Drainage solution

[MrJ]

Hello
I am in the process of extending my house at an angle. On the newly built part, I have raw timber and on the old part, there is some type of roofing felt. My question is how to best make it watertight in a smooth way? It should be added that I have a metal roof.

Hope someone has a good solution for this!

 

[andreas pelz]

I will also do something similar. The new patio roof will have raw board while the old one has thin asbestos sheets. How did you manage that?