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Unread 06-29-2004, 04:59 PM   #1
vietco
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slightly stepped hardi subfloor

My original bathroom subfloor is 5/8" flywood on support beam with 18" apart. I put down the 1/2" hardi underlayment for marble tiles. I used thinset underneath of hardi boards and also screwed them onto the subfloor at every 8". Since the bathroom floor is big, I had to mix several buckets of thinset to set all 7 full hardi boards down. On the last mix, I accidently added more water in the thinset than those previous ones. This results that a couple of hardi boards at/close-to the entrance sinked slightly lower than the others (about 1.5mm or ~80mil). Now all the hardi boards had been done for 1 week so I don't want to pull the 2 lower ones and redo unless it is a MUST.
Can I compensate for these lower boards with a thicker layer of marble thinset when laying tiles? Any special treatment needed for at the steps?

thanks,
Anh
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Unread 06-29-2004, 05:16 PM   #2
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When you lay Marble ( or any other stone) you will be making sure each tile is precision placed with respect to the other. I see no issue in compensating using thinset or a medium bed mortar. Some of your stone will likely be thicker/thinner than other as well so even if the floor was level, you would still need to adjust the tiles.

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Unread 06-30-2004, 07:10 AM   #3
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Anh,

Let's go over this again.

1) You are installing marble tiles?

2) Your subfloor consists of 5/8" subfloor with 1/2" hardi. Your spacing is 18". How long does it span?


I think that you need to remove the hardi. Then I think you need to check and be sure the area is stiff enough to accept marble.

The deflecto will help. We answer specific questions.

Again, I say stop!



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Unread 06-30-2004, 07:26 AM   #4
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easy fix...they have this bag of concrete mix called leveling compound at home depot. Just mix one bag up...pour it into the unleveled spots and it will level out just like water. Be careful on how much you poor as it literally runs like water and will drain out through the smallest of holes. But for leveling for tile, nothing beats this stuff.
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Unread 06-30-2004, 02:35 PM   #5
vietco
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Thanks all for good advices as always!
I'll make sure that I'll compensate with thicker thinset on the lower board like Jason said.
What is the best way to level these tiles? In previous bathrooms, I just precisionly placed each tile down to the same level with the other tiles (at least at the edges). I tried the 2x4" wood tap method, but it seemed a lot harder to get all the tiles level at the edges and of course I was afraid my tiles would hurt I used 1/16" spacing between tiles so any little unleveled edges is much more amplified. Overall, 2 bathroom floors I've done turned out much better than I expected, but it went at a rate 4-6 tiles/hr( Now, this masterbath floor is much bigger ~120 sq. ft or ~24 hrs of back breaking work...uhrrr...

Steve, I knew that someone is going to tell me of my worst fear..., but both my wife and I "love" the look of marble. We both have gone to great length of pain to finally have the Daino Reale marble so that is why I try to pretend that I know no better about the subfloor flex. And that is why I chose 1/2" hardi with thick thinset underlayment. I eye-ball inspected the floor flex by having a couple of people "dancing" on the finished hardi floor. I did not notice any potential problem...knock on wood The other smaller bathroom has the same subfloor except the thinner 1/4" hardi's. We're using it daily for 2 months now and it is still going...
Ok, enough marble loving...how do I find out about the span? These bathrooms are on the second floor and I saw all the screws are evenly spaced out along the support bars. This is 1999 Ryland built home if that helps.
Some pictures of a guest bathroom...still need the molding, caulking and of course decorating.
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Unread 06-30-2004, 03:02 PM   #6
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Pix's of the guest bathroom

I could not attach the pictures here due to its size. Follow the link for the before and after pix's of the guest bathroom... it is not completely done yet.

cheers.

http://f1.pg.photos.yahoo.com/ph/dta...=/150b&.src=ph
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Unread 06-30-2004, 03:13 PM   #7
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Hi Anh,

You could call the builder, they may retain a plan set and could just tell you.

Beyond that, I guess you could hope for the best and move on with the self leveling compound idea and hope for the best. Don't forget the self leveling primer.

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Unread 07-19-2004, 04:05 PM   #8
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Subfloor flex ??

