Roof Air vent
 

I am building a large 'oak barn style' building, the building has
150mm square wood posts physically bolted to steel base plates on the
concrete raft, on 2 sides (open sides) and also steel straps holding
down walls to floor on the closed sides.
Due to large open front, I still have concern over wind lift
(although structural Engr seems happy with bolts & straps)

I seem to recall that on some farm buildings they have what I can best
describe as large upturned casserole dishes ... these lie flat under
normal conditions, but if a positive pressure builds under the roof,
they lift allowing over pressure of air to vent out.

Anybody know what these are called or where to get them ?