In classical design gutters were part of the architecture of a building.
Box gutter construction.
Let me try to make this as simple as i can.
Built in box gutters are meant to be part of the architecture.
Typically a box gutter is trapped between two roofs that feed rainwater into it.
Built in gutters are part of the classical orders of design.
Box gutters are essentially placed between parallel surfaces as in a valley between parallel roofs or at the junction of a roof and a parapet wall.
The foundation of most box gutters is wood that is then covered with thin sheet metal such as tin copper lead stainless steel or some other metal that will not readily corrode.
A box gutter is actually built into the bottom of the roof or into the roof overhang.
It may be lined with epdm rubber metal asphalt or roofing felt and may be concealed behind a parapet or the eaves or in a roof valley.
A good box gutter design will have falls along its length together with sumps to collect water before it is fed into large downpipes.