Hello

I am the upstairs joint freeholder in a 2 flat Victorian conversion.
My downstairs neighbour has a Bay window whilst my windows upstairs are flush to the wall. My neighbour wants to cover the top of the bay with metal sheeting and create new drainage pipes as the rain water sits and she wants me to pay half.
Our lease says the lessee is repair jointly 'roofs foundations walls' but 'excluding all portions corresponding to the foregoing in or appurtenant to the other flat in the building'
Is the top of the Bay window considered a roof and therefore am I responsible for maintaining it?
Very grateful for your imput
Many thanks
Henny