Why does the mantel opening cover some of my profile?(i.e. marble, tile, slate, etc.)

In many cases, the facing material is a natural material, such as marble, or a rough material, such as brick, which may not be completely straight on the edge. It is recommended that the mantel overlap this material by at least 1/2" to prevent any gaps between the edge of the mantle opening and the facing material. This ensures you will have a straight, even finished edge.