A window or door sill should be _________.

A window or door sill should be sloped to ensure proper drainage of water away from the building. This design feature is critical for preventing water from pooling on the sill, which could lead to water intrusion and subsequent damage to the interior wall and framing. A sloped sill helps direct water runoff toward the exterior, enhancing the structure's ability to resist moisture-related issues such as mold, rot, and corrosion.

While a flat sill might seem adequate, it can encourage water accumulation, which is not ideal. Curved or vertical sills could also hinder proper drainage and create other complications in managing water flow. Hence, sloping is the best approach for maintaining the integrity and longevity of the building envelope around windows and doors.