Rejection Warranty - Decoupling Mat

Hello,

we are currently building a single-family house with a general contractor (GU). At the end of the month, the screed will be installed. I have now received an additional offer from my GU for the extra costs of our tile selection. For the ground floor, we chose 20x1.20cm tiles. Due to the larger format than what was originally included in the scope of services, additional costs arise. So far, so good.

However, the additional offer explicitly recommended an uncoupling membrane (Ditra membrane) because of the large-format tiles. If this service is not accepted, the GU does not provide any warranty for the tiles. According to my research, the uncoupling membrane does not refer to any DIN standard. After inquiring with the GU, they also only base their recommendation on the Tile Industry Association.

Regarding 1.3.: "On the building physics conditions"
For large-format tiles from 60x60 cm, in addition to the contact layer and the rapid adhesive, the executing specialist company also recommends uncoupling in the form of a Ditra membrane. The uncoupling membranes separate the subfloor and floor coverings from each other. They absorb movements and thus prevent stress cracks. The larger the tiles are, the greater the risk. Therefore, when laying large-format tiles, we already point out in advance the additional effort compared to standard formats (15x15 cm, 45x45 cm, 30x60 cm) in terms of installation.

The statement from the Tile and Natural Stone Association within the Central Association of the German Construction Industry can confirm this again: "During installation, the building physics conditions of the different substrate types must be observed as a mandatory condition. The coefficients of expansion of cement screeds and calcium sulfate flow screeds/poured asphalt screeds are significantly higher than those of ceramic tiles and slabs and natural stone tiles. The shape changes originating from the substrate can be only very limitedly compensated for by the small joint proportion and joint width. Therefore, for large-format tiles, additional stress-relieving measures may be necessary."

How do you assess this argumentation? I still believe that the uncoupling membrane does not refer to any DIN standard and that therefore, with a proper screed, the warranty cannot be denied.

I would be happy to receive your feedback!

Well, I know it as that Ditramatten are actually only used where there is no screed, basically on wooden floors...

DIN or not DIN. You either get the large tiles with the mats or not. The GU can decide what they offer. It's that simple. You can do 1000 DIN handstands and argue crookedly. By the way, I consider laying with the mats to be sensible.

We have similar formats ourselves, no one saw the need for such mats. There is flexible adhesive underneath and that's fine. To my knowledge, neighbors also don't have mats. 20x120 is nothing unusual anymore. What is this fun supposed to cost? In the end, you'll have to take it like that or not. Welcome to the [GU] trap.

We also install 120x20 and do not have a mat... no one recommended or tried to sell us one either. The regular flexible adhesive seems to be sufficient here.

Ditra mats must be considered regarding the thermal conductivity of the underfloor heating. (additional layer). With us, both the 120x20 and 60x60 tiles are currently being glued directly onto the cement screed. The screed will only be heated in the next few days; so far, no one has come with a decoupling, but it may still happen, based on my assessment of our general contractor...