Where slates meet a wall

[WabbitPoo]

What am I googling for to find the solution to this:

Currently a gently sloping slate roof over a utility room

and I'd like to build up off its wall to add an adjacent new roof to an extension (mock-up for illustration only):

My query is what to do where the sloping slate meets the new wall? Water can only come towards me on this view, as behind is the main house and there is existing drainage on this side.

 

[foxhole]

Lead gulley?

 

[WabbitPoo]

I figured some sort of lead work but to get it sloping back to front? Make a ply tray and angle that slightly?

 

[datarebal]

Basically yes.

 

[WabbitPoo]

Sounds so easy!

 

[foxhole]

Water will still run off the ends if level.