Boolean operation results in non-manifold body?

https://cad.onshape.com/documents/ab3332e1684958a44f290ffc/w/551681a7a09b752c48d96c39/e/cabedd45839b1fa5b9dbf6cf

Hey all,

I'm trying to extrude a roof (boss) onto this structure I've drawn, but got an error message and can't figure out why. This boss would be extruded in the same direction as several other elements, and would have one coplanar face with many other parts. I assume this is why I'm getting the error but might be wrong as I'm super new to OnShape. Used to using SolidWorks.

What's the correct method for drawing that? And yes I already know this isn't the most efficiently drawn part 😂

Thanks!