Help!: Tiling Over Cracked Screed

[mowzer]

Hello All, I've been reading through the forum and it seems like a great place to get some expert advice.

About 5 years ago, we did the first renovation on our house. We had the subfloor removed (concrete) and dug down so that we could have UFH (wet obviously) put in. The screed was pumped in - so hopefully there aren't too many voids under there, but we have a major crack in our tiles (across about 5 large tiles) that we tried to fix once but it just came back with in a few months. The crack is in front of a set of doors leading into another room.

We put in an extention about a year ago, and the room these doors are leading onto was tiled during that process. My dad tiled his bathroom and recommended Ditra - so we used that on all the 'new' areas - so the new area is about 5mm higher than our kitchen area. Good news is that with Ditra - we have had no problems. Prior to tiling in the extension we saw cracks forming in the new screed within weeks of it being laid - and that was without the UFH on. So a de-coupling membrane is a must.

Back to the big crack - we guessed there was some serious movement as one half of the crack is acutally 1mm higher - so we decided to rip up the tiles today (we have plenty of spare). In doing so we have found that the screed is particularly soft around the crack. Whilst using the STS to get the adhesive of, we have managed to get a lot of the 'soft' screed out - like a dentist drilling out a cavity - being very careful due to the UFH. (Yikes) What is looks like in our opinion is that they pumped the screed room by room and it must have been the beginning or end of the mix that was too wet or something and was weak.

Obviously we will need to level the screed again, but we are tempted to cut some 5mm lines in the screed and use the STS to get it up so we can make room to put Ditra down to de-couple the tiles from the screed. But we have to consider the height difference of the other room - so we kind of need to 'blend it upwards. So questions:

What should we use to fill where the screed was soft? More screed (Sharp Sand/Cement 3:1??) after prepping the area with PVA?

Should we consider not taking the screed down 5mm and using a thinner product like Pesilastic?

We are considering using mosaics in the area we have taken out - as we have then elsewhere and I'm thinking they would be less likely to crack - the grout would be more likely to crack which is easier to fix in the future. You can see them in the picture below.

We also have several hairline cracks in other tiles that we have taken up and again it looks like hair line cracks in the screed. We can't do ditra there due to the height difference - so should we be looking just at flexible adhesive? If so which kind? Or is Pesilastic an option?

Would be very grateful to hear any thoughts or ideas on this one - as we don't want to be fixing it every couple of years!

I should add that my husband and I are quite confident in our DIY endeavours - it has been a bit of a passion for the last 6 years - we like to educate ourselves and work out how to do things right - so we aren't complete novices or anything.

Thanks in advance for any advice.

Jayne

[Dave]

Morning Jane.

The reason it is cracking in the doorway is stress.. there should really be an expansion joint inserted there in the screed.. but if yo leave a movement( stress relief) joint in the tiling at this point then that will stop your problem..

[mowzer]

Thank you Dave for the reply. I think you have a point. However there has always been a 3 inch separation (non-tiled) in the doorway. The other room had a floating wood floor originally and the crack appeared within 6 months of installation - so it has never been continuously tiled between the rooms. So I think there is a doorway stress and it has taken the route of the weak screed - it was really crumbly. We are going to put an expansion joint in, but I was going to put it further back in the middle of 2 rows of mosaics, but after considering your post, I think it needs to go as close to the weak screed as possible - so I will put it between the kitchen tiles and the 4 rows of mosaics. Ideally, the joint would be further back into the kitchen where the crack is, but this would look really ugly.

I've had a morning of research- so the plan of attack it going to be get the current screed down far enough to make room for durabase matting plus tiles etc. We will repair screeding after bonding with SBR and let that cure. Then fit matting and expansion joint and finally complete the tiling.

Finger's x'd that will be the end of it.

Thanks again for your advice.

[Dave]

No probs,, use rapid setting cement in the screed to speed up the tiling process.