In plain vanilla there are some non-obvious ways to prevent water damage. For the large gates, you can use the angled beams atop the gates and that will stop water damage to them.

It is also possible to build a working flat roof in vanilla.. build a sandwich of 26 degree peak roof pieces between two layers of floor with short walls at the edges caped by either angled beams or 26 degree X pieces to form a low wall around the top. The topmost flooring will still take water damage but that is of no consequence just adds a weathered look seen only from above.

Valheim rewards thinking outside the box.

PS: For bench in park, make a gazebo.