Slab has cracks & is not level - need your advice on go or no-go [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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12-13-2016, 05:40 PM
Hi all,

My name is Derek and wifey and I just bought our first home in April. We're in Florida (Merritt Island) and have been renting all our lives..and are now faced with the joys of home-ownership and the "fun" DIY attitude that comes along with not having any money left over after dropping a down-payment :)

The previous owners left us with a nasty flea infestation in the 300sqft master bedroom (used to be carpet). After 3 weeks of bombing the room and brushing in Diatomaceous-earth‎ powder during this period, we finally got rid of the fleas! We were so turned off by the carpet that I hastily bought $1K worth of 6 x 12 wood-look porcelain tiles and then ripped out the carpet and baseboards in a matter of hours. I realize now, that this order was definitely not I now have this tile sitting in my garage for over 6 months while I'm still trying to figure out what to do. I doubt that HD will take back the tile...but I haven't totally given up yet. Here's why I haven't progressed yet:

1. The slab has 3 giant (i.e. spanning half the room) cracks that all meet at one point.
2. The floor is REALLY not level. Over 4 ft I found spots that revealed a 1" difference.
3. I am clueless (you probably all figured this one out a few sentences in).

I've had at least 5 people (including HD) come look at the floor. HD won't even quote because the floor isn't level. Most quotes are $2K+ just for tiling, then there's an extra to level. Some want to level and use membrane, others say that they can float the floor with thinset and don't think the cracks are a problem at all...most say "you're in Florida, you're on sand, you're gonna have cracks".

I live on the I research the hell out of everything. I quickly found out that:

1. You want the floor to be level and flat
2. You want to fix or isolate cracks
3. You don't want to use thinset to build up more than 3/4 to level low spots
4. If the cracks have vertical movement you're toast

Looks to me like #4 is really the most important, and first question I need to address. The answer is, I'm clueless. I ran a level across the cracks, there's no jiggle-wiggle. I ran a quarter over the cracks and at the point where all the cracks meet, the quarter gets stuck and it looks like perhaps one side of the crack is a third of the quarter's thickness higher than the other side (but just at that one spot). So, we're talking 0.023 inches of vertical movement. The house is 11 years old...maybe it's done perhaps no more vertical movements?

Does this already disqualify porcelain tile installation?

If the answer is "Yes". Thank you. I will beg and plead for HD to take tile back and maybe have them just re-install nasty carpet and call it a day (or half-year in my case).

If the answer is "No". Thank you. I have many more questions now.

I am one person, I doubt self leveling will work for me. I calculated that I have about 70-80 sqft of area that needs to be leveled (built up). So I'm leaning towards going the dry pack method. It's cheaper, seems to be more solid, and maybe less messy - most importantly, I can do it myself (hopefully). Then, my question is how do I deal with the cracks? Do I just patch the cracks, add dry pack over (if needed) and then install membrane over dry pack where cracks were? Is it best to maybe lay membrane over entire floor (after dry pack leveling) to ensure everything is decoupled?

Thanks so much for reading this far..hope there's some energy left for some feedback! :)

I've attached a few images to help visualize the mess I'm in.

Much appreciated.


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Tool Guy - Kg
12-13-2016, 07:37 PM
The floor is REALLY not level. Over 4 ft I found spots that revealed a 1" difference.
Yikes, that's pretty much.
I am clueless (you probably all figured this one out a few sentences in).
No, you're just not experienced...yet. And tiling is mostly a skill. Most anyone can learn. And you've come to a very good place for that. There are lots of pros here willing and able to share what you need to know.
You want the floor to be level and flat
Tile cares about flat. Tile don't give a crack crap hoot about level.
You want to fix or isolate cracks
Rarely can you fix cracks...only isolate them.
You don't want to use thinset to build up more than 3/4 to level low spots
You really don't wanna be using thinset for leveling tile. There are far better products to do that ahead of time, rather than doing it "on the fly" with thinset. Besides, you'll hang onto your sanity more if you don't try leveling tile with thinset.
If the cracks have vertical movement you're toast
Generally speaking, cracks with: Uneven spacing, larger than 1/4" wide, or if there's vertical displacement...then the cracks are considered "structural" and no products are available to protect the tile from such.

Let's ask some questions. But I'll start out simple:
There's very little vertical displacement between slabs on either side of the crack. But is the crack forming a peak or valley? What I'm trying to figure out is if the slab has cracked due to bending due to poor support from below. And if the crack is a peak of valley to the slab plane, it would be evidence of such. I suspect there isn't due to what you said:The slab has 3 giant (i.e. spanning half the room) cracks that all meet at one point.That makes me initally think that this is a shrinkage cracks due to the large 300 sq/ft room. What is the length & width of this room?

