My model looks fine in blender but does not render all faces in Unity

Question

I created a simple arch bridge in Blender and it looks fine in blender but if I import the blend file into unity, some of the faces don't render at all. Can someone check my model and tell me where I'm going wrong? I have recalculated the normals, and I was not using any modifiers when I created the mesh. Any insight would be appreciated. The blend file is here

https://www.dropbox.com/s/qw3h9ueqfm6vl9o/arch.blend?dl=1

the mesh looks like this in Unity:

https://www.dropbox.com/s/tprxy8psyupkt51/2017-01-28.png?dl=0

Answer 1

The model is only loosely connected, and the top face is completely separate from the rest of the model. You can see for yourself by selecting the object's faces in Editor mode and dragging them away:

Since the faces aren't properly joined together, Blender is unable to recalculate the normals because it can't find any edges joining faces that are pointing in opposite directions.

You can fix this very easily. First select the entire mesh in editor mode and apply Remove Doubles. Then recalculate the normals again.