Laying tongue and groove boards in the attic - tips

Sure, that works too, it's the "American" version. I don't like it that way. But it's a matter of taste. You usually don't want to remove it again voluntarily – with nailing, that's not damage-free... Personally, I'm also faster with a cordless drill and deck screws than with a hammer and nails, but it's probably a matter of practice. Or if you have a nail gun, the question doesn't even arise. But I don't know anyone in Germany except carpenters who own something like that...

The batten has the charm of being unbeatable cheap. 40x60 even at premium price-Obi 1.69 EUR/m.

It doesn’t have to be planed. Battens are also available in 40x60mm and with the wishlist you go to the next sawmill. Although I don’t find the 1.69 € totally exaggerated now, 40x60mm is obviously a lot more material than 30x50. When dimensioning, the question is also what is going to be stored. Only light things or the remaining roof tiles from the house construction?

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When sizing, the question is also what is going to be stored. Only light things or the remaining roof tiles from the house construction?

Well, as always. It starts with light boxes and after 5 years half the household is up there, including 3 children’s bikes that have become too small, the complete summer or winter clothing collection, books you want to read when you retire, and so on.

Well, Christmas decorations, less so the leftover bricks mentioned here, or something like that.

I have rough-sawn boards with a thickness of 23.5mm. But because of the tongue and groove, it does have a certain stability, right? Although I would probably choose a narrower spacing between the slats. Maybe only about 40cm or so, so that there wouldn’t be much sagging when walking on it.
the spacing of the beams is about 90cm for me too.

Just lay down 2 slats, put 3 rough-sawn boards on top, and stand on them. This way, you can try out at what spacing you no longer feel like you're standing one floor lower.

I don’t think you need 40, 60 is enough. I only covered the doubled beams with the 23mm planks (also 90 cm spacing) and besides tiles, I store a lot of heavy boxes. So far, I only have the typical cracks at the edge of the drywall ceiling, which usually appear after a year anyway. But if you have the time and money, it certainly doesn’t hurt.