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usafdrew 04-10-2012 09:22 PM

Assistance in determining concrete slab flatness
 
Hello all. First post here but I'm sure it won't be my last since I've just gotten started with the spring to-do list. Let me first say thank you in advance for any and all advice.

I am about to undertake a kitchen tiling project with 12x12 ceramic mixed with 6x6 ceramic. Should nice once done, but I'm OCD when it comes to work I do myself so i want to ensure things are done right. Already got the linoleum removed and adhesive scraped all up. Now comes time for checking 'flattness'. This is where I'm a bit unsure. I see the general rule is for 10' straightedge, no more than 1/4" deviation. Now does this mean no more than 1/4" deviation at any one point along that 10' or aggregate? I tried using a straightedge, but decided to go with a stringline for better accuracy (hopefully). So I've got a 10' section of taught stringline which just so happens to fit nicelely from wall-to-cabinets. It sits 1/2" off the ground at each end, supported by tile pieces at each end. I make 2ft passes across one end of the room, then go across again perpendicular to cover both directions. Does this sound like a solid plan? So from a few passes so far, I'm getting a drstring-to-ground height of 3/8" where there's a 'high spot' almost in the center of the room with the rest still close to 1/2" string-to-ground. This difference is under the 1/4" so I should be ok right? A goofy paint sketch outlines the method..

http://s13.postimage.org/qe2bl33ev/tile.jpg

I'm hoping not to have to use SLC but haven't finished the measurements yet. I'm sure I'll be back with more questions after I get this part knocked out. Thx

oh'mike 04-11-2012 05:35 AM

That's the hard way---I use a straight edge---nice to be able to spin the straight edge to check in all directions.

I think you may be okay---get the straigt edge out again--stack coins in the deepest low spots--see what you really have---

JazMan 04-11-2012 11:45 AM

I agree the straight edge is the best way. Make sure it's really straight with no flex. If it's wood, I think a 1x4 is the best. The floor need to be "within 1/4" in 10 ft. and 1/16" in 12" of the required plane. for standard ceramic tiles. It needs to be flatter if the tiles are large and/or smooth and shiny.

Jaz

usafdrew 04-11-2012 01:21 PM

I tried using a straightedge with most straight I could find 1x4x8, but even then it had a 1/8" bow at the end. The 1x4x10s were firewood. I gave up when nothing measured was consistent and went with the string. I've finished checking the room with it and it's been spot on. A little more difficult to move around, but I got 10ft readings every 2 ft from both sides of the room.

There are a few high spots that hit 1/4" deviation in 10ft. No more than 1/4" though, so instead of sanding down, could I just be a little more generous with a mallet there when setting the tiles?

JazMan 04-11-2012 02:20 PM

Quote:

could I just be a little more generous with a mallet there when setting the tiles?
Well sure, that, along with a beating block are part of the procedure, but the cured thickness of the thinset should be at least 3/32" or so.

Are these high quality tiles, are they regular edges or rectified? How wide a grout joint or do you have a choice with the pattern?

Jaz

usafdrew 04-11-2012 03:02 PM

Quote:

Originally Posted by JazMan (Post 896705)
Well sure, that, along with a beating block are part of the procedure, but the cured thickness of the thinset should be at least 3/32" or so.

Are these high quality tiles, are they regular edges or rectified? How wide a grout joint or do you have a choice with the pattern?

Jaz

The tiles are 12x12 Del Conca Roman Stone porcelain. Edges are slight rounded and has a PEI rating of 5 (heavy traffic). They'll be mixed with 6x6 Eliane Mt Everest Verde porcelain which also has slight rounded edges but don't know the quality. I was planning on 1/4" grout lines.

Another couple questions come to mind now that I'm moving forward. With the linoleum adhesive scraped off, there are large areas of the slab that seem almost polished and when water is poured on it, it takes a real long time to absorb. The rest of the slab is fine and water absorbs at a steady rate. Should I be worried about this before applying the tile adhesive? I asked the Lowes guy. He seemed pretty knowledgable and said not to worry, just as long as all the old adhesive is removed. Also, there is one crack that can be handled with Redgard that I have. Originally I was going to membrane the entire slab, but the Lowes guy talked me out of it since the slab is 15 yrs old and won't be cracking anymore. Does this sound right? Thx.

JazMan 04-11-2012 08:15 PM

You should be concerned about those areas in question. Moisture has to be able to soak into the slab to achieve a bond. Most sheet vinyl adhesives are water soluble, so I think it matters.

I'd be soaking the area with hot water as a first step. Even adhesive that has been removed will leave a residue in the pores of the concrete.
Quote:

just as long as all the old adhesive is removed.
That's the point, it has not been removed. That is why water won't readily soak now. I would test and not have continue on faith alone.

Jaz

usafdrew 04-24-2012 12:46 PM

Quote:

Originally Posted by JazMan (Post 896974)
You should be concerned about those areas in question. Moisture has to be able to soak into the slab to achieve a bond. Most sheet vinyl adhesives are water soluble, so I think it matters.

I'd be soaking the area with hot water as a first step. Even adhesive that has been removed will leave a residue in the pores of the concrete. That's the point, it has not been removed. That is why water won't readily soak now. I would test and not have continue on faith alone.

Jaz

Thanks for the advice. I went at the floor with a grinder to take off what must have been a sealant and now the water will absorb and all adhesive is gone. I've also now finished mapping out the floor and there is just one high hump that doesn't quite hit 1/4" deviation anywhere in that area from floor to string line. Does this mean I'm probably good to start laying tile?

JazMan 04-24-2012 03:24 PM

Quote:

there is just one high hump that doesn't quite hit 1/4" deviation anywhere in that area
If that hump falls within the specs as I wrote in post #3, then you would be OK. Better check again.

Jaz


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