Cracks in SLC UFH layer

[jshollick]

Hi I'm new and need of advice!
I have a new conservatory completed at the end of last year. After allowing the 100mm concrete to dry, I primed and fixed 50mm Fastwarm insulation boards using Ultra RS flexible adhesive, layed out the underfloor heating cable and covered it in approx 7 mm of Level It 1 SLC. This morning after 4 days drying, hairline cracks have appeared. Some of these areas sound different to areas without cracks and also vibration can be felt when tapped. Most cracking appears to be in areas where the SLC is less deep. How should I proceed before tiling? Should I pour more SLC in these areas or will the flexi adhesive under the 600 x 600mm porcelain tiles cope with this?
Any help appreciated. Julian

 

[Gazzer]

Has the heating been used yet ?

 

[Dan]

First things first, obviously don't tile it. I wouldn't cover it with another layer either. Hollow sounding SLC isn't good.

Can you upload a picture or two by chance? It would really help.

It sounds like either the SLC has been mixed wrong, used incorrectly, is a dodgy batch, or the heating has been turned on during the drying process. SLC shouldn't take 4 days to dry. I'm guessing it dried 3 days ago but you left it another just to be sure?

 

[jshollick]

No, the heating is not commissioned yet. I have not been in a hurry to tile, thought it should dry completely first, painted walls yesterday, only noticed cracks this morning. I think cracking is maybe in areas where the insulation boards are taped together using board jointing tape and above cables where slc less thick. I also used Wickes SLC with no cracks! View attachment 38483 View attachment 38484

 

[macten]

How long have you left the 100mm concrete to dry mate?

 

[jshollick]

8.5 weeks. Builder layed concrete 1st December and I put the boards down 30th Jan.

 

[macten]

100mm screed should be left for 23 weeks to dry/cure properly.
Did you notice any cracks in the screed before you put boards in?
Could also just be an Wickes SLC thing - I'd imagine it's not the best out there but never used it myself.

 

[Dan]

I'd hate to say use another bag or two and tile it and then it screw up in weeks/months time. Screed isn't my thing so macten's your man for that. But the SLC looks like it's got the cracks where two different mixes of SLC have been used. Not sure what that proves/gives us though. Then if I zoom in a bit it looks like around the crack the water has ran away from the SLC causing it to perhaps cure with less water than the rest. Does it feel like there's a bit of a dip near where that crack is above the ones that's next to the tape measure? Looks like it's putted a bit as if the water in the SLC has ran under the boards perhaps or something?

What did you stick the boards down with?

p.s. thanks for the pictures.

 

[jshollick]

I was advised the time was one week per 25mm of concrete before laying boards.

There were no cracks in the concrete. It was a ribbed finish so I primed it then 3mm of SLC, and after drying fixed boards with Ultra RS flexible adhesive. The Wickes is less cracked than Ultra SLC.

 

[jay]

You need to lift the loose slc and determine why it has failed .pics might help of the area you lift

 

[jshollick]

The cracks are mostly where Ultra has been used. There has been a bit of run between boards so there are a few low areas and where the two mixes meet. The slightly hollow sounding area has perhaps thinner layer and also needs raising. The area above the tape measure I filled with tile adhesive to level. I used the tape supplied with the boards to join them together and fixed to floor with Ultra RS Flexible adhesive.

 

[jay]

But if the slc is drummy you cant fix over it

 

[jshollick]

Are you suggesting carefully drilling down between the heating wires which are 10mm apart, through the slc layer as far as the insulation board and breaking up an area. My concern also is not to tile over a surface thats not firm. Have you ever tried this?

 

[jay]

Nope but if its drummy you may be able to lift it up not keen on the drilling idea ,if you tile over it it may deteriorate

 

[jshollick]

No way it could be lifted without breaking up surround. It is a firm flat area with a fine crack running through, that's why I thought drilling might be the best way to explore.