Exterior window sill: slope and drip groove to stop leaks
An exterior window sill needs a slope away from the window, an overhang beyond the facade and a drip groove, a channel on the underside that cuts off running water. At the window and at the sides it has to be tightly joined to the frame and the wall. Without this, water runs down the facade, leaves streaks and soaks into the wall.
Slope
The sill has to fall slightly away from the window towards the outside, so water does not stand at the frame. Without a slope, water gets into the gaps and freezes.1
Overhang
The front edge of the sill should stick out beyond the facade. That way water drips away from the wall instead of running down the render.1
Drip groove
A drip groove is a channel milled into the underside of the sill, close to the front edge. Water that would creep under the sill breaks off there and drips down.1 Without a drip groove, dark streaks form on the facade under the window.
Sides and window frame
The sill meets the window frame and, at the sides, the wall. These joints are sealed so that water cannot get under the stone. At the sides the edge is often raised or side end pieces are used.
Material
Outdoors you need a dense stone with low water absorption that resists frost. Most often granite. A brushed or honed finish shows streaks and rain marks less.
Measuring
We measure exterior window sills after the windows are fitted and, with external insulation, once the thickness of the facade layers is settled. Then we know how deep the sill has to be and how far it should stick out.
(Sources)
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Brick Industry Association, Technical Notes 36: Brick Masonry Details, Sills, and Soffits
(Questions)
Which stone for an exterior window sill?
Dense and with low water absorption, most often granite. Sintered stone also works.
Can an exterior window sill be polished?
It can, but a honed or brushed finish shows streaks less.
