Warranty for tiling works

Hello everyone, we want to have our bathroom tiled as part of a renovation. It is located on a wooden beam ceiling. Dry screed was installed here, the tiler came for an inspection and tested the substrate (jumped on it a bit, etc.) and said it was no problem, the tiles can be laid with a decoupling membrane. I'm not so sure and said I could also make improvements before the tiling work. He says that's not necessary. That’s good for me at first. But if, in the worst case, the grout cracks or something similar after a year – does a warranty then apply and do I have the right to have the tiler come back and redo it? In my research, I only found contradictory information...

Even if you have legal warranty... if he simply doesn’t show up, you would have to fight that out in court in the end; the tiler has long since received his money.

And what do you expect from the suggestion to make improvements? That’s just one more reason to deny you the warranty after a year, isn’t it?

... And what do you hope to achieve by the proposal to improve? That is just one more reason to refuse you warranty after a year?

The original poster probably means to improve the structure of the floor, particularly regarding deflection BEFORE the start of the tile laying work. That should have no impact regarding the warranty of subsequent work. Maybe consider a black tank (fully welded bitumen membranes) if it is an older wooden house / wooden beam construction?

To what extent does a bitumen membrane reduce deflection? Can only an additional layer in the transverse direction achieve that?

I can understand that reading forums makes you lose all faith in the goodness of people... but doing it yourself as a basis for the craftsman?
Not a good idea.

From my own experience:
There is no better gift than when the customer takes over preliminary work. Anything we build on that, we can blame on him.

If you use a bitumen membrane, in 125% of all damage cases the tile layer will hold the bitumen membrane responsible and won’t lift a finger ;)

The bitumen membrane was of course not intended for structural improvement but only for reliable sealing against moisture-sensitive components (beams, wooden floor, wooden walls?). My brother did it that way in his well over 20-year-old wooden house (timber frame construction) during a bathroom renovation. It convinced me technically.

A structural improvement could be a reinforcement of the wooden beam layer, an additional layer of the floor, depending on what is possible.

BEFORE starting the work, the tiler must ensure that the floor is "ready for laying" (as with parquet on screed regarding residual moisture).

The tiler cannot refuse warranty claims due to work by the original poster that took place BEFORE his trade. At least that is how I have understood the warranty law in the construction sector (unfortunately I had to deal with the topic due to defects in my house in one trade, I have a court appointment about it in mid-December :-( ).

But I could be wrong, it is only my layperson’s opinion. Furthermore, this can and should not replace legal advice.

The discussions, as I see it, always lead further away from the original question of the asker.
Basically the following:
In the bathroom, especially with timber construction of the building, the floors must be sealed in such a way that in the event of a failure no water can infiltrate the floor construction. Just imagine the damage that would result if water were to flow over the wall surfaces into the lower floors.
Now, as the asker explained, we have a walking surface made of gypsum fiber elements. The inspection and duty of care of the floor installer (here: tile installer) therefore specifically refers to a substructure that differs completely from a mineral screed of the usual type.
Such a construction must certainly be adequately and suitably checked for possible deflections under point loads.
After all, in the event of damage, proof must be provided as to which inspections were carried out according to which procedure - and which results emerged at that time.
Later, simply stating "Everything was fine" will certainly not serve as an exonerating argument for the tile installer in the event of a dispute.
In the simplest case, one will place a longer straightedge on the unloaded surface of the dry screed elements and then load the surface with a body weight. There must not be any significant deflections.
However, since there will definitely be deflections, especially with a wooden beam ceiling, the manufacturers of the lightweight construction specify a maximum tile size of 30cm x 30cm and a flexible adhesive together with a flexible joint grout.
That’s on the topic of ceramic floor covering.
Let us now turn to the preparatory work.
This consists of sealing the entire bathroom area, regardless of the water exposure class, with a suitable liquid sealant.
Anyone who comes up with the idea of using a bituminous sealant at this point should quietly sit in a corner and continue to follow the further discussion on this subject passively.
A bitumen sealing membrane used indoors, and on gypsum fiber boards (prefabricated screed) at that, is a no-go!
Once the floor area has been treated with the first sealing coat, the transitions on rising surfaces are integrated into the second application of the liquid sealant within about 12 hours using appropriate sealing tapes and system corner solutions. This second coat usually has a different color than the first to make any defects quickly visible.
In most cases, an uncoupling mat only benefits the craftsman because he can gladly pass on the resulting costs to the frightened and grateful client.
The fact that uncoupling mats are only applied to critical screeds should at least be mentioned here briefly.
A floor level based on a (of course suitable) gypsum fiber board is not a critical substrate!
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So let’s summarize:
Sealing, for example based on a brush- or roll-applied liquid sealant, together with sealing tapes on rising wall surfaces is absolutely necessary in timber frame construction of the building.
An uncoupling mat is by no means among the necessary building materials when new gypsum fiber boards are used as "dry screed".
Flexible auxiliary construction materials such as tile adhesive and joint mortar are also necessary purchases.
The tiles must not be larger than 30cm x 30cm, otherwise there is a considerable risk of breakage or cracking due to usage loads caused by (unavoidable with dry screeds on timber beam ceilings) deflections.
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I hope these notes are of help once again!
Regards: KlaRa