Tiling a slab with a hump

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  #1  
Old 08-13-07, 09:09 AM
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Question Tiling a slab with a hump

Hi,

I would like to tile (glazed porcelain 12 x 12) the 10 X 20 cement slab floor of a renovated sunroom. The cement slab is 15 years old, has no cracks and has NOT been covered with sealers, paint etc.

What concerns me is that near the fireplace where we have removed the old brick hearth there is a noticeable gradual rise in the floor up to the face of the fireplace (1/2-3/4 inches rise over 4 feet). This takes place in a corner of the room which will not be a walking area.

The local tile store owner said that if I can live with the sloping tile, I can just go ahead and tile right over thinset. I figured to ask you guys to make sure this is OK.

If this is indeed an OK idea would you please advise me on the following in this case:

1) 1/2 x 1/2 trowel vs. 3/8 x 3/8 OR other size?

2) HD Flexbond (modified powder) thinset vs. Tec FullFlex OR something else?

3) Butter the tile or not?

Thanks in advance!

Leo.
 
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Old 08-13-07, 04:56 PM
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1) 1/4 inch square notched trowel.

2) As long as the thinset is a latex modified, powdered thinset, it should work fine. We normally have that one thinset we swear by, but that's the result of doing this stuff for years and we notice the differences. Not being a pro, any of them that fit that description will work for you.

3) I trowel the thinset onto the floor, comb it, and then work a good layer of it into the back of the tile with the flat side of my trowel. It adds a little time to the job, but insures a good bond from floor to tile.

As to tiling the raised area in your floor, you can tile a tree if you do it right. Properly done, you can tile nearly anything. It is certainly possible to tile the "hump" in your floor. You may want to give it a little thought in the planning and layout stage in order to try to avoid having to split any tiles in order to accommodate the plane change where the flat meets the hump, but it is doable. You may need to use extra thinset in some areas to avoid hollow spots under it, but that you can deal with when you get there.
 
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Old 08-13-07, 05:56 PM
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Smile Grateful

Thanks for the tips!

:-)
 
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Old 08-13-07, 06:09 PM
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Hi Leo,

Half to 3/4" in 4', YIKES, that is a lot! You should fix it 'cause it's gonna stick out like a sore thumb. You will probably have a severe lippage situation unless it's corrected. The standard is no more than 1/4" in 10 ft. AND 1/16" in 12" of the required plane. You either grind down the high spots or raise the rest. Rent a machine, won't take any time at all.

1. With standard tile and flat floor, 1/4x3/8x1/4 is best. You can use larger, up to 3/4" U if the floor needs help AND you also use the proper medium-bed mortar.

2. Those two are high-end modified mortars and work fine for most installations. Neither are medium bed though. If you need a medium bed, Custom makes several. One is Marble & Granite mortar. Tec makes one called Medium Bed thinset.

3. Those tiles are not warped, have the standard grid pattern on the back. There is no need to back-butter...unless you use too small a notched trowel.

Jaz
 
  #5  
Old 08-13-07, 07:08 PM
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Machine?

Thanks JazMan!

What kind of a machine can be used to grind down a big chunk (4x4) of a concrete slab to make it flat and even with the rest of the slab floor? Is this machine going to take a professional to have the grinding done right or can one count on succeeding on the first try?

:-)

Leo.
 
  #6  
Old 08-14-07, 06:02 PM
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There are hundreds of different types and brands made. Any thing from hand held, to models for large size job. The best thing to do is to visit a few tool rental shops to see what is available in your area.

Jaz
 
  #7  
Old 08-16-07, 02:02 PM
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Question How flat?

Well, putting aside the hump for a moment I went and tested the rest of the floor by putting my 12x12 tiles down. In MANY places the tiles rock! Does the floor have to be so flat that there is almost no rocking at all, or can one use the thin set to take care of the rocking?

It seems that my slab is quite smooth but not flat. It has highs and lows all over (just like in a gently undulating pool). Even the hump is very gradual with no clear delineation of its boundary.

If one must have a very flat floor, is JazMan's grinding idea the only good recipe for my "wobble-inducing" concrete slab?

:-)

P.S. Sorry for splintering into two threads; my bad.
 
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Old 08-16-07, 04:10 PM
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When installing tiles we want flat as I have described above. We don't care about smooth. Matter of fact we don't want smooth, a fine broom finish is best. If you don't want to lower the high spots, you can raise the rest.

You should check the floor with a straight edge to determine how un-flat it is. The use of a medium bed mortar with larger notches will help too.

Jaz
 
  #9  
Old 08-16-07, 04:37 PM
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Thanks for all the help, JazMan!

Brushed finish is what I have, but the surface overall is definitely "wavy". What do I use to raise the low spots once I grind down the high spots?

Also, I know that medium bed gives a lot less adhesive strength that fine bed. Is that ever a problem for porcelain over a slab?

:-)

Leo.
 
  #10  
Old 08-16-07, 06:36 PM
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If you grind down the high spots, you won't have any low spots. You would use a cement patch to raise only certain areas, or SLC (self leveling cement) to better bring things level. Remember SLC will flow to find it's own level, so it might raise the floor more than you'd expect?

It is NOT true that medium bed thinset has less strength than regulars thinset of the same grade. (Where did you read that?) If you're going direct over concrete any modified thinset will work.

If you want to step it up a notch you could install an isolation sheet membrane such as Ditra over the concrete, then tile over that. Reduces the chance of cracks in the concrete from cracking the tiles, also isolates the tile from unequal lateral movement in the substrate. If you go with Ditra, you'll be using unmodified thinset instead.

