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.