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cement backer board question - Cooler - 05-18-2020

I am mounting cement backer board for a bathtub surround. 

I am ready to apply the fiberglass tape for sealing between panels.  The tape is self-adhesive. 

I've read that thin-set should be used to adhere the tape?  Do I have to use thin-set over the self-adhesive tape?

Is the self-adhesive OK to use on its own?  Can I just paint over it with Red Guard?

Also, I had to remove one panel and then put it back in place.  I have several screw holes that are not being used.  Do I have to fill those holes with thin-set?

What I've learned so far:

Cement board is heavy.

Cement board is harder to snap than drywall.

It is a good thing this is going to be hidden behind tile.  The workmanship before taping and Red Guard is embarrassing.


RE: cement backer board question - rschissler - 05-18-2020

The thinset between the panels is also to fill the little gaps and make it one big rigid panel, not just to secure the tape.  If you don't, you could get cracks in your tile,  between the panels.


RE: cement backer board question - Cooler - 05-18-2020

(05-18-2020, 09:51 AM)rschissler Wrote: The thinset between the panels is also to fill the little gaps and make it one big rigid panel, not just to secure the tape.  If you don't, you could get cracks in your tile,  between the panels.

So do I apply the thinset first and then wait for it to dry and then apply the tape?  Or do I apply the tape first and then the thinset?


RE: cement backer board question - rschissler - 05-18-2020

(05-18-2020, 11:28 AM)Cooler Wrote: So do I apply the thinset first and then wait for it to dry and then apply the tape?  Or do I apply the tape first and then the thinset?
Apply the tape first then the thinset.  Keep the thinset fairly thin, but not runny, so it will pass though the tape easily.


RE: cement backer board question - Snipe Hunter - 05-19-2020

(05-18-2020, 09:40 PM)rschissler Wrote: Apply the tape first then the thinset.  Keep the thinset fairly thin, but not runny, so it will pass though the tape easily.

+1

You want to push the thinset "through" the mesh. The idea is to fill any void between the backer-board panels. Then take your knife and skim off any thinset on top of the mesh. You don't want it built up on top of the mesh so you should still see all the mesh after you apply the thinset.There will be a little thinset inside the mesh web, just not built up at all. If you leave a hump of thinset on top of the mesh, it's very difficult to get your tiles to lay flush against the backer-board. The thinset you use to set the tiles will be the only thinset on top of the mesh and will lock everything into place.

Remember to use a brush in the mesh areas when you apply your liquid membrane. I use the cheap Chip Brushes and toss them when I'm done. Put on several coats over the mesh. Thinset is more porous than the cement board and. I use a roller for the bulk of it and still do 2 or 3 coats. The membrane dries fairly quickly so you can apply the next coats before your roller dries. I just stick the roller in a plastic bag between coats. You can probably re-coat after about 1/2 an hour with a roller if you have good air movement. You'll now it's ready when the shine is off. I think most of the liquid membranes are water based now so air movement is important to dry it. I set a fan on it. I've used Mapai Aqua Defense and Red Guard and liked both. IIRC, the Redguard has some goofy warranty stipulations. Like a long wait time (several days) before applying thinset and tile.
.


RE: cement backer board question - Cooler - 05-19-2020

Thanks for the input.  I will follow those suggestions.

I am not too worried about water absorption.  This is a guest bathroom that has been used perhaps 12 times in the last 20 years.  That gives it a over a year to dry out between showers. 

But I want to do this correctly, so I will follow best practices.

Best Regards (and stay healthy),

Cooler


RE: cement backer board question - splinter7612 - 05-19-2020

I used to rehab bathrooms in an elderly complex built in the 70's, they were small about 6x7 and I would pull up the old tile that was glued to the plywood subfloor ( some were a b!tch to get up) then lay down Hardie backer board over thinset and screw to ply, and tape the seams with fiberglass mesh tape and thinset. I used cement bb once and then only Hardie, lighter, cleaner doesnt't crack, cuts with a jig saw.


RE: cement backer board question - Cooler - 05-19-2020

(05-19-2020, 02:42 PM)splinter7612 Wrote: I used to rehab bathrooms in an elderly complex built in the 70's, they were small about 6x7 and I would pull up the old tile that was glued to the plywood subfloor ( some were a b!tch to get up) then lay down Hardie backer board over thinset and screw to ply, and tape the seams with fiberglass mesh tape and thinset. I used cement bb once and then only Hardie, lighter, cleaner doesnt't crack, cuts with a jig saw.
I don't know what they used building my house in 1953, but the tiles never popped off the walls like they do on HGTV shows.  And the porcelain floor mosaic would send up sparks when I chipped away at it with a hammer drill.  The small bathroom took hours to chip away and then I had to go over some of it with a diamond grinder to get rid of small bits of porcelain.  Patching the divots was easy with self leveling cement.  

They make like demo is fun.  I found it to be nothing of the sort. 

The wall panels were made from pre-made concrete panels about 1" think with wire mesh.  Over that they spread some plaster to smooth out the seams and then the tiles. 

Removing the wall tiles was less than half the work.  Removing the concrete backer was much tougher.


RE: cement backer board question - Woodenfish - 05-22-2020

You will need to apply RedGard waterproofing sealant over the he concrete boards before tiling.