cracks in thinset with floor heat

MarkMark · Mar 15, 2007

my bathroom is apx 35 sq' the joists are 2x8 16"oc with a 10' span. sub floor is 5/8 then 1/2 screwed every 6". I layed custom building products easy matt down then true comfort heating wires on top of this followed my flexbond modified thinset. its been about 4 days since the thinset and i am ready to tile but have noticed 4-5 cracks up to 20" long in the thinset starting at the edge and going towards the middle with one going the full length. Is this normal due to drying or is this a bad sign of whats to come once the tile is down? If so any suggestions of a fix before i ruin my tile

any help would be great
Thanks mark

Geniescience · Mar 15, 2007

call the manufacturer of Flexbond, immediately. Tell them how you mixed the thinset, whether you slaked, how much water you used, etc. Describe the cracks with respect to the wires (are they parallel?). Did you walk on this floor a lot in the last three days? Did you turn on the heat cables a lot, to test them?

More questions to come. Tell us what flexbond says.
David

Sponsor · 2022-01-29T12:20:58-0800

MarkMark · Mar 15, 2007

I mixed the thinset as per directions. the cracks are running parralle to the wires. infloor heating has not been turned on yet. minimal amount of walking on the floor after apx 30 hrs

MarkMark · Mar 15, 2007

recieved a reply from custom building.....

Iâ€™m not aware of any industry standard that allows for floor joists smaller than 2x10 for a tile/stone assembly.

If you have already determined that movement/deflection in the floor is what has caused the cracks in thinset bed, I would expect the addition of more weight from thinset and tile to cause more deflection in the floor. So yes, Iâ€™d expect to see cracking grout joints and/or tile.

Kory Jones CTC, Technical Service Representative

Geniescience · Mar 15, 2007

that's a cautious response. It's the movement of people, and live load, that stresses a floor the most. Not the weight of tile itself.

at the moment I don't know specifically what to suggest or to ask you about. Tell me more, before I ask. Please.

david

Jadnashua · Mar 15, 2007

Keep in mind that to calculate deflection, you need to know the total length of the unsupported span, not the room size. If things are as you say, then you should be okay for ceramic tile but not for stone. Thinset is not designed to be installed thicker than about 1/4", so if you have more than that over the wires, that may be the problem all by itself - they could be shrinkage cracks, rather than deflection induced cracks. Many people prefer using self-leveling concrete rather than thinset to cover the wires and provide a nice flat surface, but thinset works, too. Suggest you ask this question over at www.johnbridge.com. Also note that if the joists are not in good shape, your deflection may not be up to snuff (i.e., too many cuts for plumbing, heating, etc.). Did you install the second layer of ply across the joists like the first (it should be!)? Ideally, you offset the seams and the end, and avoided the joists when attaching the second layer. Are any of the cracks over the seams in the ply?

Leejosepho · Mar 15, 2007

markmark said:
... easy matt ... true comfort heating wires ... thinset ... cracks up to 20" long ...

I would say those cracks are simply from shrinkage and nothing to worry about. I am not familiar with "easy matt", but I used thinset and nails to fasten down my concrete board, then did a scratch coat of thinset before laying my heating wires, then added about four more variously-thick coats of thinset to get a smooth and flat surface before tiling and had some of the same kind of cracks you describe. You should be safe ignoring them and moving right along.

MarkMark · Mar 15, 2007

"Are any of the cracks over the seams in the ply?"

the cracks are fairly random, not by the seems in the ply or the easy matt. they do run in the same direction as the wires.

Geniescience · Mar 17, 2007

picture please

i don't know how one determines randomness if one also can see parallelism in this randomness....

Can you post a picture?

david

MarkMark · Mar 17, 2007

the cracks are random in position, they are not all in one spot but scattered unevenly throughout the floor, however they all run parralelle to the wires which is the shortest direction on the floor. The Joists under ther floor are 2x8 16" oc with a span of 10'. from what iam hearing in other forms is that where the thinset was layed rather thick apx 1/4 it cracks are forming due to shrinkage as it dryes (im hoping)

Leejosepho · Mar 17, 2007

geniescience said:
i don't know how one determines randomness if one also can see parallelism in this randomness....

In my own case, the cracks were "random" in the sense they were not all equally spaced and/or always in the same relationship to the heating wire, yet the longer ones did tend to run alongside the wire while some shorter ones ran perpendicular at the ends of loops. So, maybe we can call that "random parallel", eh?! Or, maybe "parallel at random" would be closer.

Geniescience · Mar 17, 2007

markmark said:
.... Joists under ther floor are 2x8 16" oc with a span of 10'. ...

Parallel with cracks or perpendicular? Has anyone asked you this yet? This is information needed to know.

You put thinset down 1/4" thick?

David

Leejosepho · Mar 17, 2007

geniescience said:
You put thinset down 1/4" thick?

In my own case, the first "scratch coat" was about that thick in order to cover the heating wire, then the thinset shrunk to just a little below the top of the wire and it took two more very-thin applications to finally get a smooth and flat surface slightly about the level of the wire. Looking back, thinset was not the best choice for that build, but a maintenance mechanic where I work who does tile on the side had said it would do just fine since negligable shrink cracks would not compromise the foundation for the tile.

I believe someone has already confirmed Markman's floor system is sufficient for tile, and my only concern here would be that his heating wire should not be protruding above the thinset when his tile is finally applied.

FSTConstruction · Mar 18, 2007

do not know if this will help you but I first re-screw down the origanl floor and structure it if need be, then thinset my concrete board down and screw it, then if need be thinset the DITRA (easy matt) down, then thinset the infloor heating pad down then scratch coat that then lay my tile on that.

Geniescience · Mar 18, 2007

not both.

hi FST!

All good, but for one thing. With these matts you are better off without the Concrete board, and instead of that layer, more plywood or nothing at all. The waffles in the ditra type material do the same function as a Concrete board so you don't need both.

david

bukzin · Mar 19, 2007

great tile info

You might also find a copy of 'Setting Tile' by Michael Byrne.
Its the bible for tile setters.

MarkMark · Mar 19, 2007

the cracks were all parallele with the wires...(running in the same direction)

Geniescience · Mar 19, 2007

post a picture please.

Leejosepho · Mar 20, 2007

geniescience said:
With these matts you are better off without the Concrete board, and instead of that layer, more plywood or nothing at all. The waffles in the ditra type material do the same function as a Concrete board so you don't need both.

I do not know anything about Ditra, but I am curious about what function its waffles serve in place of concrete board. In my own experience, the concrete board is a far better radiator than "more plywood or nothing at all" would be for spreading the heat from the wires, and it does not seem right to me that I would have been better off without the concrete board. Jim has explained that concrete board does not add any significant structural strength to the overall floor, but this is the first time I have ever heard it mentioned as unnecessary.

Jadnashua · Mar 20, 2007

go to www.schluter.com and watch their video on Ditra installation...review the spec sheet. It is a decoupling membrane that performs the same task as a cbu on the floor - it prevents cracking from the substrate movement. Nothing can prevent that if the deflection ratings aren't up to snuff, though. Neat stuff, quick and easy to install.

cracks in thinset with floor heat

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