Hey! I'm a beginner with SketchUp, currently using it for an student assignment in urban planning. I have made a model for an small area with proposed buildings for an area. The problem is that one surface wont split from another. its like its something wrong with the lin so when I put asphalt on the road it get asphalt on the other streets to, but does are supposed to have a different surface (pederstrian street)

When I look at this particular lines from under is like they dissapear (see attachments).

Any idea what causes this problem?