CBU Woes [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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08-08-2001, 09:33 AM
I'm having some trouble with hanging CBU in the tub/shower enclosure. I got it up there, but it did not go as smoothly as I had thought it would. I'm having 2 problems; The first problem I have is the gap between boards. It's supposed to be between 1/8" and 3/16". Using the score and snap method, I was not able to make accurate cuts and the gap is more like 3/8" along some of the seems. Is this going to cause a problem later on with tiles cracking or popping off the walls? Should I get some thinset into the joints to reduce the gaps, let it harden, then go back and tape the seems? Should I cut new pieces with a masonary blade in a circular saw?(I have enough left over) Should I just not worry about it and tape the joints as normal?
Next problem - I can't seem to counter sink about 75% of the screws. I'm pretty sure that will cause me a problem when trying to spread the thinset. The CBU I'm using is Wonderboard but the screws I bought are for Durock. I really didn't think it should matter. Last night I picked up some different CBU screws at Home Depot. The bottom of the heads of these screws are a little different, more "V" shaped, where the bottom of the Durock screws are flatter. I tried replacing 3 screws and 2 of them went in better. Did I get the wrong screws, or am I doing something wrong? :confused:

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08-08-2001, 11:18 AM
It's not as bad as it seems.
If the gaps are over a stud,as they should be,just fill them with LATEX MODIFIED THINSET then tape using fiberglass mesh.

Wonderboard and Durock are closely related, however, Wonderboard is a little bit more dense,thus the problem of "proud" screws.First,don't use the more "V" shaped screws.They are much more likely to "pull through" the Wonderboard causing structural damage and allowing a spring action in the board.The Durock screws are the right choice as they are made with the wider,flatter head for added support.......just like drywall.
Now for screws that don't want to set flush just give them a good tap with a hammer.Not TOO hard...just enough so that when you place a tile over the screw it doesn't rock.The head should be flush with the surface of the Wonderboard.

*TIP* Use a 1/4 x 1/4 x 1/4" notch trowel instead of a V-notch.This will give you some room to adjust the tiles in the event that a screw just will not set properly.....and you get a stronger installation too.

Bud Cline
08-08-2001, 01:17 PM
The screws I use made for CBU's are "double thread", they have a very "sharp but square point", they are "coated" for anti-corrosion, and they have "two little nibs" under the head to dig at the cement board and promote "setting" of the screw head.

With a 3/8" gap between the boards I can't imagine how you are even getting the boards up there with less than 1/2" of stud remaing to work with. These board edges "break out" easily as it is, even when the boards are tightly butted together.

I wouldn't deny that I have "smacked" a proud screw with a hammer to set the sucker and move on, but with 75% of the screws "stopping proud", I don't think I would beat on that many screws.

If you have installed a vapor barrier as all these guys (and specs) recommend then you don't have the option of glueing the boards to the studs, as I do. I glue all cement board.

Hot shots around here (my town) laugh at me because I also take the time to saw cement board with a diamond blade. Rarely do I "cut & snap" cement board, it isn't accurate and it creates a helluva mess to contend with. And you end up with just what you've got now. So in my opinion it is faster and cheaper to saw that stuff.

If your board is breaking out severly becase you don't have much overlap on your studs then what happens when the weight of the tile is added to the mix? Now your going to hammer on 75% of those screws? Sounds risky to me.

Any screws made for this purpose should work. Try backing the screws out a little and "taking another run at 'em" to see if they will "set". Don't break too many edges though.

Is your screw gun tuff enough? If you are using a gun with a clutch turn that sucker off and "you" control where the screw stops turning. Having that many "shiners" is going to be a problem I think.

[Edited by Bud Cline on 08-08-2001 at 03:23 PM]

08-08-2001, 01:43 PM
The seems I am having a problem with run horizontally. Picture the back wall where the window is. CBU runs horizontally from 1/4" above the tub rim up to the bottom of the window sill and from the top jamb up to the ceiling with a 1/4" gap. That leaves a rectangle about 17" tall x 11" wide on each side of the window. Does this make sense so far? The seems at the top and bottom of these rectangles are the 3/8" seems. I can definitely see where cutting the CBU pays off. I did use a membrane - 30lb roofing felt behind the CBU. I'll try backing the screws out and having another run at them with an A/C powered drill instead of the cordless and see if that makes a difference.

08-08-2001, 01:47 PM
Hey Bud,
I use a Skil-saw to cut cement board too.I found a 6-tooth(carbide) blade at HomeDepot that is designed just for cementious backers.Works great and cuts fast and smooth.

