My buildings are fine until I import them to Unity. None of the formats I've come across( FBX, DAE, 3DS Max, Blender Files, any of the Autocad formats, OBJ, PRJ, WRL etc.), have roofs when importing the buildings as one cohesive file into Unity. In Sketchup, Blender etc. they all appear fine. Does anyone have suggestions?
The most likely cause of your problem is with the way your 'normals' are facing in your 3D-modeling program. If you are using Blender, you can select your faces, press w, and select 'flip normals'. It should turn your darker colored faces to a normal lighter color again.