I'm about to tile marble on my upstair bathroom, but stop short when I read kokopup's thread.
My original subfloor is either 5/8 or 9/16" flywood sitting on support beams that are 16" apart. I don't know how far the beam span, but base on the structure, I says at most 12' (with the tub and the vanity sitting on both ends). I did hear some quirky sound as walking on it. I did fasten it down with a few screws and also had 1/2" hardibacker board on top (with about 1/4" thinset underneath). The floor seems very solid now. I'm worry now since I have not measure the floor flex yet. Where do I rent a deflectometer? HD has no idea about the defectometer.

thanks,
Anh
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Unread 07-19-2004, 04:09 PM   #9
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Let say this subfloor is not good enough. Can I lay another 1/4" hardibacker board on top? I really try to avoid ripping the whole thing out. We are light people (170lb max max.) if that help
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Unread 07-19-2004, 05:42 PM   #10
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Hi, Anh:

Is the same floor we've discussed before on another thread? If so, please don't make any more threads for this same project, just keep asking your new questions on this same thread. Otherwise, we'll all forget what we've discussed and what we have not and we'll all be confused and perhaps fail to give you the information you need.

If we have not told you before, you will need a floor that is very rigid to install that natural stone tile. If you have only one layer of 5/8ths inch plywood over joists on 16 inch centers, you already have a problem. We also need to know the size and span of those joists. No maybes here, actual measurements.

The deflection can then be calculated for your floor joist structure from this information. There is no actual measurement tool called a deflectometer for this, that's just what we call the calculator we have here on the site (little icon at the top right of this page). We plug in the dimensions you give (or you can do it) and it will tell you if your floor is suitable for tile, or for stone.

So get us all the information and we'll give you all the advice you need for a successful installation.
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Unread 07-19-2004, 07:05 PM   #11
vietco
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Sorry that I started a new thread... I will merge it back if I know how.

So from what you are saying and from the deflectometer from this page, it is only the joists that matter, right? So if my subfloor joists don't meet the specs, then adding more backer boards on top to increase overall thickness won't help. Am I missing something here?

thanks,
Anh
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Unread 07-19-2004, 07:15 PM   #12
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No, Anh, the subfloor thickness is more important than the joists. We don't lhave a calculator for that because the underlayment manufacturers specify the plywood thickness in their installation instructions.

5/8" plywood is not thick enough for stone tiles. You need 1-1/4". You don't add the underlayment to the thickess. CBU (backerboard) does not add anything significant to the subfloor stiffness.

To solve your subfloor problem, you have to add plywood, either above the existing subfloor or below. Is this floor over finished living space?

Bob
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Unread 07-19-2004, 07:36 PM   #13
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Bob, one good news from your reply is that my subfloor could still support marble tiles with modification. So with my current 5/8" flywood + 1/4" thinset + 1/2" hardi, can I do anything or I have to rip hardi off and start from fresh. Let say 5/8" plywood + 5/8" plywood + thinset + 1/2" hardi, will this be ok. If I really have to rip the existing hardi board, what is the best way? How about the thinset residue?

thanks,
Anh
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Unread 07-19-2004, 07:39 PM   #14
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Bob, yes the bathroom floor is directly above the downstair family room... I don't have a good way of finding out the structure beneath. I tried to contact the builder, but not much success.

thanks again,
Anh
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Unread 07-19-2004, 08:16 PM   #15
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Don't get so overjoyed with Bob's good news that you forget that your joist still must meet deflection standards, too, Anh. That means we still need to know the size and unsupported span of the joists. To find the size, you'll need a hole in the floor or the ceiling below so you can look in there or at least stick a tape measure in to gauge the depth. I'm going to speculate that in a Ryland home with spacing of 16 inches on center, the joists are sawn lumber rather than an engineered joist. That could be wrong, but we can start there.

You can get a close estimate of the span by looking at the walls in the room or rooms below the floor you're working on. In most cases you can say that every wall in the rooms below is a support wall for the floor above. Determine what is the maximum distance between supports in the direction you know the upper floor joist to be running.

I don't mean to be causing you undue concern or additional work, but if your floor structure - both joists and subflooring - is not adequate, you may be wasting a lot of effort and money on a marble floor that is likely to fail. No matter how much you love it.

When we get the floor structure issues resolved, I want to talk to you about your CBU installation. I don't see nearly enough screws in your photos, if I'm viewing them correctly.

My opinion; worth price charged.
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