Faces are Missing when Import from Blender to Unity

I am having a very weird issue with Blender. I have created a model of a mansion, with each face on the roof being a flat, solid object. However, when I import the model into Unity, the outer faces of the roof are missing.

I have tried subdividing and triangulating the mesh as other posts have recommended. However, the faces are still not showing in Unity.

Any help would be greatly appreciated.

Have you tried finding the faces that are missing and flipping the normals.