Q:

How can I add a triangular shape with the thickness of my loop cuts?

I'm trying to make a simple shape but can't seem to do it right.
It's basically a right-angle shape with edges. Hard to explain, here are the pics:

http://imgur.com/vB0sT16
This is where I'm stuck:

http://imgur.com/GrUy4G4
All I need to do now is add in the triangles on the sides with the thickness shown by my loop cuts.
Any ideas??

A:

Create additional geometry and use Bridge Edge Loops to achieve the final result.

Bridging won't work from the beginning as there aren't 2 edge loops; however you can split existing faces and bridge the result. For that select edge between faces which form the base for the desired triangles. Press V to rip it and Esc to cancel moving. You have 2 edge loops now.  
Switch to Face Select mode, choose both faces and execute Ctrl+E > Bridge Edge Loops. Internal faces will be deleted for you.
Remove doubles (which were made with that ripped edge) via W menu. Also in 3D View header find Select > Non-Manifold operator (switch to Vertice or Edge Select mode to use it). Edge in the corner will be selected, delete it.

Note that tris may be undesired topology (e.g. if subdividing this surface or adding more loopcuts) so use it if you know this part of model will be mostly flat and not needing further subdividing. 

