (its ON there). Had thought I measured the existing Firecode Drywall at 1/2" but then realized, nooooooo.... its 5/8". Now, I have 1/2" cement board butting up to 5/8" Firecode drywall. You are starting to see my problem.

I read another post/thread on here where the suggestion was to remove the cement board and use shims to meet the 5/8" board. In my infinite wisdom, I didn't really see the problem until AFTER I secured everything :censored: (again, its ON there).

Several questions given the above situation:
1) Now, as I plan to do subway tile (3" x 6") all the way to the end of the cement board, and prime/paint the butting Firecode board, is it safe to assume that I can lay the thinset and with the tile on top of it, will close that 1/8" difference in board thickness?

2) Again, assuming I keep that difference (remember, the cement board is ON there), how should I tape the uneven joint? Paper Joint Tape? Fiberglass joint tape? Using thinset/mortar on the cement board half and drywall compound on the Firecode side?

3) On the Firecode edge, should I use a corner bead to protect it, or would you say "Levi, easy... you're going overboard"?

Also, as a follow up to #3, I'm a wee bit annoyed at myself for not doing the ONE thing everyone says: "measure twice, cut/buy/install once" - I'm a perfectionist. So, any encouragement or "Levi, easy... you ARE obsessing over this a bit much, it's happened to me plenty and not a big deal" would be greatly appreciated. :thumbup:

Ininkus 04-16-2009 09:29 PM

I've never done anything with fireboard/cement board, but I can tell you I have seen drywall finishers mud 1/8" surface differences more than a handful of times and make it look perfect.

Mop in Hand 04-16-2009 11:10 PM

On point #3, corner bead will do more harm than good if there is any chance for getting it wet. As for the rest your screwed. (not really). This is what I would do. First I would tape the inside corners of the cbu with cbu tape and thinset. It's a special tape, not just the fiberglass stuff. Next, I would take a 1/4" x 1/4" square nothched trowel (doing 1 wall at a time) cover the cbu only with thinset then smooth out the notches. This should give an even coat of 1/8" over your cbu. Next tape over the flats using the same tape and thinset. A thin coat of thinset on your drywall isn't going to hurt a thing. Last, I would Redquard over the thinset with 2 coats.

LeviDIY 04-17-2009 10:07 AM

Ininkus - yeah you are right, except I'm no professional :)

Mop In Hand - you actually scared me for a second with the screwed comment. What you are describing makes sense.. I'm a bit concerned about getting that initial layer of thinset smoothed out well enough... but just to clarify: once I get the thinset as close to equal with the butting drywall board, tape those again (In other words, the initial tape UNDER the thinset layer, then again above it?)? ALso, not familiar with Reguard... is that available at HD/Lowes or should I try a local building supply?

Thanks both for the responses... even just the initial responses help me breathe a bit easier :)

Mop in Hand 04-17-2009 10:38 AM

No, do not tape them twice, tape the inside corners and flats of the cbu first. After you have added the 1/8" thinset then tape where the drywall and cbu meet. Sorry about the mis-spell. It should read "Redquard", available at Home Depot or Lowes. It's a waterproofing membrane in the tile section. Use a magnesium trowel to smooth out the notches (works better than a regular float). Be sure your thinset is dry before Redquard.

LeviDIY 04-17-2009 10:50 AM

OK ok... now I think I'm picturing it... get the 1/8" of thinset along the WHOLE cement board... I was thinking just at the joining of the drywall and cbu. so, thinset across the board... smooth it out... tape with redquard... 2 coats... then the regular thinset layer to lay tile on?

Sorry to be such a pesky questioner... gotta be sure I get ya on this :) And I really appreciate the advice.

Mop in Hand 04-17-2009 06:25 PM

Yes, except "tape with Redguard?" Redguard is a membrane that is rolled on. (I use a foam roller) Cbu tape is also available at HD. Good Luck

Rory Read 02-09-2010 12:45 PM

If you plan on covering the entire cb and only the cb with tile, could you look around for slightly thicker tile, or create a border, either with some sort of edge tile or wood or pvc trim over the 5/8 rock?

I think I would be looking for a way to avoid building up the 1/8th difference. No matter how it's done, there is going to be a transition with a reveal. Why not build it into your design, taking advantage of it to create a cleaner and easier install?

Just a thought.

Good luck,


RDG Read Development LLC
Portland, OR

naehring2000 02-09-2010 09:40 PM

I like rory's idea of an accent to hide it, (something dark) For future reference all firerock is 5/8" :thumbsup:

Scuba_Dave 02-09-2010 09:41 PM

This thread is from April 2009 & was bumped by a spammer

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