bumpy Wonderboard seam

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Old 03-26-09, 02:18 PM
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bumpy Wonderboard seam

Hi all,

I laid down Wonderboard over a plank subfloor (thinset, then screws), taped the seams with fiberglass and thinset, then let it dry for 24 hours.

When I came back to the job, I noticed some of the thinset raised up in the seams, making the seams bumpy (like a speedbump, with a smooth profile). It's enough to have a square of 12" tile rock back and forth when laid directly over the seam. I'd say the height was about 1/8" or so.

Will the thinset used for the tile help to smooth this out, or am I going to have a problem?
 
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Old 03-26-09, 02:23 PM
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You have worse problems.

First, you should not have installed the board over planked flooring. You should have first determined if your current floor joist structure is suitable for tile.

Can you please tell us your floor joist sizes, center to center spacing of them, and the unsupported span?

If you get us those #'s, and they are good, you would then have to install at least a layer of 1/2" exterior grade plywood, then the cement board.

From the sounds if it, you got some movement in your floor, which is very bad for tile.
 
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Old 03-26-09, 02:35 PM
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Originally Posted by HotinOKC View Post
Can you please tell us your floor joist sizes, center to center spacing of them, and the unsupported span?
The immediate subfloor is 1" x 4" tongue and groove. Below that is what looks like 2" x 10" planks laid diagonally. All that sits on top of joists of 2" width spaced about 16" apart. I installed the Wonderboard directly over the 1" x 4" on the advice of several friends and as well as articles I read.

The floor, with Wonderboard installed, seems extremely rigid and, other than the bumps in some of the seams, is fairly level.
 
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Old 03-26-09, 02:46 PM
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Your friends are wrong, and do not listen to anything they show on those DIY shows.

This is directly from Wonderboards installation instructions:
Subfloor and countertop
base should be an exterior
grade plywood with a
minimum thickness of 5/8"
(16 mm) when using
1/2" WonderBoard. When
using 1/4" WonderBoard,
base should be a minimum
of 3/4" (19 mm) thick.
The total wood subfloor
including the plywood and
WonderBoard should be a
minimum of 1" (2.5 cm)
thickness.
http://www.custombuildingproducts.co...er=arc&lang=en

If you still want to continue the installation and risk failure, then you should remove that "hump." I always do the board taping as I tile so I don't have this issue.

Your call.
 
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Old 03-26-09, 03:19 PM
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Roger

Cement board needs to be installed over something more stable that planking. The planks will expand and contract independently as moisture conditions change, and that movement can cause grout and tile to crack. The cement board will not be enough to protect against that movement. If you have 2 layers of planking, your best bet would be to remove 1, and then install 1/2" exterior glue plywood bc or better over the remaining plank subfloor.

I know this is not what you want to hear, ouch.
 
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Old 03-26-09, 03:48 PM
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Let's assume for now I'm willing to take the risks. I know the planks are subject to expansion. I went ahead on the installation on the advice of my father, who installed concrete backer board (not Wonderboard) over hardwood several years ago (so far so good) and the advice of a carpenter friend with tiling experience. Both said the same thing—tighten the planks before starting, bond the backer board strongly to the joists with screws, and use an acrylic modified thinset to help with any expansion/contraction.

So, yes, I know I am up against a huge risk but at the moment I'm willing to plow on.

Having said that, does anyone have an idea of what to do about these seams? Or should I hire one of you guys to rip everything out and start over?
 
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Old 03-26-09, 03:50 PM
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What is the height of these seams? We are just talking about thinset, and not the actual board? If just thinset, mechanically remove them with putty knife.
 
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Old 03-26-09, 03:56 PM
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The board is level, it's just the thinset. I didn't think a putty knife would be strong enough to chip the thinset after it cured…I'll give it a go.
 
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Old 03-26-09, 06:16 PM
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YIKES!

I need a break from this.Beer 4U2

Jaz
 
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Old 03-26-09, 06:23 PM
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Obviously this has been an issue around here. :|

So I have to ask—are there any verifiable cases of a major floor failure due to this sort of installation? Or is everyone just mentioning out of precaution?

The chance that I might have to replace a few cracked tiles in ten years is far outweighing the time and cost of ripping up several hundred dollars worth of backerboard and modified thinset and starting over from scratch.
 
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Old 03-26-09, 07:58 PM
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Roger,

You are absolutely right that at this point you may as well continue if your only mistake was installing the CBU over the planks. It's your house, you did the work and if you get some minor cracks or some tiles shearing off the floor, you will still be happy with your handy work.

Again, if that is the only thing I believe you've got a good chance of never having a big problem. I hope you'll understand that we can not recommend others making the same mistake as you.

Your Father's single positive experience and your carpenter's qualifications, do not impress any of the tile setters here.

Jaz
 
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