underfloor heating Concrete screed for UFH cracked

[Rowly]

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

 

 

 

 

Edited May 23, 2021 by Rowly
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[LA3222]

I'm no expert but I would say it's fine - look like shrinkage cracks to me. Concretes dries out and it shrinks a bit, if there are no precut bits for it to crack into, it will crack where ever it fancies! I'd say carry on?

[saveasteading]

Agree with LA3222.   All concrete and screed cracks. This has no contraction joints , but does have fibres so will crack at almost random.

Are the cracks very, very narrow?  Good

Are they random, and fizzle out rather than go a distance? Good.

 

My concern would be hardness. You say it is sandy, and coming away. That may just be the very surface, try a little deeper, and I expect you will find it is harder.

 

A loose sandy surface will not accept the adhesive so may simply need to be rubbed back and given a coating.

 

Not saying this is Ok but first impression is that it is.  Your builder must see it and comment.

[nod]

If it’s a sand and cement screed

 Not concrete 

You need to leave it 12 weeks minimum to dry out and finish shrinking 

 

[Redoctober]

1 hour ago, nod said:

You need to leave it 12 weeks minimum to dry out and finish shrinking 

 

Yes, I agree. We ran our UFH via the ASHP for a good 4 weeks on a specific cycle which aided the slab to dry out correctly. Prior to that it had been left to dry for a few weeks anyway. 

 

A top tip would be to put down a sheet of clear plastic, taped to the floor and then leave it to see if any moisture is evident on the underside of the plastic sheeting a day or so later.

[Rowly]

13 hours ago, saveasteading said:

Agree with LA3222.   All concrete and screed cracks. This has no contraction joints , but does have fibres so will crack at almost random.

Are the cracks very, very narrow?  Good

Are they random, and fizzle out rather than go a distance? Good.

 

My concern would be hardness. You say it is sandy, and coming away. That may just be the very surface, try a little deeper, and I expect you will find it is harder.

 

A loose sandy surface will not accept the adhesive so may simply need to be rubbed back and given a coating.

 

Not saying this is Ok but first impression is that it is.  Your builder must see it and comment.

 

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 :-)

[Carrerahill]

14 hours ago, Rowly said:

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

 

 

 

 

Looks like normal shrinkage to me. 

[Rowly]

12 hours ago, nod said:

If it’s a sand and cement screed

 Not concrete 

You need to leave it 12 weeks minimum to dry out and finish shrinking 

 

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.

[Carrerahill]

Just now, Rowly said:

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.

Yes, it also cures from the outside in, so surface readings are not illustrative of the whole section. 

[Carrerahill]

7 minutes ago, Rowly said:

 

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 ?

Personally, I would not call the builder, I think he will, with all due respect, think you are being pedantic - you now have  good dated record of this on this forum if you ever did need to show him.

 

Unless you need to for testing reasons and at that I would only do a short test, I'd leave the UFH now until I needed it in the autumn/winter. 

 

Cracks in concrete/screed are totally normal. Keep an eye on them, if by say middle of July things are looking terrible then maybe start to worry, but so far that is just what screed does, it is like paint, it is inevitable it is going to dry! 

 

If you have any to look at, go and look at a dried out concrete block wall, note various little cracks in the bond between the mortar and block, this is all shrinkage and is normal, can look a bit distressing to many but it is normal. Also bear in mind this is just a floor screed, even if it were a structural slab I still wouldn't worry at what I can see above.

 

If you can start putting £1 coins into the gap though!

 

 

[Mr Punter]

The screed looks fine.  Your flooring fitter may want to prime it before laying the Karndean / LVT.

[nod]

2 hours ago, Rowly said:

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.

Normally a day per millimeter 

[saveasteading]

As Carrerahill suggests, look at some other examples for complete peace of mind.  I suggest go to a newish B& Q , Dunelm or similar and look at the concrete floor. This will have been done to high spec and finish, but I bet will have lots of cracks, worse than yours. Nobody looks  though.

On the subject of drying out. presumably this was quite a dry mix, so nearly all the water goes into chemical bond with the cement, and it will dry quite quickly.

 

The weather isn't helping, but generally floors dry much more quickly than the rules of thumb say. (Unless the material was much too wet when laid)

A month per inch is stated but this is very cautious in my experience of concrete floors up to 200mm thick, and a thinner, damp-mix screed will be quicker.

 

If we get drier, warmer days then letting air through will dry it quickly.

 

As Mr Punter says: Flooring contractors often like to sell an extra of a seal coat. It seals the damp in, and costs you. 

There is a nice mark-up on it. I have even seen them fiddle the moisture reading, , and their annoyance has been clear when they lost the extras. Instead, check the moisture content (yourself) until it is ok.

[Rowly]

1 hour ago, saveasteading said:

As Carrerahill suggests, look at some other examples for complete peace of mind.  I suggest go to a newish B& Q , Dunelm or similar and look at the concrete floor. This will have been done to high spec and finish, but I bet will have lots of cracks, worse than yours. Nobody looks  though.

On the subject of drying out. presumably this was quite a dry mix, so nearly all the water goes into chemical bond with the cement, and it will dry quite quickly.

 

The weather isn't helping, but generally floors dry much more quickly than the rules of thumb say. (Unless the material was much too wet when laid)

A month per inch is stated but this is very cautious in my experience of concrete floors up to 200mm thick, and a thinner, damp-mix screed will be quicker.

 

If we get drier, warmer days then letting air through will dry it quickly.

 

As Mr Punter says: Flooring contractors often like to sell an extra of a seal coat. It seals the damp in, and costs you. 

There is a nice mark-up on it. I have even seen them fiddle the moisture reading, , and their annoyance has been clear when they lost the extras. Instead, check the moisture content (yourself) until it is ok.

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.