Installing a stone window sill requires careful planning and execution:

The surface finish is equally critical. A highly polished sill reflects water efficiently but becomes dangerously slick when wet. A honed (matte) finish offers a compromise between water runoff and pedestrian safety. A thermal or flamed finish—created by intense heat that pops crystals on the stone’s surface—provides maximum slip resistance for ground-floor sills that might be sat upon or stepped over.