Timber won’t rot if it’s kept dry to a moisture
content below 20%. When it’s used outside, this may not be possible.

There are two answers: either you can use a
naturally durable timber, like oak or western red cedar, or you can use a non-durable softwood that’s been treated with a suitable preservative.

Guide on which components treated timber can be used for, the four different use classes and the different preservative treatments are: 1. Internal, dry; 2. Internal, risk of wetting; 3. Outdoors; 4. Direct soil or fresh water contact.

Natural durability and treatment considerations go accordingly the use and timber species