screed instead of latex

[bluesky]

went to deliver tiles of a job i`m starting on monday to discover that the builders have decided to lay screed over the top of electric cables 30mm thick .this was instead of laying 20 mm insulation board then cable then latex to build up to the correct height . they have told my customer that it will all be dry in time for monday as they have added pva to the mix! normally i would allow minimum of 3 weeks for this screed to dry but also i am concerned about the heating up times and costs .normally latex will only cover the cables my 2-3 mm.if i am correct with regards to screed not being dry could i get round this by using ditra.?
many thanks for any input.

[garythetiler]

tyco have got an electric ufh cable that can be screeded over with sand and cement but that 30mm screed wont be ready for tiling on monday and i am not sure why they put pva through it, with wet system ufh its normally a 60mm fibre screed that is used so i would be a bit wary of future problems unless they can show you some technical data to back up their preparation method

[Alan.P]

I wouldn't so much be concerned with the laying of the tiles, 30mm of screed to heat up before getting to the tile might be asking too much, I await to be corrected, Bal green screed could be used, site circumstances permitting.

[whitebeam]

I'd be concerned about the cable being able to heat the floor with 30mm screed on top

[Dave]

The screed is to thin imho...If it was a leveller with finer sands so that it binds better but normal type screeds with coarse sands will more than likely crack at that depth..50/60mm is usually the minimum...and no modifier in the screed...

[RockCeramics]

seems to me that the builder is trying to cut corners and costings down.
& the qoute of the week is.
"we have added pva to the mix! "
why does every builder feel the need to add pva?

[Ajax123]

is the screed straight onto the concrete subfloor or is there insulation underneath it?

[bluesky]

screed is straight on to concrete base no extra insulation- looks like another postponed job!

[Ajax123]

One assumes that the conrete slab was not mechanically prepared - it would be very un usual to see this done to say the least. BS8204 treats this as an unbonded screed although the industry terms it partially bonded. This means that the minimum depth should have been 50mm. 30mm is simply too thin for sand cement and there is a real risk that the screed will crack when subjected to heating. Additionally if the concrete underneath is new this can take an age to dry properly (100mm concrete takes in excess of 12 months to dry) so the residual moisture in the concrete will want to rise through the screed. Additionally PVA degrades when in the presence of alkalis (cement is alkaline) so over a period of time the screed will potentially begin to fail although this is relatively low risk and not something you as the tiler need really worry too much about.
The best idea would have been to use an anhydrite screed at this depth if they had to screed it but I cannot think why they did not go with the original plan except to cut costs cos insulation would have been more expensive than the screed at this depth. The other problem of course is that the efficiency of the underfloor heating will be seriously impaired cos they will now be trying to heat up the entire slab and not just the screed. This one is a deffinite candidate for an uncoupling mat in my opinion.
Why can't people just build things properly!!

[bluesky]

am i correct then with drying time of 21 days for the 30mm screed then use ditra mat with spf ?

[Ajax123]

am i correct then with drying time of 21 days for the 30mm screed then use ditra mat with spf ?

3 weeks will get you somewhere near dry I would think. On the basis of the risks previously explained I would certainly not be entertaining this one without uncoupling

[Diamond Pool Finishers]

dear bluesky you need to leave an (opc) mixed screed three weeks to cure (not dry out) its a chemical reaction according to BS standards if your installation fails you are open to litigation POSTPONE MATE GOONER 59

[grumpygrouter]

am i correct then with drying time of 21 days for the 30mm screed then use ditra mat with spf ?

You can use Ditra as soon as the screed has set enough to walk on. it does not affect the drying time or process.

[Ajax123]

dear bluesky you need to leave an (opc) mixed screed three weeks to cure (not dry out) its a chemical reaction according to BS standards if your installation fails you are open to litigation POSTPONE MATE GOONER 59

