Most efficient way to turn an open polysurface into a solid?

how do I turn the polysurface that I have into a solid? I basically just wanna make the surface thicker. I tried using gumball to make a bigger copy of the polysurface that I have, then cap the edges and the use Create Solid but that is messing up my geometry as when I use gumball and make a bigger copy of the polysurface it scales it up in three dimension. I want it scaled in 2 dimensions only.
I uploaded a file of the object that I’m trying to make a solid out of.
PPC simple building - 2.3dm (8.2 MB)

Never mind I used the offset command. :smile: