Building a stone staircase against the facade - how to solve water and moisture?

[Trafikverket1]

I'm going to build a stone staircase up to the entrance with stone blocks. How do I prevent the horizontal panel behind the blocks from getting moisture damaged? Should I replace it with metal sheeting?

Is it enough to put something like a threshold flashing from brick to brick?

The idea is that the staircase will occupy the entire space between the bricks and extend a bit along the facade on the left side of the door.

 

[pette]

I would place a platon strip between the wood and concrete there. I guess you have a little eaves that offer some protection from rain there as well.