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-   -   Overkill? Durock over wall studs: Add Construction Adhesive? (https://www.johnbridge.com/vbulletin/showthread.php?t=130860)

SWB04 04-15-2021 07:24 AM

Overkill? Durock over wall studs: Add Construction Adhesive?
Question: Should I add Construction Adhesive (probably a poly version) on the wall studs before installing the Durock CBU with CBU screws?

Project: Quartz ledgerstone panels over Durock CBU, for an 18" high kitchen backsplash, (between upper cabinets and a granite countertop).

1) I realize that I probably could have just run thinset over the wallboard (per manufacturer's instructions). However, due to the weight of the stone, I prefer the stability of CBU with CBU screws (versus drywall screws) over a wallboard substrate. Plus, there were issues with the existing wallboard, including insufficient horizontal stud support.

2) Durock installation instructions (http://pdf.lowes.com/howtoguides/081099032585_how.pdf) are clear, i.e. CBU screws to studs, 16" OC maximum, etc..

3) I reinforced the wall with horizontal 2"x3" (i.e. non-structural) studs top and bottom (of the 18" backsplash area) between the 16" OC vertical studs, where I could. Where I could not, i.e. due to plumbing vent or drain pipes, I did the best I could (i.e. putting a stud above the pipe in the middle of the 18" area).

4) It bothers me that there will be places where there is no bottom or top horizontal support for the CBU between the vertical structural studs, to prevent flexing, because I have no way of getting a horizontal stud into those areas. Also, I left a 1 inch "tab" of wallboard top, bottom, and sides where I was able to, to be able to tape & thinset between wallboard and CBU. However, there's about 10' of area where there's a 1.5" gap below the back of the granite countertop, where the wallboard had already separated, (presumably because the drunk on the drywall crew was responsible for that joint). I will extend the CBU below and behind the countertop, and obviously, cannot tape that joint (without pulling out the countertop, cabinets, and destroying a bunch of stuff in the process).

5) Regarding the original question: Should I add a bead of construction adhesive to the studs prior to installing the CBU? The purpose would be to add strength, again, considering the weight of the 1/2" troweled thinset and quarts ledgestone panels that it will support? Or, should that CBU be free floating on the wall, with only the mechanical CBU screw fasteners for support, to prevent normal expansion and contraction from communicating through to the CBU? (Note: I'd like to use the poly version of Construction Adhesive, but may just use Liquid nails from Walmart, so I don't have to make the 1 hour trek into town to a big box store)


cx 04-15-2021 11:15 AM

Scott, if I understand what you did, I wish you hadn't done it.

The existing drywall would have been far more than adequate for installation of your ledgerstone panels.

Removing a section of that drywall and replacing it with CBU might sound like a positive step, but if you cannot provide blocking for all the new edges, I think it's a net negative.

But to your question, no, there is no need nor real advantage to using construction adhesive along with your mechanical fasteners in that application. It won't hurt anything if you've already done that, but it's not necessary.

My opinion; worth price charged.

SWB04 04-15-2021 12:42 PM

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Thanks for the advice.

The dry wall wasn't contiguous. It was cut and separated (and therefore, flexed) at the countertop. I think that happened during the countertop installation, but as I said, I had a problematic drywall install (long story).

As far as CBU versus sheetrock behind stone, I realize that pro's would run this stuff over sheetrock seven days a week. I'm just not comfortable with that. I've seen so many snapped drywall screws (and in this situation, missing screws).

Blocking is a problem. I'll slip a backing in a couple of areas where blocking is weak, to meld the drywall to the CBU. I can't do that where I can't reach the drywall (below the countertop). There are three "cells" where I have vertical pipes preventing me from adding a block, and one that has a vertical 2x4 (i.e. leading edge of a rear wall) doing the same. It's just two very busy walls. I've got a third section yet to do, where the sheetrock is in better shape. However, I have to open part of that to do some electrical work, so I may or may not replace the sheetrock with CBU there.

I'm going to literally lay the CBU down on the kitchen island, and map out my screw pattern, to ensure I don't put a screw through an electrical line, drain, vent, water pipe, HVAC vent, or any other place that doesn't like screws (there are so many options to choose from! :drool2:).

(Also, don't worry, the countertop, stove, and cabinets will be covered before I install anything.)

cx 04-15-2021 04:04 PM


Originally Posted by Scott
...to ensure I don't put a screw through an electrical line, drain, vent, water pipe, HVAC vent, or any other place that doesn't like screws...

Where's your spirit of adventure? :)

I would want blocking all across the bottom of the wall at the countertop, whatever it takes to get it there. An unsupported CBU edge span there would be in serious jeopardy of damage, I would think.

My opinion; worth price charged.

SWB04 04-15-2021 09:10 PM

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Where's your spirit of adventure?

