Due to a dormer roof usually being on the top of a house, maintenance would be very tricky requiring scaffolding which then becomes expensive.
For these reasons you really only want to do a dormer roof once.
The best material for this would be an EPDM Rubber Roof whereas you can install it and just forget it.
The construction of the roof may vary depending on whether it’s a ‘warm’ roof construction or a ‘cold’ roof construction.
Wherever possible we do like to install furring strips to provide a slight fall to the roof to the gutter.