Crack repairs to stone floors

[markcollings]

Hi everyone. I am a Building Surveyor and have been reading your forum with interest. I am currently dealing with two properties where limestone tiling laid over heated screeds has cracked. I have uncovered the screed in both properties and found the cracking indeed extends through and seems to be as a result of shrinkage due to insufficient drying and heat cycling of the under floor heating prior to laying the stone tiling. In both instance no separating layer such as Ditra Matting was installed.

In both instances I need to specify a repair to the stone floors. In both properties this will require cutting out and replacing approximately 15 stone tiles. I am happy that the cracked tiles can be cut out and removed and new tiles bedded back in after with the floors refinished to avoid the new stones looking out of place. What I am concerned about is continued movement of the cracked screed below causing the new tiles to crack.

I have read with interest the threads relating to crack repair techniques including epoxy resin and steel needles which seems a sensible option to me. Hopefully this coupled with a flexible adhesive will help to prevent any further cracking of the replacement stone tiles. Has anyone used this technique as a repair for a damaged floor before and if so was it found to be successful? Does anyone have any trade info on products that are suitable for this type of repair.

The only other option in both instances will be to lift and relay the entire stone floor over Ditra Matting which would be incredibly costly as in both instances expensive kitchens would need to be removed.

I would appreciate any views members have on appropriate and lasting repairs.

Many thanks,

Mark

 

[jay]

hi only real way to fix is rip up and do right as cracks connot be patched with one tile span any pics:8:

 

[Ajax123]

The important thing is to determine why the screed has cracked as undoubtedly these will have reflected through the tiles. Is the screed flexing? Is it simply thermal expansion? Is it curling? there are lots of things which can cause screeds to crack and heated screeds are quite complex animals. The usual thing is that the screed has not been jointed into suitable sized bays. What type of screed is it?

If it is a cement based screed the bay sizes should not exceed 15m2 (Chapter 8 NHBC handbook) or room size whichever is smaller. If anhydrite it can be up to 300m2 but in both cases it is important that the screed is jointed at the interfaces between underfloor heating zones. This is something I coe across as a problem in many installations as installers don't seem to realise that the screed needs joints.

If the screed is repaired and it has cracked because it needs a joint then the crack will simply re-locate to the next weakest point.

I disagree with Jay in that the whole thing should be ripped up. It does depend however on the reason the cracks have appeared as to whether a resin stitch will be successful.

 

[Terry Cottar]

I have in the past retro fitted expansion joints. It involves tiles coming up but not a total refit you can usually see the weak point especially with natural stone. Limestone requires the correct filler.

tel

 

[Floor_Restorer]

Underfloor heating and cracks in Limestone go hand in hand!

Being based in The Cotswolds we see quite a lot of this. Ajax123 is right in identifying the cause and putting in measures to make sure that the repaired floor can cope with the constant expansion and contraction of underfloor heating.

With a standard screed or concrete floor that is not jointed between floor areas causes a number of instances to happen with Limestone. Often you will see cracks in doorways or narrowings, as well as in floors that do not have a floating element to them want to expand out of the weakest point. We used to install floors and any thing with heat involved had to have Ditra matting and jointing in the screed Easy to say that now..........

Also, I know this sounds stupid, but always use flexible adhesive and grout with underfloor heating. You'd be amazed at the number that haven't.....

Could you post some images?

Thanks,

Tom

 

[DHTiling]

Mark...this thread will help you understand how substrates behave...

http://www.tilersforums.com/tiling-forum/14730-understanding-tiling-shift.html

 

[markcollings]

Thanks all of you for your input. The primary reason for the cracking seems to be insufficient drying of the screed and heat cylcing prior to laying the stone tiles. The screed was laid very thick (around 125mm) and given little less than 6 weeks to dry before tiling. Also the underfloor heating was not turned on prior to laying the tiles but some 3 months later.

When the heating did finally come on it was in an uncontrolled manner with sharp rises and drops in temperature. This I suspect caused some rapid curing of the undried screed and resulted in some significant cracks appearing relatively quickly. I will post some pics next week.

In answer to AJAX123 comments about jointing, the screed is a cement screed and critically no movement joint was provided between two rooms, both of approximate size 6m x3.5m. There is a 2.5m wide opening between the 2 rooms and cracks have appeared in a Y formation extending out from either side of the opening, meeting approximately in the middle and then extending through the centre of one of the floors. I also confirm that the two rooms have independantly zoned heating.