Ahh, ya mean like when my plumber forgot to leak test a bathtub during construction, and I found out about that the day after drywall was complete, and the tub leak took out half my ceiling over my kitchen, the day before the kitchen cabinets where scheduled to be installed. That was after the 3 day drywall job required 13 days, and buried 15 electrical receptacles, including all the bath fans. Oh, and they nailed an electrical line in my garage, requiring surface mounting BX wiring, because we didn't have the time to tear out the double sheetrock ceiling. Of course, the drywall debacle was preceded by the ONLY city building inspector taking an unschedule two week vacation during the middle of the summer building season, with no relief. I'm kind of adventured out, by now. ????

Yeah, agreed on the blocking. I've done some creative things to make it happen. I finished the first section where there's a gap under the pipe (but I put blocking above it. It feels pretty solid. I think I can do something to mitigate the 3 other areas where blocking is difficult.

Let me finish with a rant.

I would have been a terrific major league pitcher. I mean, I have the memory of a gnat. I'll finish a project, and forget all about things I hated while doing that project. Take, CBU, for example. I love working with Hardieboard. I don't like cutting it, but I rarely have problems with breakage.

Now, contrast that to Wonderboard and Durock. They are made of some nylon netting and some sort of concrete akin to loose gravel. The attached picture show's a problem that started when I lifted the board, and it must have ended up on that corner, starting a slight fracture. It was barely noticeable, but I said, "Hmm, I've this movie before, and it's a horror film. Better be REAL careful with that corner so it doesn't break." So, after trimming another area and adjusting the receptacle opening a half inch, I carefully lift the panel in place. Perfect fit, all except that corner, which was at most, 1/8" too high. The entire six inch corner crushes. I had the rest of the board in, and thought if I could flatten the area, I'd cover with nylon mesh and reinforce with mortar when I'm sealing the gaps. I tapped a piece of 2x3 against it, and ... now it's dust. I can't cut and replace that corner. I doubt I can even repair it with tape and mortar, but I'm going to try. This stuff is just garbage. If you get every cut correctly, and install in nice, rectangular sections, sure, it'll work. If you have any problems at all with a corner, a punch out, whatever, the entire piece is basically trashed.

Hardie was $15 at the big box store, and Durock under $10. I knew I didn't like the stuff, but I didn't remember all the reasons why I hated it. Just like a MLB pitcher: Blow the game last night with a hanging curve ball, forget about it, gotta move on, there's another game tomorrow. Then go right back in the next night and throw that curveball again, to blow that game, too! :bonk:

I HATE HATE HATE HATE Durock and Wonderboard.
I SWEAR I WILL NOT USE IT EVER AGAIN (like the last 10 times).

Tool Guy - Kg 04-16-2021 01:31 AM


Originally Posted by SWB04
...Wonderboard and Durock. They are made of some nylon netting and some sort of concrete akin to loose gravel.

That “loose gravel” is physical damage, not a defect. Could be an impact from a forklift. Yes, Durock (and especially Wonderboard) require a little more work to cut, but that’s true about all sorts of different building materials. In your case, Hardi may have served you better.

If it makes you feel slightly better, I dislike the properties of Hardibacker’s incredible thirst for moisture, inability to bury its bottom edge in a shower mud pan, and inability to use it at exterior locations quite as much as you dislike Durock and Wonderboard. I prefer Durock over Hardi for the ease of obtaining great bond strength. And I also prefer Durock when adjoining 1/2” drywall, as Hardi is just too darn thin.

ss3964spd 04-16-2021 07:23 AM

While horizontal blocking would be nice we're not talking about a high impact area, unless the cooks are prone to slamming a heavy food processor against the back splash.

Indeed, I would focus my attention on getting those electrical box mounting plates and pipe protector plates recessed into the studs so that their faces are even with the face of the studs. If you're using Hardie you might consider some drywall shims, which are 1/16th thick, to fur the Hardie out so it is even with the drywall.

IMO, natch.

SWB04 04-16-2021 06:49 PM

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@KG (Bubba)

That "gravel" CBU damage was caused by me. Post rant, after I slept on this, I decided to just treat this like a drywall repair. Cut out an 8" x 6" rectangle, and replace, probably by overlapping the adjoining cell which has yet to be cut. Add a plywood backing plate to knit the drywall and/or CBU together. It's not as big a deal as I made it out to be.

I was mostly ticked at the time because I was trying to wedge the Durock into the "slot" where the damaged drywall sat between the granite countertop and studs, and it didn't compress to fit (concrete board doesn't compress, who knew?). While I was jockeying to cut down the board to fit, I damaged the opposite corner. I used Durock to build a fireplace surround last year, and I agree, it is easy to stick stone to it (with the help of a sticky mortar like Ardex x77).


The electrical boxes will all need to sit proud to come even with the eventual Quartz ledger/stack stone, so I'm adding box extenders. The plates will sit close to even with the stone (which I may have to grind a bit to fit). That look worked out OK for the fireplace, and a prior backsplash project (see pix).

To all: As far as blocking in general, my goal is to ensure there is no more than 8" of space between two screws. So, where a vertical pipe (Or other item) prevents putting a horizontal blocking stud at the bottom of the backsplash, I'm adding several 6", 2x3 vertical blocks attached to structural studs, stacked, to close the 14.5" gap between studs to something closer to 10". It won't be near as strong as a proper block, but it'll help prevent flexing of the bottom of the backsplash, between the two studs.

