ᐅ Durable hardwood, locally sourced decking boards

I also don’t like these plastic solutions, such as WPC and the like.
The idea of thermally modified wood sounds reasonable to me and is certainly a viable option, and of course your suggested variant is helpful.
On the other hand, I think the aging process can only be delayed to a certain extent when the material is exposed to the environment. Depending on the processing costs, I then consider whether the wood will outlast me anyway or what replacing the decking in a number of years would cost, if it is even necessary.
I believe that in these natural regions, the locally available and most affordable wood is used, and when it eventually breaks, it is simply replaced. We tend to be much more sensitive here.

Larch lasts a very long time and does not need treatment if you appreciate the natural weathered gray look. A distance of over 50cm (20 inches) to the next support point is acceptable for normal loads; the extra effort for a larger spacing is minimal.

Our carpenter built a terrace surface using thermally treated beech as a test. Even after more than 5 years, it still looks perfect. Beech is generally not suitable for outdoor use. I draw the unverified conclusion that thermal treatment has a very positive effect on durability and feel confident about it.

Nevertheless, we chose larch – also for the facade and the carport.

"Just use larch" .... 🙄 (yes, you are right)

I have also considered oak (robinia is said to be even better, but I have never actually handled it knowingly). Although I prefer wood in its freshly planed color over gray, I don’t complain when wood turns gray. I find the dark brown on the south side of wooden houses particularly beautiful again ... especially where there are such color transitions.

For me, conditions are more interesting than taste. Every material has its own characteristics – not to say: peculiarities – and experiencing those is more fascinating to me than imposing my opinion.

Finding the "right" material for specific purposes is one of the nicest and most exciting tasks for me in design (of whatever).

@manohara: In my opinion, proper installation of the decking boards is much more important than the type of wood itself. Good ventilation, no standing water, and the end grains of the boards swelling, etc. With that, you can basically achieve a long-lasting terrace from any type of wood. I would define "long-lasting" as at least 15-20 years.

The wooden terrace resembles a dock. The decking is basically ventilated underneath, which is very good. The substructure must be well protected. Applying plenty of Carboline helps. Critical points are also where the deck board touches the substructure and both are nailed together. These areas are prone to trapped moisture and rot. Therefore, apply Carboline thoroughly in these spots and additionally place some tar paper as a spacer between them. Be sure to use galvanized nails.

In my case, the substructure will be made of galvanized steel. It will be a metal construction attached in front of the existing small balcony, approximately 5 x 2 meters (16 x 6.5 feet), creating a walkable surface that must be waterproof underneath (since there is a second terrace below, which serves as my workspace).

This likely means it will be fastened from below using stainless steel screws. The top bearing surface of the steel frame (where the wood will be placed) is probably 60 millimeters (2.4 inches) wide. When screwing it in, I plan to place something in between to help the moisture dry more easily.
I think thick stainless steel washers would be the best option. Maybe I can glue them over the pre-drilled holes before laying the wood...?
... but that’s less of a problem here.
It’s clear that the "proper construction method"—protected against constant moisture—is important. 😎