Tile cracking, any ideas?

[ubofbc]

I have an installation we did about a year ago and it is cracking (small cracks)
The subfloor is strong, 2x10, with 1x8 shiplap diagonal to joists, and 3/8" plywood installed cross joists screwed on a 4-5" grid. Above the plywood there is heating cable on a 4" spacing, Mapei primer and self levelling underlayment poured to the depth of the cable. Finally tiles are set in acrylic modified thinset.
The odd thing is the cracks in the tile appear to be along the joints in the plywood in both the 8' and 4' direction.
We can chip out the tiles and replace them but my fear is that the cracks will continue in the new tiles.
Any ideas on what caused this and what can be done to fix the problem?

Thanks for your ideas.
Brent

[The Kid]

that 1x8 shiplap is extremlly unstable. Did you add fasteners to it or insure it was fasterned good? alot of the shiplap sub. that I see is split from the old timers hand driving the nails or loose from withdraw over the years, usually i will go through and re-screw the stuff done at minimum, or tear it out if budget alllows.

3/8" ply is pretty skimpy, id want 1/2" at the least. Im not sure about the mapei SLC put I do know alot of the other manufactures require lath on a woood substrate with their product.

Is this stone? Does the deflection of said floor meet specs.?

[Edthedawg]

The odd thing is the cracks in the tile appear to be along the joints in the plywood in both the 8' and 4' direction.

Nothing odd there at all - sounds like it's failing just the way a lot of folks would predict

You have motion in the floor - probably from the 1x8's moving a little, or due to the unsupported free span of your floor joists, or maybe something to do w/ the style & size of tile (your details TBD)

You also have the poured underlayment over all that moving deck and plywood - it's obviously cracked thru now too. You'da probably been better off w/ something like Hardi bedded in thinset over the plywood, aligned offset from the plywood edges. That woulda likely restricted the translation of the floor movement substantially.

So there's lots of recommendations one might make (assuming you don't wanna live w/ all the cracks) such as ripping everything out entirely, at least back to the 1x8 decking and lay a proper subfloor and underlayment along w/ new heating mat and tile.

But all that is still dependent on your deflection info...

[bbcamp]

Above the plywood there is heating cable on a 4" spacing, Mapei primer and self levelling underlayment poured to the depth of the cable.

SLC needs metal or plastic lath if you are using it as your underlayment. Electric heating cable doesn't count.

I don't know if there is a way to salvage this installation. Applying Redgard under the removed tiles, and caulking around the replacement tiles may give you some time to recover financially.

[ubofbc]

Thanks for the information all.
The tile installed is a 12x12 ceramic, there is no cracking of the grout, just the line of the plywood joint below.
The 1x8 was screwed to the joists with 1-3/4" decking screws and the plywood fastened to the 1x8 with 1-1/4" drywall screws.
One of the first things I measured was deflection in the joists which was about 1/16" over it's 13 foot span with a medium man leaping on the floor. This seems reasonable as it calculates to about 1/2400 deflection over length.
The idea of uncoupling the tile from the cracked substrate seems like it is worth a try in the areas of the joints. I am not too enthused about tearing up the whole install if avoidable.
Is there a thin antifracture material that will allow the occasional tile to be replaced level with the surrounding tiles. Is Redgard the ticket?

Thanks again for the help

[Kman]

I've not personally installed a heating system in the floor, but everything I've read here says that a membrane such as Ditra should have been installed on top of the SLC in your case.

I don't know if Redgard or one of the other anti-fracture membranes could help in your situation, but it wouldn't be that big of a job to find out. Try it out on three or four tiles and see if it works. If the tile is cracked but the grout is not, you've got some significant pressure there that an antifracture membrane may not be able to handle.

[bbcamp]

13 foot span with a medium man leaping on the floor. This seems reasonable as it calculates to about 1/2400 deflection over length.

We do it a little differently. We calculate the deflection based on a 50 pound per square foot distributed load. In your case, we get a deflection of about 11/32, or L/458. It's still good.

As far as using Redgard and caulking the tile goes, you don't have much to lose to try it. The correct repair is to redo the floor, and you'll probably lose the heat wire in the process.

[Edthedawg]

I dunno - i still see plenty cause for concern.

1x8 shiplap installed diagonal to 2x10 joists - sounds like an old house. I'd think it unwise to put "SYP / Doug Fir" into the Deflecto calc.

Running "unknown wood, but in good condition" along w/ 13' span, nominal 2x10's @ 16" OC, you get L/350. Inadequate for ceramic.

Deflecto also assumes good construction, can't see old plumbing and electrical cutouts/notches, etc.

I'm also concerned about the screws. 1 3/4" decking screws sound awfully short for socking down 1x8's. Old 1x8 can be nearly 7/8" thickness, so you only have another 7/8" - 1" of thread into the top of those old joists. 2 1/2" screws minimum would be better. Then sheetrock screws have no business doing anything to hold flooring down. For starters, they can rust inside your floor and cause their own problems from that. Second, the heads are designed to snap off under certain loading conditions. Can't say your floor is doing those things, but in general they are for drywall use only

All that said, I'll go along w/ Bob and Kman - might as well try the RedGard here. It might work. I think it won't. At the end of the day, I still think there is woefully inadequate underlayment to support a crack-free future for your tiles

[johnfrwhipple]

Did you allow any spacing between your 3/8" plywood sheets when you installed them.

I like to leave a fare gap between the 1/2" sheets we install and then we fill the gap with silicone. The following day we cut back the silicone, prime the floor with self leveling primer, lay our cable heat, cover cable heat with Versa bond scratch coat, top with 5-1/4-1 mix and level/grade floor. The next day or two we come back and set our Ditra over this mix with non-modified thin set.

I think there is a chance your plywood is installed to tight and it is expanding and contracting with no give anywhere.

Not much you can do to stop this. The wood will move when it wants too.

Good luck.