Post 564 –by Gautam Shah
A sill is the bottom part of a window opening and is relevant till it serves some functional purpose, such as a protective barrier, for resting hands, or works as a reflector of light or rain. It has dual reference, to the outside and inside faces of the window. Functionally it is valued for its height against the floor level, its profile (shape in section), and its depth from the face of the wall. Sills have specific profiles for the intended use.
Sills also serve the function of a ledge. A ledge is a horizontal projected band, running over the external walls. It indicates the edges of floors, but sometimes represents windows’ bottoms. Window ledges other than decorative function serve many functional needs, like protection to the floor and wall below, and act as a support base for servicing. Window ledges provide a baseline for pilasters, half columns and engaged columns placed on sides of a window opening. Ledges, as projections called ‘label-stop’ work as a weather baffle, gargoyle, decorative carving or form the springing point for arches.
Sills are dominantly connected with internal faces of windows. Doors have sills in the form of thresholds. French doors have characteristic bottom ledges. Old shops-fronts were raised off the road level and from the shop interior floor level, by means of a stall-riser. Stall risers protected the shop-window from ramming by the cyclists. Bow or bay windows have sill-seats and ledges for flower pots and planters. Parapets have wide sill tops, to restrict view downwards. Banks and post offices’ windows have sill-shelves for transactions. These shelves are sometimes of sunken shapes to prevent the insertion of a gun nozzle.
The windows’ frames on interior and exterior sides have articulated surrounds to increase the apparent size of the opening, which also accommodate a deeper sill. On the interior side, the deep-set windows-sill works as a shelf. It is used to keep small things like combs, hair oils, adornments, and importantly, an oil lamp for night illumination. Similar sill ledges are also formed over bottom of niches in the walls, and are called Gavaksh or Gokh (India). The Gavaksh as a form was the forerunner of the Indian window. Gavaksh with extended sills are used for installing minor deities on internal and external walls of the temple. A Gavaksh is like an aedicule (a minor shrine) of Greek or Roman architecture, a microcosm, an abode for the minor god.
Sills of stones or wood planks, for doors and windows housed the bottom pivot. Sills maintained the width of the frame and provided integrity (square shape and the plumb-line) of the frame. Exterior window sills are often inclined and width-wise projects out to drain of the rain water.
Ground floor windows have taller and lower sills depending on the conditions of the surroundings and the security concerns. Low sills are employed where openings are latticed for security. Low sills for upper floor windows require protection bars at least up to safe or half height. The protection bars are mounted on the outer face or even beyond, so that one can lean out and see sideways. A low sill in tropical building provides a body level circulation of air or laminar air flow. Low plinth buildings with low sill openings gain high reflectance of light and heat from the surrounding ground surfaces. Low sills on an interior side warm up the areas near the window. Sill and plinth level manipulations work better on some orientations then others. Ground surfaces on the South side (in Northern hemispheres) receive solar radiation nearly throughout the day, whereas East and West receives it only for a part of the day.
A low sill or zero sill window ‘opens’ a space as in case of traditional Japanese houses or upper floor windows of a glass curtain high rise building. Openings like clerestory windows and skylights have no functional sills.
A high sill from interior space cuts off the view to the outside as in medieval cathedrals, and reduces the cold draft over the seats of congregation. A high sill in a high plinth building cuts off the interiors from the effects of high surface temperatures of surrounding ground surfaces. A high sill at any plinth level protects the user from cold drafts. Sill levels are also determined for the nature of privacy desired. Higher plinths to an extent compensate the need for a high sill over a street side in comparison with the buildings located within a private estate.
Externally, window sills are chamfered to allow greater view, and add to the perceptible tallness. Internally a chamfered sill illuminates sections close to the floor. A rule-of-thumb is that the depth of daylight penetration is about two and one-half times the distance between the top of a window and the sill. Store and other minor rooms have smaller size of openings with raised sill level but with a tapered ledge. The outside tapered ledge allows clear view of the street below, and the inside sloped sill allows more floor level illumination. Confinement cells of jails have openings at very high level and with a flat bottom sill to cutoff the view across, but may have tapered top for greater sky-component. Windows for domestic use require a sill level that an infant cannot jump out that is at least 850 high. Window sills in this respect function like a parapet, barricade or a railing.