Adding edges based on one axis

First hello to you all as I’m new around here and wish everyone happy modeling.

Now to my current problem - As you can see on the attached image I would like to continiue to add edges on the selected face horizontally all the way to the roof the same way I’ve placed them on the lower part of the wall. I’ve tried CTRL+R(it makes a mess) and a Knife tool(it won’t snap to a diagonal edge) without success. I can duplicate an edge and then grab it for placement but it makes a mess of triangels at the end of the line. I would delete those and connect a line (vertex) to the edge but this is already another problem I’ve encountered…

Please include image.