Curing is where the moisture is trapped within the screed in order to promote the hydration reaction betwee the cement and the water. It is a separate aspect to drying. According to BS8204 sand cement screeds should be cured by covering with a sheet of polythene for a period not less than 7 days following instalation in order for the screed to go hard. Once cured the screed can then be dried. This is the process by which surplus residual moisture is removed from the screed by evaporation in order to equilibrate the moisture in the screed with the moisture in the atmosphere. If the two are not equillibrated then the vapour pressure in the screed being higher than that in the atmosphere (normally accepted as 75% although this includes a 5% testing error) then the upward movement of the residual moisture can cause a failure of the bond between the adhesive and the screed. This is for two reasons, firstly the primer (usually acrylic or SBR ) will soften and reduce the mechanical bond between it and the interface. Secondly the excess moitsure can cause the adhesive to soften again causing a failure. This is when he tiles delaminate. WIth anydrites there is a third mechanism in place and that is the chemical interaction between the cement in the adhesive and the sulphates in the screed which promoes the formation of Ettirgnite and Thaumesite sulphate salts which disrupt the bond again causing delamination. Ditra is one way to prevent this bit but there are other methods as well.
A fully compacted 1:4 sand ceent screed will have a dring rate of around 1mm per day. Trapping moisture on a screed based on portland cement is not detrimental to the screed as the hydration reaction will continue ad infinitum whilst there s moisture present. The Ditra acts as a vapour equalisation layer thus equilibrium with the atmosphere is not necessary and as grumpy rightly says it can be placed as soon as the screed is hard enough to walk on. It does not matter if it becomes delaminated lightly because the tiles are completely divorced from the screed.

[biffa]

when you say ufh do you mean a mat or a cable, because a cable under 30mm screed would be fine even if it was a 10watt per metre. a 30mm screed would'nt be very efficient for storage heating should be at least 50mm, unless a liquid screed.
ditra a must tho

[Diamond Pool Finishers]

yes ditra looks good idea or tile backer board , ardex eb2 is a rapid hardening cement for floor screeds instead of (OPC) that can be tiled on in 3 hours!!!!! for future ref good luck mate

[bluesky]

cheers gooner and all others , will order ditra on monday as floor has limited height available due to those bi fold doors and they want to basically walk straight out onto patio.wish they`d spoken to me about options etc!ta again

[bluesky]

update.builder now wants to put 3mm latex on top of screed instead of having cost of ditra knocked off his bill !i`ve said i dont believe this is making a solution and still wouldn`t be able to be laid until screed is dry anyway .so my original solution still seems to be the way forward - ditra which can be laid asap .can any one back me up ?

[Ajax123]

Yes

If you put the tiles or latex down whilst the screed is still wet they are likely to blow.

If the underfloor heating is not comissioned before tiling commences the tiles are likely to blow

If the screed is too thin (which it is) and subsequently cracks with the tiles in place the tiles are likely to blow

You could look at an Epoxy DPM but these are not recomended for heated screeds

If you use Ditra all of the problems above are avoided and the tiles won't blow (I can't offer that guarantee of course - litigation litigation blah blah blah but you get the picture)

Simple...

Oh by the way if you do it his way and it goes wrong you could well find yourself liable

[grumpygrouter]

Bluesky, sounds to me as if this job is going to bring you nothing but grief and could well cost you dear in the long run. Might be better to cut and run now before it gets messy!

[smurf21]

or.........you could tell the builder to wait till the screeds dry that wont cost as much as replacing a blown floor

[Ajax123]

or.........you could tell the builder to wait till the screeds dry that wont cost as much as replacing a blown floor

Given the depth and dimensions of the screed and the fact it is heated I am not at all convinced that it would remain stable in operation so even if it is dry you could end up with the tiles blown due to substrate movement.

[bluesky]

hi again ,ta for all the help .update:had now agreed to use ditra until the bulder has told the customer that i need to put down bal bond(sbr?) product prior to tiling to guarantee good bond!i explained that i didn`t want to fix the tiles directly to the screed .and already there is a tiny crack inthe screed after the heating was tested .would love to walk away but this is for a friend of a friend.am i correct about bal bond?
ta

[Ajax123]

hi again ,ta for all the help .update:had now agreed to use ditra until the bulder has told the customer that i need to put down bal bond(sbr?) product prior to tiling to guarantee good bond!i explained that i didn`t want to fix the tiles directly to the screed .and already there is a tiny crack inthe screed after the heating was tested .would love to walk away but this is for a friend of a friend.am i correct about bal bond?
ta

Balbond wont stabilise the substrate so could not make that reomendation - Sounds to me like the builder is just trying to penny pinch.
Balbond is not necessary on sand cement - a standard Acylic Primer (From memory Schluter recomend Bal APD or Ardex P51) stick the ditra down with a standard flexi addy then tile on top. It does not matter if the Ditra delams a bit due to substrate movement.

[bluesky]

exactly what i said - many thanks for confirming what i said ajax and all who have helped.