The present invention provides an apparatus for use on a roof or the like which incorporates an insulation material. The invention allows workers to shave the insulating layer to smooth, remove high spots, or create slopes, valleys or sumps, in order to achieve positive slope for water drainage. As part of the invention, there is provided, an apparatus for milling the surface of a low slope substrate, comprising a milling device comprising a cylindrical tubular structure having an outer rotatable milling surface, said surface positionable adjacent said substrate and drivable in a rotary motion to mill the surface of the substrate; a milling device support upon which said milling device is mounted for movement in a milling direction; a mounting assembly for mounting said milling device to said support; said mounting assembly including means for adjusting the angle and depth of engagement of said milling surface to said substrate.