Hairline cracks in self levelling - okay to tile over?

11 Jun 2013
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United Kingdom
Hi folks,

A few months ago I removed all bathroom floorboards and replaced with 22mm tongue & groove plywood, fastened down with about a zillion screws and glue in the joints, then primed with watered-down SBR and applied Screwfix No-Nonsense SLC.

It's the kind that comes with a tub of white milky stuff. Mixed thoroughly with a drill. Laid about 3-4mm thick, then a second coat about the same thickness just in certain spots. This was done over the top of a loose wire electric underfloor heating, which was stapled down (not taped).

A few months later I'm ready to tile, but fine hairline cracks have appeared dotted throughout - mostly a few feet apart. Some run through both layers.

I tried gently chiseling a bit up and it seems well stuck, even the corner where two cracks meet you can't get a chip/chunk to come up... doesn't creek or crack when you walk over it. The floor was sturdy as hell before the SLC was poured.

Should I just ignore it and tile over? Or is this definitely going to result in cracked tiles/grout?

I could pour another layer of SLC possibly, so long as it was thin.

Would appreciate any advice. No idea why this has happened.

Unfortunately I can't add tile backer boards because I've got to tile to the height of a shower tray which is stuck down.

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when you worked on the B/R was it a shell striped of all fixtures?
why did you replace the floorboards?
did the ply at any point touch an abutment like a wall? the ply sub-floor should have an allround expansion gap.
how many pieces was the ply in?
why the watered down SBR?

any chance of a photo
Yeah it was a shell, no fixtures or anything.

I replaced the floorboards as a lot were damaged, and I had to pull up quite a few to replace the soil pipe, before long there was more gaps than floor.

Pretty sure the ply has a gap all round, though I didn't do that deliberately... More it was just not possible to get it tight. It was in boards approx 600 wide and the room is about 8m long so must be about 12 boards. Each board spans the full room depth of the room though so all the joins run in one direction only (and the cracks are random in all directions).

I was told SBR slightly watered down was a good primer for self levelers, especially latex ones? I painted it on and waited for it to dry.

Photos attached. Thanks.


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the t&g boards you laid sound like chipboard flooring not ply.
the first suspect lookin at the pics is have you spread the SLC to thin esp over the UFH?

is ther dfinitly no sway or bounce in the floor joists - was they all fixed well firm?
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100% definitely 22mm ply, not chipboard, it cost a fortune and it's really strong.

Definitely no sway in the joists, they're really solid, can jump up and down on the floor and it does nothing.

The SLC is reasonably thin - in most places just enough to cover the wire and then a bit more - so about 3-4mm - but about 6 or 7mm in others - but the spec says it can go from something like 10mm down to nothing? The cracks are still prevalent in the 6-7mm area too... (although that was poured in two layers).
Here's a photo of it before pouring.

In case it's not clear how the UFH wire is fixed down, I've laid strips of scrim tape and stapled the tape either side of the cable. Prefer this way over gaffa tape as the SLC can penetrate the scrim tape through to the ply underneath.

You can see the expanding foam around the edges of the ply too where I've sealed the gap to keep the SLC in, so I guess there was a gap! (Can't remember it that well though).


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have you activated the UFH? if yes, when for how long?

youve done it the right way - primin, an a good slc an waited a bit.
i guess you used the c/w water an mixing buckets an correct amounts etc.

i wonder if you poured the slc in the hot summer an it dried out too fast.but it would probly have shown cracks before now if it was a suction or fast dryin issue.
UFH hasn't been activated. It was fairly warm when it was done, but the room has no direct sunlight, it took a good 2-3 hours for it to stop leaving fingerprints when I touched it...

Just don't know whether to

a) tile over the top and hope for the best
b) cut a groove in each crack, scrim tape them all and pour another layer of SLC
c) smash it all up and redo with a different brand
switch on the UFH - maybe more cracks will appear or it will stay the same?
a)smaller tiles will take up movement better than the v. large modern tiles.
b)dont be cuttin anythin you could damage the UFH.
c)dont be smashin stuff for same reason.light hammer taps will free SLC.

if i got cracks in SLC like yours id sack the lot an do it again with a different SLC an primer but i cant afford to be goin back to fix for free.

im told, by the way, that elec UFH is well expensive to run.
Well after doing some more tests today I'm fairly happy it has a good hold, so I've laid out a matt of what I can only describe as giant scrim tape (approx 600mm wide), which will hopefully add some extra strength to the adhesive bed, and started tiling it.

Fingers crossed.

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