Water gutter or fallanca[1]​ is a construction element made of stone, ceramic, zinc, wood, etc., which, forming a conveniently inclined surface to drain rainwater, is placed covering the window sill, the projections of the walls, the lower part of the exterior doors, etc.[2]​.

Its mission is to prevent rainwater from penetrating the building, which is why the flashing has an outward slope, ensuring rapid evacuation of the water and usually protrudes outwards forming a drip;[3]​ for the same reason of watertightness, the sill must penetrate the jambs of the opening. Due to its importance in watertightness, the design of the flashing is one of the critical points in the construction design of buildings.

From Latin vertĕre, turn, turn, 'destroy', 'change, convert' and aqua 'water'.[4]​.

The flashing is an important part of the windows, since it prevents water from entering the building.[5]​ In some countries there are regulations on the specifications of the flashings. In Spain, they must have a slope of at least 10 degrees, be waterproofed and have a drip on the underside of the protrusion.[6]​ There are different types of flashings according to the materials used for their construction, the most common being stone, concrete, ceramic, metal and polymer concrete.

In many constructions, windows have a shelf-like piece of interior trim, often made of wood, ceramic, or stone, that is distinct from the window's exterior flashing. These are popularly known as window sills or pedestals,[7] terms that are also occasionally used as synonyms for window sills. The technical term used by carpenters, window makers, and other professionals for this molding work is mantel.[8]​ In residential buildings, some people use it for decoration or support of houseplants, books, or other small personal items.