The Durock screws are made like you described except for the "nibs" under the head.Where do you get them?
Oh yea,while I have it on my mind,What are the numbers on that stapler by Senco you got?I want to take a look and want to see the same model you have.

A cordless drill eh? I hope it ain't a Craftsman.I have one that I'm less than impressed with.Definitely use the A/C drill Johnny.Just be careful not to over drive the screws.

Bud Cline
08-08-2001, 02:13 PM

I see what your saying, wasn't thinking about horizontal seams. For those gaps where there is no backup between the studs use some fiberglas joint tape. Most of it is self adhesive but it won't stick to cement board for very long. The dust causes it to release but you do have time to "bed" your joint with thinset. I would tape and bed all at the same time, I think it's the only way your going to get thinset to stay in place.

I've been sawing cement board forever. About eight years ago I bought a cheap 4 inch "Skil" circular saw and a $40 diamond blade. I bought the cheap saw because I know power tools don't last very long around cement dust and I considered it a throw-away. Well about two months ago I replaced the diamond blade for the second time and that damned saw is still going. Eight years man and I didn't pay more than $30 for the saw back then I don't think. It's a little noisy now and after saying this I'll probably burn it up tomorrow, but it's been a good deal.

The cement board screws I used to buy mail-order from McFeeley's Catalogue then they turned up at my local Big Box, I'm not sure of the brand right now but I can look later and let you know.

The Senco I'll also have to get a look at. My truck is in the shop today for yet another front-end-alignment, this makes about a million and six front end alignments. Back before the warranty ran out the shop manager at the Ford dealership would just shake his head when he saw me walk in.

There has been a time or two that I would leave the truck on thier lot at night, before I could call them the next morning they would call me and say "Hey Bud, alignment"?

08-08-2001, 05:03 PM
I saw them in McFeelys.There not cheap.
That old saw I've got is a Skil.I've had it for about 10 years and used it for cutting just about everything even vinyl underpinning for mobile homes.It has seen Craftsman saws come and go.I paid $39.99 for it.

08-09-2001, 10:15 AM
Well I seem to be back on track. I used a $2.99 masonary blade in my old and trusty B&D circular saw to finish cutting the CBU to line the window jambs. Cutting it with a saw is definitely the way to go! Forget that score and snap method. I wonder if that's why the tool was only $2.00:) While I was at it I even replaced the two pieces on the sides of the window to get the seems tighter. I wish I would have used the saw to do the whole thing. My problem was impatience; I didn't think my neighbors would take too kindly to me cutting CBU out in my driveway with a circular saw at 11:00 at night. I should have just waited until the next day.
As far as the screws go, I tried replacing some of the Wonderboard screws with Rock-On screws using a corded drill. Worked much better. A few of them I had to back out and then run back in, but I was able to counter sink them. So I'll just go around and clean things up.
Hey Kalford, you had asked me about my cordless; it's a DeWalt 18V. I do a lot of woodworking, building furniture and stuff. I have pretty much given up on Craftsman power tools. I'll still buy their hand tools, but not anything with a motor. I'll go with Porter-Cable, Bosch, DeWalt, etc.
Here is my question of the day; I'm going to set a marble sill under the window. The sill is 9/16" thick. The space between the bottom of the window frame and the rough opening is 1 1/8". This leaves me with a gap of 9/16" between the top of the sill and the bottom of the window frame. That's a heck of a lot of caulk :D I'm thinking a bed of thinset 1/2" thick under the sill is too thick, especially under the weight of the sill as it sets up. I mentally picture the sill slowly sinking and thinset oozing out all over the new tub :eek: Should I lay down a bed of thinset 3/8", wait 24 hrs and then set the sill with thinset and a 1/4 notched trowel like you would set tile? Should I use something else instaed of thinset for the initial bed? Am I way off base and there is a better/more correct way to do this?

Bud Cline
08-09-2001, 11:49 AM
"Marble Sill"

How about cutting the back of the sill (marble) about 5 or 10 degrees, then sloping the sill to do two things; 1.) shed water back into the shower, 2.) take-up some (if not all) of the 9/16" gap.

The sill should slope some anyway. Even if you can't cut the marble slope the sill and caulk it at the window. The sill should go-in before the jamb-side tile to also promote proper water shed.

I use a Dewalt 18 volt for CBU's, when the battery is low it doesn't want to "set" the screw heads either so I back them out and do it again in low gear, this usually works.

Bud Cline
08-09-2001, 12:10 PM
kalford: "Oh yea,while I have it on my mind,What are the numbers on that stapler by Senco you got?I want to take a look and want to see the same model you have."