By the way, Home Despot will absolutely take the tile back.


12-13-2016, 07:37 PM
Unbonded mud bed looks like it would be your best option. Doing that you'll be able to get the area flat and take care or your crack problem in one step. An uncoupling membrane can be used on top or you can install directly over your mud bed.
Couple notes...
While a level floor is great, all you need to aim for is flat
No floating with thinset
Steer clear of big box store installers, I'm sure there are a few good ones out there but find a qualified installer and go that route.

12-13-2016, 10:08 PM
Welcome, Derek. :)

While I agree with most of what's been said, I gotta take issue with the statement that an unbonded mud bed will protect your tile installation from cracks in your foundation slab that have vertical displacement from one side to the other. While the unbonded mud bed (minimum thickness 1 1/4-inch with welded wire mesh in the vertical center) Is an excellent tile underlayment and provides a good bit of protection, vertical movement below it is still likely to telegraph through the surface.

My opinion; worth price charged.

12-13-2016, 10:15 PM
Looks like you have had some vertical movement so it might be best not to tile that floor. But I have had customers take that chance without problems a few times.

A mud bed would need to meet the other floor surfaces in the doorways. How much mud thickness can you stand to have?

12-13-2016, 11:44 PM
Raising the whole floor to put in a sufficiently thick mudbed that has a chance of surviving means a whole bunch of things to consider:
- the entry doors will have a significant step
- the doors will need to be cut down resulting in the door handle and actual door opening being lower than the other rooms in the house
- the height to the receptacles, windows, switches, etc. will be lower
- if you have minimal ceiling height, it might just make things a bit uncomfortable (but if you have say 9-10' ceilings, you'd probably never notice)

A bonded mudbed could be thinner, but if what it is bonded to is moving, it won't help any and then you'll still have the above issues.

A floating mudbed (unbonded) is hard to integrate into a home that wasn't designed for the added thickness of the floor surface without issues. It certainly works great as a tile substrate and to flatten and/or level the floor, but it's not for everywhere.

Todd Groettum
12-14-2016, 06:19 AM
Return the tile and Get a Floating Laminate Floor...The End!!!:neesie:

12-14-2016, 08:16 AM
First off - thanks so much to everyone for your responses, questions, and suggestions. I'm amazed at how active this community is and wanted to thank you guys for taking the time to look and respond to my situation - I truly appreciate it.

Let me try to answer some of the questions you all posted.

1. Do the cracks form a peak?
I believe so. I've attached another picture to identify some of the low/high spots. I was hoping that it was just a really poor concrete pour that caused the cracks. But the fact that there's a much bigger gap between drywall and floor towards the exterior left corner of the house vs the interior walls, and the low spots being concentrated towards that side makes me believe the cracks all meet at this peak - albeit with very little vertical displacement at the peak. So, in the picture [excuse the big mess] (1) is to give your an idea where the cracks meet. (2) is where one of the thinner cracks ends, (3) is to show where the low spot/area ends. So it seems that the top-left corner just sagged, causing the issues (probably). Also, you can probably tell that the crack thickness changes from (3) to (1) - it gets thicker as it approaches the peak - which logically seems to further indicate it's the sagging corner causing this.
2. Room dimensions?
When I said 300sqft I should have mentioned that this included the walk-in closet and transition to the master bath. The dimension of just the master bedroom are 18.25 ft x 12.5 ft
3. Floating mud bed items
Right now, the bottom of the doors are actually 1.5 inches from the I wouldn't need to cut doors. However, it would still be a step up into the bedroom, which I think would be a few hurt toes and a frequently tripping toddler. I don't think I would have the stomach for all that dry-packing, blood, and sweat just to have the tile in there..and potentially STILL have cracks transcend to the tile if there's another shift in a few years. I was hoping I could go the bonded route to just fill in the low spots if it was just a matter of a crappy concrete pour. If my level was just sitting on a high spot it could have explained why there is a 1" drop over the 4 ft. But it seems like it really is just that the sand pad under one corner of the house has let the foundation sag a bit.

I will note though that I did not see any indication of this on the exterior - there's no staircase (house is concrete block with stucco) cracks or sticking doors etc. But then again, I'm not an engineer - although the inspector who looked at the house before our purchase didn't mention anything structural to us either (we still had the carpet and baseboards so he didn't see the crack or the gaps).

If tile is truly not an option - can I at least consider wood flooring? Or will this pose similar issues? I would hate to "downgrade" to laminate.

