Do I need internal angle wall tiles for an external dormer wall

I will shortly be tiling the external dormer walls of a loft conversion and for the particular tile I have chosen, it is possible to purchase external angle tiles but not internals. My thoughts are that I can use "normal" tiles to create the internal angle but do I need to put anything extra (in addition to the breather membrane) behind them in order to make it watertight?

 

Lead soakers or concealed gutter Craig.

 

Thanks Alastair - I thought soakers might be the answer.