OSB or tongue and groove boards for flooring in an expandable attic?

Tongue-and-groove boards are wonderful and also make a pleasant floor. They often look much better than any laminate. Underfloor heating on it? Better not. You can also heat with a partition wall with a low supply temperature. Impact sound is fantastic with simple tongue-and-groove boards nailed onto beams; it is amplified and not insulated. So it also depends on what is going to be up there later and what happens underneath. We have tongue-and-groove boards as both ceiling and floor on the gallery above our cooking-dining area. It looks great and is at the same time very basic and inexpensive. Of course, you can hear when someone is walking above, which doesn’t bother us in our single-room setup.

On insulation. Do it. We did it too. The result is now, despite minus xy and an east storm up there, a tolerably tempered attic compartment where I do my Pilates exercises and a bit of weight training every day. Drinks are also kept cool there but frost-free. Like it used to be in the basement. My Fleischmann train set also handles it well.

Great. That's exactly how I imagined it. I will implement it that way. But I still don't know how to deal with the masonry and unplastered gable walls. But basically, I will implement the rest just like you. Do you have a substructure there now, or is the rough boarding screwed directly onto the rafters?

Plaster gable walls? Practice up there, the result is not so important.

That’s exactly what makes it complicated if I have to saw each board to length. Staggered joints means I only saw when there’s an obstacle.

You can buy the boards in almost any way you want. What do you want to do up there? Store or keep? Otherwise, you can just lay OSB or even boards without tongue and groove if it’s just storage. Is there actually going to be underfloor heating installed later up there (storage?) or am I misunderstanding? Nothing needs to be ventilated behind, just directly on top; although I generally prefer screws. Underfloor heating is of course possible with wooden floors if it’s a living area, but then you should consider the floor construction. There are also simple systems that can be equipped with wood. We used rough-sawn boards back in the sauna area; for me, that’s a natural wooden floor without any fuss.

Plastering gable walls? You’re practicing up there, the result isn’t that important.

I’m just wondering if I should really plaster it. Otherwise, there’s no air circulation up there anymore. As it is, the whole thing is a bit leaky and the air can be exchanged. Or am I misunderstanding something?