My carriage roofs are multi-functional.
They're made as a styrene laminate.
Part of its purpose is to help keep the top of the bodyside straight and give the shell some strength.
Where there is a plastic skin being curved over the top then I also mount some ribs which help the top surface to keep its shape.
Lastly, I glue some strips on the underside which the hold the top edge of the glazing in place and, along with the seats and tables, keep it up tight against the inside of the windows.
That's what you can see in the picture here.
It's usually the last job I do before bonding the roof in place but not this time because, as has become our way with the Superbarns, I shall ask Himself to cut, bend and fix a brass roof to the carriage.