Jaz
 
  #11  
Old 08-17-07, 06:42 AM
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Get yourself an 8' straight edge and lay it down on the floor in various different directions around the rooom. Mark all the high spots that youll need to grind down with one color chalk and any low spots you may need to fill with another color chalk. Grind the high spots first and then repeat with the straight edge to see if you still have to fill any of the low spots. As Jazman pointed out some of the low spots will go away when you grind down the high spots. The medium bed mortar will help some, but it'll take some good setting skills to get the finished floor nice and flat if you do nothing to that wavy floor. Starting with a nice flat floor will definitely give you better finished results.
 
  #12  
Old 08-20-07, 07:36 PM
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Question

I am still pondering my options, feeling a bit reluctant about grinding the slab.

1) Can one put cement board over a concrete slab?

2) A local contractor offered to put down a decoupling membrane and then 2 inches of concrete over that, finished "very flat". Said it would be a lot less expensive than SLC. Is this a good solution? (Our drywall goes down to the present floor, so this way we get drywall below the level of the new floor. Can this too be a problem?)

3) SLC? Is Ardex the way to go in this case? 200 square feet at 1/2 inch can get very expensive I reckon...

What do you think?




Still conundrumed,

Leo.
 
  #13  
Old 08-20-07, 09:51 PM
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You can't install cement board over concrete. What would it change anyway? The finished floor would be the same.

Obviously a new mud bed would be the best way to level that floor. Let that contractor do it.

Jaz
 
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Old 08-21-07, 03:17 AM
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The contractor has it backwards-----The mud goes on first and then the uncoupling membrane!!!
 
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Old 08-21-07, 04:37 AM
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Question

I am not sure that "mud bed" is the same thing as a new layer of concrete (which if I recall the contractor called a "skim coat"). The contractor is proposing a 2 inch layer of concrete over the existing slab. Since the new layer may shrink/expand at a different rate from the slab he wants to "decouple" it.

Perhaps I should then also decouple the new layer from thinset as well? Hmmm...


Thanks for all the help so far, guys!

:-)

Leo.
 
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Old 08-21-07, 06:11 AM
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Tilepro, the contractor was probably meant a cleavage membrane with lath above then the mud, as if going over a wood structure.

llivshi, a mudbed over a cleavage membrane would only need to be about 1.25" thick. That is where the 2" thickness probably comes from (1.25" min at the top of the 3/4" ramp up to 2" thick at the other side) It can also be directly bonded to the existing slab with thinset at a minimum of 3/4" thick so the max thickness given your 3/4" rise would be 1.5" thick. An uncoupling membrane or an anti fracture membrane could then be used over the bonded mud. Would you need it? Maybe, maybe not. It's like a vaccine...If you get it, you never know if you needed it, if you don't get it, you always know if you should have.

You will need an expansion joint in there somewhere too. You don't mention any expansion joints or saw cut control lines in the existing slab but I would expect to see them in a slab that size and the unbonded mud bed would be the way to isolate the mud bed from those movement joints in the slab.


Hey, could you post some photos to a site like flicker or photobucket? Maybe you ISP gives you web space?
 
  #17  
Old 08-21-07, 01:37 PM
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Just curious. You say the slab is in good condition. Whats the square footage of this ramped area? Im just trying to understand if it makes sense to add 2" of mud to a 200 sq ft room rather than grind down a small area in the corner of the room?
 
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Old 08-21-07, 02:39 PM
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Tilebri, thanks for pointing that out. When they say decoupling or uncoupling I automatically think along those lines.
 
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Old 08-21-07, 06:50 PM
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Geeze Johnny, why not add 2" of mud to 192 sf instead of grinding down 8 sf???

3/4" in 4' in one corner of the room, since you asked
 
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Old 08-21-07, 08:20 PM
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Unhappy

Seeing how grinding down the hump seems to be a favorite, I went and took the time to "survey" precisely the relative floor elevations across the whole room. I should have done this earlier (rather than trusting the measurements taken by the tile store rep), because I am ashamed to say "the hump" turned out to be "a sag". The elevation near the fire place on top of the hump equals that on the opposite side of the room, while in the middle it dips down as much as 1.6 inches. Near the other two walls there is variability, higher on one side than the other etc. I guess I really do need to have someone come and build up the cement bed on top of the slab to even it all out.

Tilebri, what is a cleavage membrane and lath? When you folks say "mud", do you mean concrete or some other special thing?

This project sure has turned out a lot more than what it seemed at first... I really do appreciate all the help!

:-)
 
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Old 08-22-07, 08:14 AM
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A cleavage membrane acts to isolate the mud bed from the substrate. That way the mud bed can basically float above any movement in the substrate. It's also sometimes referred to as a slip sheet. Often you see tar paper used because it helps to prevent moisture from leaving the mud bed which would weaken the strength after curing. Over a slip sheet and lath, at it's minimum thickness, it's a monolithic structure that pretty much just does it's thing on it's own. In your case, unless there were cracks that needed to be isolated or control joints that needed to be relocated, there isn't any real reason not to bond it to the existing slab.

Mud is often used as a reference to thinset but mud in the sense of this thread referes to a mixture of about 5 parts sand to one part portland cement mixed with enough water to make it clump, like sand castle sand, where it's packed and shaved resulting in a very dense and flat surface. It's low water content makes it less prone to both shrinking and cracking when kept within thickness range but when needed thicker, reinforcing wire can be used in the bed. Unlike a regular pour of concrete, mud stays where you place it and doesn't have the 1 month cure time that a regular pour would have prior to being able to set your tile. So keep that in mind if the one contractor was going to actually pour concrete and not do mud.
 
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