Give the above it seems sensible to try and introduce a movement joint between the 2 sections. If anyone has any experience in retro forming movement joints in screed it would be great to here your comments.

Where cracks have formed elsewhere in the rooms (they are fairly numerous) I am suggetsing a straight epoxy and needle stitch repair.

 

[Ajax123]

Thanks all of you for your input. The primary reason for the cracking seems to be insufficient drying of the screed and heat cylcing prior to laying the stone tiles. The screed was laid very thick (around 125mm) and given little less than 6 weeks to dry before tiling. Also the underfloor heating was not turned on prior to laying the tiles but some 3 months later.

When the heating did finally come on it was in an uncontrolled manner with sharp rises and drops in temperature. This I suspect caused some rapid curing of the undried screed and resulted in some significant cracks appearing relatively quickly. I will post some pics next week.

In answer to AJAX123 comments about jointing, the screed is a cement screed and critically no movement joint was provided between two rooms, both of approximate size 6m x3.5m. There is a 2.5m wide opening between the 2 rooms and cracks have appeared in a Y formation extending out from either side of the opening, meeting approximately in the middle and then extending through the centre of one of the floors. I also confirm that the two rooms have independantly zoned heating.

Give the above it seems sensible to try and introduce a movement joint between the 2 sections. If anyone has any experience in retro forming movement joints in screed it would be great to here your comments.

Where cracks have formed elsewhere in the rooms (they are fairly numerous) I am suggetsing a straight epoxy and needle stitch repair.

All sounds sadly familiar. Where the 2 rooms meet it could be possible to retro fit a joint at this point. It would simply be a case of uplifting the tiles and saw cutting carefully - you don't want to cut into the pipes. To form a joint effectively it needs to extend approx 1/3 the depth of the screed so if they are 15mm pipes in 125mm screed you should be reasnably safe. This of course will then be reflected through the tiles. You can resin the y shaped cracks as the resulting bays will be in the right ball park and aspect ratio for this depth of screed.

Interesting point is that Y shaped cracks across dooorways often appear as a result of restraint as much as drying shrinkage suggesting the inadequacy of or the absence of the perimeter edging. Typically with underfloor heating this should be a minimum 8mm and etherfoam is best for the job.

The heating should have been commissioned prior to tiling to condition the screed. This should be not be done with sand cement for 28 days following installation. As you said, doing this in an uncontrolled manner is disastorus especialy when the screed is still significantly wet.

 

[jay]

hi if floor heating is turned on at a high setting to start with you will crack your substrate floor heating should be run very low to start as to dry out all moisture slowlly ive seen a bit of it down here with electric heating built into house slabs :8:

 

[markcollings]

Thanks Ajax,

I think cutting the movement joint in across the door opening is the best option. Can you elaborate on the perimter edging comment? I believe in this particular scenario insulation was turned up around the edge of the screed but I think this was just a thinner version of the expanded foam insulation used beneth. Can't recall the thickness.

I found some information in BS 8204-1 on screed curing. It seems that for a 125mm screed the suggested curing time is 5-6 months, therefore it seems the curing time in this instance was woefully under by about 3 months. I note that the BS recommends that heating should occur slowly once the screed is fully cured and dried i.e. early heating is not a subsitute for allowing the correct drying time hence artifical drying is not recommended.

I have also obtained the following advice about heating sequence:

"The screed should be maintained at the opertaing temperature for several days before cooling down to room temperature (but not below 15 degrees) before installing the flooring. BS EN 1264-4:2001 states initial heating up shall be carried out at least 21 days after laying the screed as follows:

- Use supply temperature of 20 degrees to 25 degrees for 3 days
-Set maximum design temperature for at least a further 4 days"

I am in the process of obtaining the BS referenced above as it seems crucial that this information is adhered to, to prevent problems from occuring. In future I will include an outline of this information in any specifications.

It seems that in order to get things right and prevent the risk of cracking the sequencing and process for forming the screed, curing and drying and heat cycling prior to laying the stone floor is essential. On busy building sites when quite often time pressures means that work is undertaken out of sequence and timings are condensed it seems that the risk of getting it wrong is quite high. I presume therefore the use of products such as Ditra Matting reduce the risk if the correct sequence and timing is not followed.

Regards,

Mark