SLATE on uhf and trav on fibre screed help!

[Ajax123]

50mm is just fine for a screed that's fibre reinforced.
50mm is fine if the screed is unbonded I.e. on a solid substrate. If the screed is floating as i assume it will be in this instance with underfloor heating the minimum depth should be 65mm. The design depth should be 75 to 100mm in order to take account of undulations in the substrate and the surface. This is a specific recomendation of the British standard 8204:1:2003. the screed shold be cured for a minimum of 7 days following installation under polythene and should not be heated for a minimum of 28days following installation. It should be reinforced with either fibres as in this instance or with d49 steel mesh. Bare in mind that the 65mm recomendations the MINIMUM not the nominal depth so this would normally be at the top of the pipe which assuming it to be 15mm the screed depth from insulation to screed surface should be a minimum of 80mm. I know full well that people do not lay screeds this deep and in smaller installations e.g. Small extensions will probably get away with it. It is not my habit to offer advice based on what you might get away with but on what is correct and best practice. think about the screeding projects I deal with if you lay 50,000m2 of sand cement screed in hospital and it is too thin and it begins to break down or fail that is a massive issue. In fact generally speaking sand cement screeds are a massive issue in the contract market.

I'm interested in the BSF and Tiling Standards you mention. Do you have a link to these I'd like to read up on them

BS 5385 Part 3: Design and installation of internal and external ceramic and mosaic floor tiling in normal conditions - Code of practice, under section 6.7 Isolation of tile bed from the base, states "That failure arising from variable stresses should be avoided by isolating the tile bed from the base by a separating layer that prevents the two elements from adhering to each other and thus allows each to move independently." Whilst this statement is not directly aimed at uncoupling membranes or heated screeds, it does recognise the existence of stresses and the benefits of isolating the tile bed from these stresses.
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It's quite a few years since I was last installing and at that time neither were the case (as you can see from the technical advice Schluter gave me back then) so it would be interesting to read up on modern requirements.

Are these suggestions? Advice or British Standards?

My answers in bold 🙂

 

[bugs183]

Sorry to disappoint Sir, i totally see Ajax's point, but i'm just testing whats out there!

 

[Ajax123]

I get your point bugs. No issues with testing for yourself but I can tell you that it has all been done previously so comparative results might be quite interesting. Epoxy primers work very well with cement adhesives on gypsum. SBR also works well provided a good key is obtained and you put enough on. I guess whichever primer you use the key will be preparation. Also I think it would be unwise to use an epoxy primers conjunction with a gypsum based adhesive. The epoxies tend towards waterproofing if you are nt careful so if at a later date you have a major spillage or something like that the water will sit at the tile adhesive and soften it leadin t a potential failure that you wild be less likely to see with gypsum direct to gypsum or cement to epoxy. Will look forward to your findings though.

 

[Ajax123]

Ooooh I feel a :lurk5: moment coming on !

I got to keep in practice whilst I am off work.... 🙂

 

[Concrete guy]

My answers in bold 🙂

Thanks,

This makes so much more sense when applied to commercial sized areas.

The degree of shrinkage on say 50,000sq/m as compared to 50sq/m will be identical in percentage terms, but when applied in real measurable terms will be so small that it's more than overkill to uncouple.

I get where you're coming from though.

As you probably know, fibre reinforcement in screeds is used to dissipate shrinkage and cracking locally and transfer this to the edge of a slab.

Out of all the isssues with the OP's floor the glaring problem in my eyes is that 10mm travertine has been specified for flooring in a reasonably high traffic area. I think that's more of an issue than any of the substrate concerns.

 

[bugs183]

I think i got off lightly there! :13:
That is a good point about the epoxy primer and the gysum, that could be disastrous!!!!

 

[Ajax123]

Thanks,

This makes so much more sense when applied to commercial sized areas.

The degree of shrinkage on say 50,000sq/m as compared to 50sq/m will be identical in percentage terms, but when applied in real measurable terms will be so small that it's more than overkill to uncouple.

I disagree having seen literally hundreds possibly thousands of screeds over the last few years which have cracked, big ones small ones all sizes really
The issue here is not so much that the screed will not perform it is more that the installation has been done incorrectly. So the tiler is faced with a potential issue. THat being the case the tiler is risking his own money if the tiles fail as a result of the screed failing as inevitably he will burden some of the cost and responsibility. It is generally not practicable to remove screed which is incorrectly laid in these sort of jobs uless it can be done early n. That needs to be done by the screeder who is likely to be long gone. The tiler therefore has a choice... Walk away or tile. If the tiler lays the tiles knowing the screed does not meet the minimum standards and there is a subsequent failure and it gets taken to court or arbitration the client will have justifiable cause to hold the tiler responsible allbeit possily jointly with the screeder. That can be expensive whatever the size. At least in uncoupling even in small projects the tiler is making a significant contribution to the long term durability of the floor.
In terms of performance and design sand cement screeds should be jointed at maximum 40xdepth so for a 50mm screed that means joints every 2m x 2m. For heated screeds this should be approximately halved. Again this is completely impractical so the tiler is once again faced with a screed which may not meet the necessary best practice design standards. The maximum ba size for a heated sand cement screed at 75mm should be 15m2 to meet the requirements of the NHBC. The other issue is that of curling. Sand cement screeds curl. The thinner the screed is the more pronounced the curling tends to be. The flexural strength of a 50mm sand cement screed even well compacted will only be around 0.5kN/m2 so placing an impact or dynamic load or even a heavy point static point or line load on a thin screed is highly likely to force it beyond its flexural capacity and thus its fracture point. If it has curled this will be exacerbated by the voids left beneath the screed. Of course I wish Had a penny for every time I heard the phrase I have always done it tha way and never had a problem.

As you probably know, fibre reinforcement in screeds is used to dissipate shrinkage and cracking locally and transfer this to the edge of a slab.

The fibres are not designed to transfer cracking to the edges as such but to help to absorb horizontal stresses caused by shrinkage helping t prevent cracks from opening up. ALL sand cement screeds crack. It is the width if the cracks that matter. The fibres also help to offer additional plasticity to the screed so that where there are cracks the fibres "knit"across the joint. Fbers are generally better than steel as they cannot be misplaced so easily as they are throughout the depth whereas steel will only in the plane of it's placement wich should be near to the top of the screed "typically within the top third. Also fibres don't rust.

Out of all the isssues with the OP's floor the glaring problem in my eyes is that 10mm travertine has been specified for flooring in a reasonably high traffic area. I think that's more of an issue than any of the substrate concerns.

I am not qualified to comment here although I do know there are may different grades of trav so I guess it depends on the psv of the particular stone in question. You may well be right however.

Again my comment in bold... Please forgive my typng. I am better at screeds than I am at typing 🙂

 

[afright]

In the 70s tiled floors were laid as a floating floor,the method was screeded floor then building paper with 1/2 inch sand & cement bed and cork expansion .

 

[Ajax123]

In the 70s tiled floors were laid as a floating floor,the method was screeded floor then building paper with 1/2 inch sand & cement bed and cork expansion .

Ie uncoupled.

Also there was very little underfloor heating around in the 70s. Heated screeds changed the rules considerably.

 

[Shortio]

Shutter advice can be inconsistent and confusing, they have told me that the membrane does not affect heat transfer, they also told me to apply tartan paint ha.