Is there an "easy" method to determine if the floor truly dropped vs it being a horrible concrete pour/screed and shrinkage? Is there some self-adjusting laser level gadget I can hang from the ceiling or the wall that will give me a definite answer?

Thanks again to everyone for your advice!!

Best regards,


12-14-2016, 09:32 AM
You probably don't want to hear this, but that's a floor that's begging to be carpeted again. I'd probably still try to flatten the floor first and then carpet it again. There's no way I'd put tile in a room that big when you already have very apparent slab issues.

12-14-2016, 05:21 PM
FWIW, laminate can't handle a non-flat floor, either...but, because it is floating (most of the time), cracks won't project through it. You still have the issue about potential vertical motion, but laminate might not be as susceptible to that considering the foam underlayment it tends to use.

IOW, you'd still have to flatten the floor. The last laminate I looked at called for about the same flatness specs as tile. I have a real wood, floating floor, and it has the same sort of requirements on flatness as tile as well. But, with a floating one, you might be able to do a bonded mudbed (much thinner than a floating one in general) to flatten things, then install wood. I've had really good luck with Kahrs stuff. They only make wood flooring and have been doing it for about 150-years. They pioneered the click together stuff that most of the laminate companies now also use so they have a good idea how to make it all work. They make a huge assortment of styles in numerous wood species with various choices of finish, installation methods, and thicknesses. I have the Linnea stuff (in a variety they no longer appear to make!) that has held up well. This stuff snaps together, you cannot feel the joint (i.e., it's smooth across it), and they use high density particle board in between the top real wood layer and the bottom real wood layer. That stuff is super hard, and the impact resistance is actually higher than solid wood floors as a result.

12-14-2016, 05:26 PM
Last time I installed Kahrs click-together, Jim, their substrate flatness requirement was the same as for large format ceramic tile at 1/8th" in ten feet.

Really well engineered stuff, that.

My opinion; worth price charged.

12-14-2016, 05:33 PM
I was told that their joints are milled to such a high accuracy, that, unlike most wood products, you CANNOT and do NOT want to open the packages to let them acclimate to the room prior to installation...if you do, things will likely not be able to snap's that precise. Once locked together, it grows and shrinks as a unit. As long as you use their approved underlayment and tape the seams with their tape, you'll manage any ground moisture that might overwhelm it otherwise.

12-14-2016, 06:47 PM
Like the others said, you could bond a mudbed to the slab and have it thinner than a floating mud bed. If there is any movement, the cracks will come up thru the mud for sure. I would apply a membrane wall to wall over the mud which would help with minor movement and hopefully not let the tiles crack. It all depends how much the slab moves. Do you fell lucky? :)

12-14-2016, 07:58 PM
Wow - I didn't think the wood floors had such similar requirements to tile installations. The other living areas of the house (i.e. formal living/dining, family room, kitchen, kitchen nook) have engineered wood (Mohawk Flooring). The previous owner left the box with a couple extra pieces in the garage..and it looks like it's the snap-together type. I'm not going to say that there aren't a few spots where it feels a little hollow/wobbly underneath a plank or two...but for the most part it all looks and feels nice and there's no cracks :)

I highly doubt that the rest of the house does not have cracked concrete or is perfectly level (which is probably also why there's a few little squishy spots in some places). I'm not a big I'm tempted to do one of the following at this point:

1. Most likely option. Bonded mud bed to level/flatten the floor. I'm going to add enough mud to cover up the cracks as well (any insight as to how much the minimum thickness of mud needs to be?). Then install carpet. Give it 3 years until it's time for the carpet to go again...pull it all up and see if the cracks have come through. If they haven't, then I'll likely be bugging you fine gents (and ladies) again about how not to cut off my fingers with a tile saw. I'll add ditra or similar wall-to-wall just for good measure.

2. Next best option. Bonded mud bed as above - but then put down engineered hardwood. That should probably last 20 which time I'll be more concerned with my body cracking than tiles.


Thanks again,


12-14-2016, 08:28 PM
Derek, in the ceramic tile industry the minimum thickness for a bonded mud bed is three-quarters of an inch.

argile tile
12-14-2016, 10:42 PM
while everyone else tells you how to tile (perhaps over slip sheet, or perhaps intently install a transition where the crack are if "your scared" or just cheap)

THINK ABOUT FIXING THE FOUNDATION PROBLEM (before you forget it by cover over it) if things have moved allot. have someone who knows about foundations and if house is still square take a look. hope that helps.

(you could have a drainage issue (ie, a gutter with no proper runoff, or soggy soil in an area of the yard. or perhaps an issue people on this forum know of like wood bonded in the cement somewhere))