Decals on geometry brushes?

Hello all,

I made a hollow box brush to serve as a room. I also made a deferred decal material that I want to place on the floor of the room. However, when I drag the decal material over, it becomes the surface material of that the floor. I would instead want it similar to if I had added a cube and was placing it on that. What gives?

Thank you,
H.

I needed to drag the decal to an empty area and then move it onto the geometry brush.