Franklin Sensors studs finders are designed for finding studs through interior surfaces.  They are particularly designed for finding studs through sheetrock. They are not designed for finding studs through exterior surfaces such as roofs and exterior walls, or tile. 

However, you may find success in finding the studs through the opposite side of the wall.  For example, if a surface is covered in tile, check to find the studs on the opposite side of the wall.  Alternatively, you may try taking your reading from an area without tile, either higher or lower on the wall.