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Unread 10-06-2016, 03:10 PM   #1
zmusashi
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Installing new tile over old mastic?

Hello all,

Is it okay to install subway tile for a kitchen backsplash over sheetrock that has the old mastic from tile demo still on it? This kitchen got refaced and if I can avoid destroying it in scraping the mastic or installing new rock or tile board, I'd be happy. I read in here somewhere that modified thinset will bond to old mastic? But that was a floor job with cutback adhesive I think that was being referred to. Budget clients. Thanks for any insights.
I suppose I could install 1/4 tile board over this substrate but I'd have a few bullnose side runs that'd be jutting out from the wall.
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Unread 10-06-2016, 03:36 PM   #2
rmckee84
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It really doesn't take that much time to cut that old drywall out and stick new up. 1 or 2 sheets of drywall and in and hour you'll have a perfectly flat surface to set on. I'm sure there's probably another way but in the end that's the only way I'd do it.
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Unread 10-06-2016, 04:13 PM   #3
HooKooDoo Ku
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Quote:
Originally Posted by rmckee84
It really doesn't take that much time to cut that old drywall out and stick new up. 1 or 2 sheets of drywall and in and hour you'll have a perfectly flat surface to set on. I'm sure there's probably another way but in the end that's the only way I'd do it.
That sounds good on paper, but to a DIYer like me, I see replacement of the drywall being a whole lot longer than an hour.

For starters, just removing the existing drywall seems problematic because kitchen cabinets have been installed over it. Since the drywall is now acting as a spacer between the back of the cabinets and the studs, I wouldn't want to just start ripping out and banging up the drywall for fear of damaging part of the "spacer" for the cabinets. So that means I've got to find a way to cut the drywall all the way around the perimeter without doing damage to the cabinets or the studs. My personal estimate for a DIYer like me would be one to two hours to "safely" remove the existing drywall.

Next, you've got to cut the new drywall to fit the space, and that includes an inside angle (so you can't just score and snap). Not only that, but you've got to also deal with electrical outlets (which I think building codes might require that you don't have more than a 1/4" gap between the drywall and the electrical boxes). I've hung drywall as a DIYer, and details like these always made things more difficult than any of the DIY instructions seem I ever tried to follow made it sound.

Finally, odds are low that studs will be properly located along the vertical sections of those inside corners and you'll need to add some sort of bracing so that all outside edges of the new drywall (old and new) will be supported.

So I'm thinking 1-2 hours to safely remove the drywall, 1 hour to get the new drywall cut, another 2 hours to figure out how to install bracing, and finally an hour to mount the new drywall.

So if I were doing the work in my house, I'm thinking replacing the drywall would be a 5-6 hour job (and that doesn't include getting the drywall home).

I don't disagree that replacing the drywall is perhaps the best coarse of action... but when I think thru all the details and all the things that can go wrong, I think it's going to take someone who might not have much drywall experience several hours at a minimum to get the drywall replaced.
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Unread 10-06-2016, 06:23 PM   #4
rmckee84
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1. Oscillating saw ran along the bottom of the cabinets and above countertop, and pull out drywall. No cabinets affected.

2. Nothing difficult about an inside corner, the pieces just butt into the corner...

3. Cut the old drywall just shy of where the backsplash will end and a couple pieces of 2x4 blocking, or cut the drywall at the nearest stud that will be behind the backsplash.

Yes I do this everyday, and the O.P. said this was for "budget clients" so if they are being paid to install a backsplash I'm hoping that they at least have experience in constructin and tiling. It might sound like a lot of work to a DYI'er but if you're in the construction field and doing a remodel odds are you have everything you need to accomplish this quickly and easily. I may be overestimating the OP's experience but tear out to new drywall installed should take no more that 1 1/2 hr for someone getting paid to do a backsplash.
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Unread 10-06-2016, 10:00 PM   #5
jadnashua
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IF you don't have an oscillating saw, even a simple hand drywall saw can cut the old drywall at the top of the counter. Just be careful about the wiring! Cutting through that would not make your day!

If you add a new layer over the top, you'd need box extenders for the electrical boxes...not expensive, but easier to avoid altogether.
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Unread 10-06-2016, 10:13 PM   #6
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Welcome to the forum, Fenton.

With all the mastic, I wouldn't be content installing a backsplash of subway tiles over it. I'd at least grab a razor sharp 4" wallpaper scraper and peel a thin layer of drywall paper...taking the mastic with it so I had a clean surface to work off. Just don't go too deep and remove the paper completely. But if you want leave the mastic, go ahead and test it. Throw a piece of tile up there with your intended thinset and test the bond after at least a week. If your happy with it, go for it.

By the way, if that's non-metallic sheathed cable I see stapled to the front of that stud from a wiring fish-job...that's a code no-no.

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Unread 10-08-2016, 08:07 AM   #7
zmusashi
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Hi everyone, thanks for the replies. Yes I am a contractor and do plenty of tiling but I always feel short of being a tile pro. I can replace all that rock fairly easily, I would use my Multimaster oscillating tool, I just thought how great it'd be if I could tile right over the existing.

I pointed out the electric to my client and if I do replace all the rock it'd likely be fairly easy for him to drill and pull the wires through the studs, or at least notch them and install nail plates. He's coming to install under cabinet lighting anyway, so i will recommend it.

Interestingly, one half the kitchen has a brown dried mastic and other has a black, the black side being much more smooth. I like the idea of a tile test!

I asked the client to look at bullnose and pencil tiles for exposed edges. If she finds a pencil tile I could install 1/4 Hardboard or rock right over the existing and the pencil tiles would cover, then perhaps box extenders are warranted.

Thanks everyone
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Unread 10-08-2016, 10:05 AM   #8
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Even if you decide to install Hardiboard over the mastic, go over it with the scraper Bubba mentioned just to get the surface as flat as you can. That might avoid bulges behind the Hardi.
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