Code of practice for fire safety of buildings (general): Exposure hazard
1988 Edition

For residential buildings, minimum open spaces vary with building height to ensure adequate ventilation, lighting, and fire safety. Detached structures up to 7 meters require side open spaces of 1.5 meters (one side if frontage is less than 12 meters). Semi-detached buildings of similar height require 1.5 meters on one side if frontage is under 9 meters, otherwise 3 meters. Row-type houses have no side open space requirements. Rear open spaces should be at least 3 meters average width, never less than 1.8 meters. For buildings exceeding 10 meters in height, side and rear open spaces increase progressively according to height, ranging from 3 meters at 10 meters height to 16 meters for buildings 53 meters or taller. Additionally, front open space must be at least 6 meters for buildings taller than 24 meters. If rooms lack sufficient ventilation from the exterior, these open spaces can be reduced by 1 meter within a minimum of 3 meters and a maximum of 8 meters.

IS 1643 addresses fire protection in high-rise buildings primarily through classification of occupancy and fire separation principles. For mixed-use tall buildings, the design must adhere to the fire safety norms of the most hazardous occupancy present. When different occupancies are divided by fire-rated walls with a minimum four-hour fire resistance, they may be treated as separate entities for fire safety considerations. The code mandates controlling exposure hazard by evaluating building construction type, occupancy, spacing, and elevation to reduce fire transmission risk. It references related standards for construction classification and fire grading to effectively mitigate fire hazards in tall buildings.

According to IS 1643, open spaces must remain unobstructed and open to the sky except for limited projections. Permitted projections include cornices, roofs, or weather shades up to 0.75 meters wide, and sunshades over windows or ventilators up to 0.75 meters wide. In congested or built-up zones, projections other than sunshades are restricted to a maximum of 0.23 meters below 4.3 meters height over roads or drains. Additionally, balconies or projecting rooms are allowed only on alternate floors with the projection not exceeding the height of the story immediately beneath, ensuring that lower floors still receive adequate light and air. Such constraints maintain proper ventilation, daylight, and fire safety while allowing minor architectural features.

IS 1643 connects Floor Area Ratio (FAR) with fire safety by regulating building bulk and height according to occupancy type and construction fire resistance. FAR limits differ by occupancy—such as residential, educational, and industrial—and by construction classification, reflecting fire risk levels. Buildings separated by fire-resisting walls with at least a 4-hour rating can be considered distinct for FAR and height calculations. Factors influencing FAR include minimum street width, fire-resistant construction, firefighting capabilities, as well as parking and sanitation requirements. Certain accessory spaces like basements and utility rooms permit a 20% increase in FAR. More restrictive or prohibited FAR values apply to hazardous or storage occupancies. Overall, FAR controls building mass to reduce fire load and enhance safe evacuation and emergency access.

IS 1643 stipulates that fire-resisting walls separating mixed occupancy buildings must have a minimum fire resistance rating of four hours. These walls must extend continuously from the foundation up to at least one meter above the roof level. Any openings within these walls must be protected by fire assemblies also rated for four hours. This arrangement permits treating each occupancy as an independent building for calculating permissible maximum height and floor area. When multiple occupancies exist in a building, the structure must conform to the most stringent requirements of the highest risk occupancy unless separated by such fire-resisting walls, in which case each can be assessed separately. This ensures effective compartmentation and limits fire spread between distinct occupancy zones.