Heavy stone sills need to be secured. Typically, this involves non-corrosive metal anchors or dowels set into the masonry below. For lighter sills, a high-quality construction adhesive and mechanical fasteners may suffice, but weight should always be calculated.
The "look" of your sill will depend on the architectural style of the home. stone window sill detail
Beyond water, the stone sill performs as a thermal break and a structural lintel in miniature. Stone possesses high thermal mass—it absorbs heat slowly and releases it gradually. In winter, a dark granite sill can absorb weak solar radiation and radiate it back into the room, slightly reducing heating loads. In summer, a thick limestone sill stays cool, preventing the “hot bridge” that a metal or untreated wood sill would create. Heavy stone sills need to be secured