Stapler: Senco Model No: SNS45
Uses: 1" to 2" 16 guage, 7/16" crown staple.

The Screws: Midwest Manufacturing (MM), Rochester Minnesota 55904
1-5/8", square drive, cement board screws.
5 pound box, 580 count, $24.95.

I can tell you that they're all there too because I dropped a new box the other day and those suckers spread themselves all over a customers garage.

I must have already trashed my McFeely's so I can't compare cost but at $.04 each seems like a good price to me, I've paid more.

I just did a search for Midwest Manufacturing and found nothing, no website, no business directory listing (AOL) so who knows who they really are.

I buy the screws at Menard's, our local Big Box, but they are only in the north I think.

[Edited by Bud Cline on 08-09-2001 at 02:21 PM]

08-09-2001, 12:49 PM
Hey Bud,
The wall with the window is a block wall and the bottom of the opening that the sill sits on is already sloped towards the tub, so setting the sill parallel to it will automatically slope it. I hope I'm explaining this correctly. There is enough clearance that I don't have to bevel the back of the sill in order to slope it. I just need to build up the rough opening underneath it about 3/8".

Bud Cline
08-09-2001, 02:09 PM
It's becoming clearer. Already sloped huh? Well so much for that idea.

I think we are attempting to hide this little space above the marble simply for cosmetic reasons, right or wrong?

Of so, how about a couple slivers of tile above the sill below the window? Yes/No???

08-09-2001, 02:34 PM
Yes Bud, it is cosmetic, but also you need to seal the seem between the sill and the window. That seem would be 9/16" wide as it stands right now. s far as the slivers of tile, they would end up being about 1/4" I guess, but there is nothing to attach them to. The 9/16" space between the bottom of the window frame and the surface of the sill is just that; space about 1 1/4" deep from the face of the frame. Have I made it clearer or just more confusing now? I wish there was a way I could draw a little picture and post it.

08-09-2001, 03:57 PM
Thanks for the info.I'm gonna take a look at the stapler,maybe keep the roofer for backup.Just FYI,I bought a Craftsman 14.4volt drill to use on the CBU screws....wrong move.It's a good "around the house" drill but the charge doesn't last long under regular(steady) use.I have a Milwaukee 12v that I've had for 8 or 9 years and it will easily outlast my new Craftsman......and has more tork.My dad bought the Dewalt combo kit,saw,recipricating saw,drill and light.(18v) He loves it.Says they hold a charge for a long time.I'm a Milwaukee man myself.....except for my old Skil.

Bud Cline
08-09-2001, 04:08 PM

I've got a Makita also, the original 9.6v and I've had it for 10+ years. By far the absolute best cordless I have ever seen. I've probably bought 10 batteries for it over the years (if that many). I needed two from time to time and one day that big fat heavy DeWalt jumped off the shelf into my lap and said "buy me, buy me" so I did. The ten year old Makita still gives the new DeWalt a run for its money.

Bud Cline
08-09-2001, 04:11 PM

So pack that gap with thinset and tile it anyway. If I had just one dollar for every 1/4" sliver I've used over the years I'd.....................

John Bridge
08-09-2001, 04:11 PM
Thin set a piece of half inch CBU to the sill and then thin set the marble sill to that. Or did I miss something?

Bud Cline
08-09-2001, 04:15 PM

Just happened to think, if you buy that stapler be sure to get it with the "slapper trigger" whatever thingamacallit gadget thing.

If you buy it like that it will be in the original price of the gun. If you buy the little plastic conversion gizmo later it costs about $30 extra.

08-09-2001, 04:30 PM
I always have 3 Makita 9.6v drills in the truck and others in the shop, some 10+ years old. Can't live without them. Have a couple saws and flashlights that use the same battery, so I always have 8 or 10 batteries. Their only weakness is their newer chargers. If you only own one cordless drill, that's the one to have.

But I also carry an 18v DeWalt drill for the heavier work and the companion circular saw will make you look for things to use it for. Even for a tile-only kinda contractor, that drill/saw combo would be well worth the money spent. That way you have two batteries to start with and the carrying case is excelent. I will bet the drill will screw anything you could want to install for a full shower with a single charge. Try it, you'll like it.

08-10-2001, 02:13 PM
O.K. Bud,thanks.I've found two so far.One is a 1 1/2" and the other is a 2". Both just above $300.00.....twice as much as the Senco Stack Tank compressor I want...imagine that.

Bud Cline
08-10-2001, 03:13 PM

Ain't she purdy?