Construct attic floor

Thick tongue and groove >19mm. We have 19mm and that is good for storage of boxes, cartons, etc. But for larger furniture or people, I would go =>22mm.
Under the tongue and groove, I would install frame legs at intervals of <50cm.
You can first set up a piece (1m x tongue and groove length in m = test floor in sqm) and test whether it springs too much.

One question about the picture:
This is an unheated cold roof, right?

I had googled, I would definitely also take the 22mm. With the frame legs, that is a great idea because the beams are actually a bit far apart, I don’t know exactly how far but you can see it in the pictures.

About the cold roof, well, I hope I understand this correctly, there is no floor installed in the attic, there is only a radiator hanging but it is never used at least as long as it currently serves as a storage area.

If you mean whether the roof is insulated? Yes, on the outside the underlayment membrane is laid, then insulation wool between the rafters, and sealed from the inside with a vapor barrier foil and as cladding we used OSB boards with 12mm thickness.

okay that fits then in the attic. The important thing is that warm moist air coming from below (living space) probably cools down in the attic and thereby reduces the air’s capacity to hold water, causing water to condense. This water can then cause building damage. The additional space should therefore also be heated or it should be ensured that the dew point is not fallen below.

Can one access the attic via a fixed staircase or via a folding ladder?

okay that fits then in the attic. The important thing is that warm moist air coming from below (living space) probably cools down in the attic and thereby reduces the air's water holding capacity and condenses water. This water can then cause building damage. The additional room should therefore also be heated or it should be ensured that the dew point is not fallen below.

Can one access the attic via a fixed staircase or via a pull-down ladder?

Yes, we have already said that when it gets cooler, we will turn on the heating there as well .... you can get to the attic normally via a fixed staircase.

Do I understand this correctly: The ceiling to the attic is insulated, has a vapor barrier and OSB. The roof is insulated and also has a vapor barrier and OSB again? So, two times vapor barrier/retarder?

Do I understand this correctly:
The ceiling to the attic is insulated, has a vapor retarder and OSB
The roof is insulated and also has a vapor retarder and OSB?

So twice vapor retarder/barrier?

Yes exactly, as I said, we really got badly fooled by the stupid construction company, that could definitely have been solved better.

Yes exactly, as I said, we really got badly screwed over by the nonsense construction company, it could definitely have been handled better

I also have to add that the rooms under the attic have slants (knee wall 1.5 meters) and the slants were insulated and a vapor barrier installed, which I didn’t understand and reacted too late when they let the vapor barrier foil from the slant run into the ceiling ‍♂️ instead of pulling it all the way up to the ridge.