The primary function of a window sill is to serve as a channel for draining water away from the building as the window sill is a horizontal ledge that is made for this basic purpose. However, with the changing trends and newer discoveries in the construction industry, homeowners have woken up to the finer aspects of building and aesthetics and looking at it in a new light. This has brought about a sea change in the face of the ordinary window sill as more and more decorative and detail-oriented sills are being ordered and constructed for windows.

What are the window sill details that designers look out for

The most important feature of a window sill is that it should drain water away so the related point here would be that the top surface of the window sill should slope downward and away from the building to ensure this at all times. In the case of brick window sill, this detail should be at least 15 degrees from the horizontal as any less would be futile and make the function of the window sill, redundant.
If concrete or stone window sills are used, they too must be sloped from the ends towards the center, with the slope from the ends at least 1/8th of an inch towards the center of the sill. Those window sills that are longer than 4 feet, must have a slope that extends for at least a distance of 2 feet.
If there is a cavity or an air space that is interrupted at the window sill, at the base of the walls, like lintels over opening and at shelf angle supports, then flashing should be provided in the wall. This flashing serves as a collector of moisture that may penetrate the wall or sill and this should ideally extend through the brick to the exterior face of the wall at the lower end, then be turned down to two inches so to form a drip.

Drips and weep holes are important window sill details

The drip in a concrete wall can be cut to different shapes and the function of the drip is to prevent water from returning to the exterior face of the wall, so every window sill must have one. The presence of weep holes allows water to escape that would otherwise have accumulated in the window sill and are as important as ensuring structural, economical and aesthetic elements granted to a window sill.

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