Rowly

yes the mix was damp, not wet. I think it was about 70mm deep from what I understand. Certainly good to know that I was freaking out for no apparent reason.

yes I will leave for a few more weeks to dry out. I have a fairly basic damp meter and it reads between 17 and 18% across the floor. the normal reading on the decades old existing concrete for reads 8%. I guess it has a way to go before it's fully dry.

Thanks. Yes the gaps are very small at the moment. I'm going to put a call into the builder so he can see them. Provided I have sounded him out and something happened later, I have full recourse. I think to be on the safe side I'm going to ditch the glued down vinyl tile idea and stick to 'plan B' which is to go with good quality Laminate. I'm going to leave it for another two weeks and then I will ask that we fire up the manifold and see how the floor reacts to a very low heat cycle. if it hold together I think it will be fine. once all the checks are complete and the floor doesn't start cracking up even more, I will lay the floor on top happy. I think it was just the initial shock of seeing cracks as it wasn't explained to me that the flor would crack, but having thought about it, it makes sense that when the floor shrinks that small cracks would appear. Thanks also to everyone for the comments. they are all very helpful :-)

Hi All, I was wondering if at all possible i could obtain some advice 6 weeks ago my builder put underfloor heating (wet) in my new extension in place of radiators 6.5m by 4m. I remember asking him on the day what mix he was using and he said it was a 6 to 1 with fibre glass to aid strength. The floor was laid on a dryish mix by a professional screeder and the floor underneath is block and beam. Unfortunately yesterday i noticed the floor has many cracks. Anything from 5 inches to 2 feet. the cracks are in the middle of the floor in a couple of other places nearby. The cracks are scraggy(like lightning) and not straight. They spur off in multiple directions. If I run my finger in the crack I can run off the top layer so it doe appear sandy. I know the floor hasn't dryed too quickly as the weather has been aweful these last few weeks and we didn't walk on it for at least 3 days after it was laid. We haven't even had the manifold installed so i know the floor hasn't been heated up too quickly. It's my aim to lay Karndean vinyl glued floor tile on the top of the screed but I don't think it's possible with the floor cracking up. Is this right? The questions I have are. 1, as it's already cracking should I request the floor be taken up and relaid using a heavier mix? 2, can the floor still be repaired in sections or should I even consider allowing this? 3, it is normal for this to happen. My limited knowledge says it probably isn't? To make matters worse I have the kitchen arriving in 10 days but I'm reluctant to take delivery if I know the floor needs to come up and be relaid again. thanks again for listening. Any advice you could provide would be very appreciated. Rowly