Comments and answers for "Physics.Raycast - Not Hitting Backside of Mesh/Poly?"
http://answers.unity3d.com/questions/126196/physicsraycast-not-hitting-backside-of-meshpoly.html
The latest comments and answers for the question "Physics.Raycast - Not Hitting Backside of Mesh/Poly?"Answer by Bunny83
http://answers.unity3d.com/answers/126373/view.html
All primitive colliders have no "interior". It's the default concept how nearly all collision systems work. Only if you use a non convex meshcollider and like Eric said you've modeled both side you can collide with both sides. Convex mesh colliders behave like every other primitive collider, so there are no "backfaces".
In your case you could start your raycast with an offset. Just shift the starting point upwards say 1.0. If you want to "measure" the distance you have to subtract the offset from the returned distance.
But watch out: The resulting distance can be negative (since you are slightly below the surface). Also keep your offset as small as possible. If you offset it 10m you will hit the second floor inside a building :DSun, 05 Jun 2011 15:28:31 GMTBunny83Answer by Eric5h5
http://answers.unity3d.com/answers/126200/view.html
It's not possible to make raycasts check both sides. If you need both sides to be hit, then you have to model both sides.Sat, 04 Jun 2011 20:43:08 GMTEric5h5