Damp cement base limestone and UFH, DPM, Ditra questions

[1960bus]

Hi,

I'm new to the board and I wasn't sure where to post my questions as they seem to cover a few forums.

I have a tiler coming to lay some 825x550x20 Mandarin Taj Grey Limestone tiles but we have a problem with one of the rooms the tiles are going to be laid in. The room has a damp floor of original flagstone covered with a sand cement screed and no dpc.

To complicate things we also have some Screwfix electric underfloor heating to go under the tiles. I don't want to try and save money and get the preparation on this wrong and have 20m2 lift or crack.


We can't afford to increase the floor hieght anymore than we already are doing so...

I have been recommended to use:
grout ardex dpm
ardex 82 primer
ardex 7001 adhesive white or grey
ardex mg grout

Should I be using Ditra matting as well? Or could I use the Ditra on top of the damp floor and not bother with the DPC? Should the UFH go on top of the Ditra or underneath?

Thanks,

 

[basshunter]

Hi,welcome etc

Has the s+c screed been laid ontop of the flags recently?if so,why?... confused

 

[White Room]

And with no membrane, flag stones where normally laid on earth.

How old is your property

 

[1960bus]

The screed is about 10mm on top of the flagstone. (The flagstones were laid in earth.) It was laid recently (4 weeks ago) by a builder to level the surface before we realised there was a damp issue.

 

[White Room]

The 10mm screed will be no use at all, there's a good chance it will break up and cause more problems.

What is on the the other floors

 

[1960bus]

The flagstone and screed is on 5m2 of the area the other 15m2 is a 20 year old concrete slab (200mm deep I would guess) that I assume has a dpm beneath as it doesn't appear to be damp. The 10mm screed must have had some additive in it as it was black drying to grey (latex?) the damp bits remain black.

 

[White Room]

Some photos would be handy, the screed could have a heavy cement mix which make it appear very dark

 

[Time's Ran Out]

If its a seperate room you could have lifted the flagstones and put a decent screed with damp course down.
A 10mm (1cm) sand and cement screed over flags is asking for problems and is most likely breaking up as we type!
Ardex 7001 is a very good adhesive and excellent for natural products with its fast drying however it is primarily for TIMBER substrates so Ardex S20 in white/grey would be better suited.
Ditra matting as a waterproofing and de coupling product is one of the best I have used and should be used where stress/movement is expected.
The electric ufh can be used above or below the Ditra.
Visit the web sites of all the products you've mentioned for a full explaination.
However you need to sort your sub floor 1st.

Timeless John.

 

[stevee]

hiya mate ,if its only 5m2 ,why dont u take em up ,and as said above start again in this area?

 

[grumpygrouter]

If its a seperate room you could have lifted the flagstones and put a decent screed with damp course down.
A 10mm (1cm) sand and cement screed over flags is asking for problems and is most likely breaking up as we type!
Ardex 7001 is a very good adhesive and excellent for natural products with its fast drying however it is primarily for TIMBER substrates so Ardex S20 in white/grey would be better suited.
Ditra matting as a waterproofing and de coupling product is one of the best I have used and should be used where stress/movement is expected.
The electric ufh can be used above or below the Ditra.
Visit the web sites of all the products you've mentioned for a full explaination.
However you need to sort your sub floor 1st.

Timeless John.

Ardex have recently reformulated and reclassified 7001 as a general purpose adhesive IDEAL for timber substrates but can now be used on other surfaces too.

It gives surface prep info in their data sheet.

broken link removed

 

[1960bus]

The reason we didn't lift them is because the house is 200+ years old and the internal brick walls rest on the flagstones. I guess i can stihl saw round the edges.

One of my mates puts in founds, slabs, screeds for new builds for a living I'll get him to check.

The Ardex recommendation was straight from their techline.

I guess if I put a new screed down I'll need the Ditra matting as it will take weeks to cure.

 

[1960bus]

What would the ideal base be? I've pretty much decided to dig out the whole 20m2 and put down a new slab as I was never happy having UFH with no isulation.

Thanks

 

[Ajax123]

As has already been posted several times - 10mm s/c screed is insufficient. Should be at least 50mm deep in this type of application. However if it is very dark in colour it may not be a straight forward sand cement screed. It could be a polymer modified screed which may well be suitable at this depth. It would be worth finding out from the builder first exactly what he used cos it might not need to come up. As for the Ardex products I would have no issues but there are plenty of other options out there so again it is worth phoning round for the most cost effective products.

The fact there is not dpm under the screed or the slabs is not so much of an issue because you can use a two part epoxy paint on DPM on the top prior to tiling. Ardex do one of these also.

The over riding factor though is still what is that black stuff on top of the slabs. If it is sand cement screed just made with dark cement it will have to come up because it is proibably already breaking up. It is mis guided to think that using a hogher cement content would make it stronger because it would also encourage shrinkage and curling at the same time and so will still be unfit for purpose. As I said find out what it actually is and report back to the forum and andwers will be forthcoming.

I would not assume that the concrete has a DPM under it just because it does not look damp. If you are not certain then deal with it in the same way i.e. surface DPM.
These mebranes are around 300 micron thick so you have no need to worry about increasing the height as a result of there use. I find the one coat systems such as Tremco or Laybond tend to be the easiest to apply.

You will need a joint in your tile face at the same point as the two substrates meet to take account of vertical movment or differential expansion and contraction particularly if you are installing underfloor heating otherwise your tiles may crack. Ditra is not designed to take account of this type of movement.

 

[Ajax123]

What would the ideal base be? I've pretty much decided to dig out the whole 20m2 and put down a new slab as I was never happy having UFH with no isulation.

Thanks

If you do decide to do this then it depends on the overall floor height you have left over. If you use hand compacted sand cement screed you need to look at an overall depth of 65mm (BS8204 part 1 : Floating Screeds) and a minimum of 50mm cover to your underfloor heating. The depth of insulation under this is self evident i.e what ever is left.

I would recomend a double membraned system. A minimum 1200g DPM polythene placed under the insulation where it will remain intact and unpunctured by the underfloor heating system or any traffic or trade (e.g. wheel barrows) Then a second "slip" membrane of minimum 500g on top of the insulation prior to the installation of the underfloor heating system and the screed. This membrane acts to divorce the screed from the insulation thus preventing over rapid drying of the screed as the moisture is sucked into the insulation in the case of polystyrene and prevents un desirable chemical reactions which can cause long term durability issues in the case of foil backed insulation. It also allows the screed to act independently of the insulation in terms of thermal expansion and contraction.

If you use a flowing screed (although on such a small area I think it would be quite expensive to do so) then all of the same rules apply except the over all screed depth can be reduced to give a minimum of 30mm cover to the underfloor heating conduits and the insulation thickened up accordingly.

Hope all this helps.

 

[1960bus]

The screen was a cement only screed apparently. So I guess I'm lifting the whole floor.