Hump in Concrete Board

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  #1  
Old 01-21-08, 02:19 PM
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Angry Hump in Concrete Board

First off, this is the first time laying tile. I am installing 12" ceramic tile in the main bathroom House was built in the 50's and the subfloor and underlayment is nearly 1 3/4" thick.

My first, maybe minor mistake, was when I was at Lowes I picked up 1/2" concrete board by mistake. There is enough clearance for doors and everything else so I decided to go with it instead of lugging it back to the store.

Anyways I was dry fitting the tile and noticed one tile just slightly higher than the others. Upon closer inspection I noticed a slight hump in the concrete board (maybe up to a 1/16 of an inch high and smaller in diameter than a CD). This was also causing the tile to wobble some. I know the floor was clean before I placed down the concrete board and I didn't notice anything wrong with the floor before. I am at a loss with what could be causing this. The floor is stable, with absolutely not movement.

A possibily dumb solution would be to sand this hump down. Otherwise I am looking at removing the board.

Any suggestions or comments would be appreciated.


Edit: After posting this I decided to bit the bullet and pull up the concrete board. Under the exact spot there is a ever so slight hump on the underlayment. At this point I am thinking of using a sander and a good level on this problem...
Scott
 

Last edited by supton80; 01-21-08 at 02:38 PM. Reason: Update
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Old 01-21-08, 04:08 PM
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2nd Question

I used Phillips II Cement Board Screws on the cement board. Even though they are suppose to screw in flush, a few just will not screw flush and are just slightly above the board. This of coarse is causing a tiny bit of wobble for the tiles. Will the thinset compensate for this or should I remove the screws and try again elsewhere. (Sorry, from all the threads enforcing the idea of a flat mounting surface to avoid cracked tiles, I am just trying to be overly cautious)

Thanks for all the help!

Scott
 
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Old 01-21-08, 05:37 PM
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Did you thinset the CBU down to the subfloor?

Sometimes the screws will not go down all the way like you experienced, and that is why some people recommend using roofing nails instead.
 
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Old 01-21-08, 08:26 PM
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Hi Scott, Mark,


Sounds like you didn't thinset the board to the plywood, if you had, you wouldn't have been able to remove it after a while. That step is critical, you must install the board into thinset, fasten with proper screws or roofing nails and tape the joints etc.

Jaz
 
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Old 01-22-08, 12:00 AM
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Opps...

Well I'm good at admitting my mistakes. Actually I read about placing thinset down under the concrete board, but my inlaws basically said "aw we've installed ceramic tiles for years without thinset under the concrete board, and never had problems". Well now is the time to correct the error if it is to be done right.

Anyways, I will unscrew all the boards and place the thinset down first. So when attaching the board with concrete board screws, can I use the existing holes, or should I make new ones?

Second, back to the question of flush screw heads. I am assuming that I need these screw heads absolutely flush or recessed, otherwise I will run into problems. Is this correct? Any suggestions for those stubborn screws?

This isn't my house, it is a church parsonage, so I want it done right. So I hope you don't mind all the questions.

Scott
 
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Old 01-22-08, 08:26 AM
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Don't you just love them inlaws....?

What brand cement board are you using? Some tend to crumble a bit. Try the same holes if in good shape, but more holes won't hurt anything.

Screw heads should be flush within 1/16". You can substitute 1 1/4" roofing nails for some of the screws. Do not over drive the screws, recessing them will break through the fiberglass mesh and weaken the board.

What kind of thinset mortar are you using and which trowel?

Jaz
 
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Old 01-22-08, 10:10 AM
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The brand of cement board is 1/2” Durock. The boards are in good shape, the cut lines and the screw holes are fine. And thanks for an exact number to go by with the clearance on the screw heads, I believe they are all 1/16 execpt for maybe a few.

The brand of thinset mortar is Laticrete and it will be applied with a 1/4” trowel. Everything I have for the project I picked up at Lowes (Home Depot is too far away).
The room that I am working on is about 59 sq ft, so unfortunately I will have to pick up some more thinset for the tile work. The bag says it only covers 80-95 sq ft with a 1/4” trowel. I don’t know what brand of thinset that will be until I go to the local building material store (they are expensive but high quality merchandise).

One last question then, when the concrete board is installed correctly, how long should I wait before laying the ceramic tile. The set time on the thinset bag (24 hrs) or could I possibly get away with working with it sooner?

Appreciate the help though.

Scott
 
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Old 01-22-08, 10:32 AM
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Be sure to use a modified thinset to install the tiles. Don't forget to tape the joints with proper fiberglass mesh cement board tape. The 1/4x1/4 is correct for the Durock, but I recommend 1/4x3/8x1/4 to install the tiles.

You can start installing tiles as soon as you've finished with the CBU. No need to wait, unless there's a football game on or something?

Jaz
 
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