For areas where I can reach the drywall (sides, under cabinets), I'm just handling blocking like I would a dry wall patch, i.e. putting 4"x6" or 10" x 6" plywood plate behind the joint. Secure with drywall screws to the drywall, and CBU screws to the CBU, to lock in the patch. The result is pretty strong.

So, adding the structural studs, the blocking, backing plywood plates, the 1/2" Durock, a whole bunch of CBU screws (plus a few drywall screws), 1/2" of combed mortar, and the back buttered 1"quartz panels, there's a lot of stability in that backsplash wall.

I'll post some progress pictures, uhhh, once I make some progress. ????

Thanks for your answers, and interest.

(Note: For some reason, there's no "reply" button after each post, so quoting and replying to person's post via HTML is problematic. That's why I referred to each person by "@".)

Tool Guy - Kg 04-16-2021 07:20 PM


:D :D :D


Originally Posted by SWB04
(Note: For some reason, there's no "reply" button after each post, so quoting and replying to person's post via HTML is problematic. That's why I referred to each person by "@".)

Yeah, the "Quote" button was removed years ago because it was severely overused. But take a look at this short YouTube video for easy instructions on how to do it.


SWB04 06-23-2021 02:40 PM

Been meaning to do a final update on this ....
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Never quite got it "finished" finished, always a little more to do, at least on my projects. However, I'll update with a few pictures in case they're of interest to anyone.

Here are some of my CBU installation pictures. You'll notice in that spoiled corner I mentioned earlier, I cut it out, added blocking, and put in an new corner. Same applies to the receptacle openings, which I mysteriously seemed always to be a couple of inches off (I'm sure a bad tape measure is the cause). I also patched them up in different ways. The CBU was so fragile on some of those smaller pieces, i.e. screws could cause them to crumble. So, I just glued them into the backing and then ran tape and mud over them, i.e. whatever it took to get the job done. Not much else of note in these pictures (I covered blocking earlier).

SWB04 06-23-2021 03:23 PM

More pixs
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These are the tile install/installed pictures. Notes:

1) I used left over unmodified thinset for mudding the seams, i.e. the very gray stuff. I used ProLite High Bond Mortar (Home Depot, only carries white), almost as good as Ardex X77. Short of it is, my stone was much lighter when I bought it, and spec'd the white mortar. After sealing it, it was much darker, and I should have just used the Ardex mortar (gray). Seems like for one reason or another, I always to too light on my mortar color, and with stacked stone, mortar pushes out thru the stone joints (including the resin bonded ones, i.e. we installed pre-fabricated stone panels, and there are gaps between most stones), and I have a clean up mess afterwards. This project was no exception. Ironically, I ran out of ProLite with about 85% of the project complete, and I could bear to buy (and store!) another bag of mortar, so I just too my chances and switched to the Ardex X77. It made ALL the difference. Any mortar that pushed through, just blended in with the rest of the rock. I'd have saved myself a lot of pain if I'd have just remembered the axiom I repeat, after I complete every one of these kind of projects (but alway seem to forget) "GO DARK ON MORTAR!!". Unless I was installing something like Carrara marble, something really translucent and bright, I'd never use white mortar again.

2) We decided to try enhancing the quartz stone color, which was a bit washed out. We've used 511 Seal & Enhancer on prior projects with great effect, but since the price was 1/3 more than Aqua Mix Stone Enhancer, I decided to give it a try. I like both products, to be sure, but they are distinctly different. The 511 was like a thin oil, while the Aqua Mix was almost like an emulsifier, i.e. much thicker. In fact, I placed my stone panels on 2'x10' scrap boards when I sealed them, and I couldn't get the darn things off the boards without prying them off with a putty knife. The Aqua Mix product pretty much GLUED THE STONE TO THE BOARD. I wire brushed the backs of the stone to ensure good mortar fusion to the CBU, and was more careful on subsequent batches. Also, I can't imagine ANY food or grease sticking to this backsplash. The product is so effective, I couldn't even tape off the stone when I applied caulk. I tried the expensive masking tape, the cheap stuff you can never peel off, but nothing would stick. Caulk would not stick. I still swear by the 511 product, but I'm sold on this one, too. The look was beautiful, slightly darker but richer, i.e. enhancing the natural beauty of the quartz.

3) Final note: You may notice shims on the countertop. I shimmed the base of the backsplash wall about 1/16th to 1/8th of an inch, to leave a gap for caulking. Since the backsplash and granite are different materials, cracking at the joints would be likely if I took the stone down to the granite, and sealed the based in with mortar. I used a 40 year, colored silicone caulk to caulk the base, and (since I was using a syringe to apply fine lines) the sides of the stone where it joined with wall trim, kitchen cabinets, and etc..

Not much to tell, really, but I sure wrote a lot of words. :scratch: :blah: The wife likes the result, and that's about as successful a project as one can have. :rofl:

Here are the pix:

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