IS 1742:1972 Part 1 is the Indian Standard (BIS) for drainage (outside buildings) - part 1: surface water drains. This code lays down the design, layout, construction, and maintenance guidelines for surface water drains and sewage systems outside buildings. It details the requirements for pipe sizing, gradients, manholes, and the hydraulic principles needed to ensure self-cleansing flow.
Provides guidance on the design and construction of surface water drainage systems outside buildings.
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
IS 1742 (Part 1) is the code of practice for drainage of buildings — surface water drainage outside buildings. It covers the design and construction of storm-water drainage systems on plot-level + building-perimeter — gutters, rainwater downpipes, rainwater drains, soakaways, and connection to municipal storm-sewer.
Use IS 1742 Part 1 when designing: - Roof drainage system (gutters + downpipes for new buildings) - Plot-level surface drainage (open channels, gully traps, catch pits) - Soakaway / rainwater harvesting pit - Connection of plot drainage to municipal storm sewer - Renovation / replacement of existing roof drainage - Industrial premises with large impervious area (warehouses, parking) - Institutional campuses (schools, hospitals)
IS 1742 Part 1 covers SURFACE water (rainwater) drainage. Companion codes: - IS 1742 Part 2 — drainage works inside buildings (sub-surface, sanitary) - IS 2470 Part 1:1985 — septic tank design (sanitary disposal) - IS 1172:1993 — code of basic requirements for water supply, drainage, sanitation - IS 4985:2021 — uPVC water supply pipes (some grades for drainage) - IS 13592:2013 — uPVC pipes for soil and waste discharge (DWV — sewer connection) - IS 458:2003 — RCC pipes (large-diameter drains, culverts) - CPHEEO Manual on Storm Water — broader urban drainage framework
Roof drainage: - Gutter — collects rainwater at roof edge - Material: GI, copper, PVC, aluminium, RCC integrated - Cross-section: half-round (most common), rectangular, custom architectural - Slope: 1:100 to 1:200 for water flow - Downpipe (rainwater pipe / RWP) — vertical pipe carrying water from gutter to ground - Material: cast iron, GI, PVC, copper - Diameter: 75-200 mm depending on roof area + rainfall intensity - Outlet at roof — connecting gutter to downpipe; with grate / strainer - Rainwater shoe (at base) — directs water away from building wall; 90° elbow + flow guard
Surface drainage on plot: - Open channel drain (kuccha or RCC) — collects surface runoff from paved areas, green areas - Cross-section: V-shape, U-shape, trapezoidal, rectangular - Cover: kerb / metal grate / PCC slab cover - Gully trap — collects surface runoff at low point; trap prevents back-flow of sewer gases - Catch pit — collects sediment + debris; periodic cleaning - Manhole — access for inspection + maintenance of sub-surface drains
Sub-surface drainage: - Drain pipe — collects + conveys water to disposal point - Material: PVC (small), RCC (IS 458:2003) for large, salt-glazed stoneware (legacy) - Diameter: 100-200 mm (residential plot); 300-600 mm (campus) - Slope: 1:60 to 1:100 (gravity flow) - Inspection chamber — at every junction + at every 30-50 m along run
Disposal: - Connection to municipal storm sewer (preferred where available) - Soakaway pit — pit filled with rubble; rainwater seeps into ground - Sized to handle peak flow - Pit volume: 1-3 m³ for residential plot; larger for commercial - Rainwater harvesting tank — store rainwater for reuse (toilet flushing, garden, recharge) - Discharge to street drain (where municipal sewer not available)
Roof area + downpipe sizing (per IS 1742 Part 1):
| Roof area drained (m²) | Min downpipe diameter (mm) | |---|---| | Up to 10 | 50 | | 10-50 | 75 | | 50-100 | 100 | | 100-200 | 125 | | 200-500 | 150 | | 500-1000 | 200 |
Based on typical Indian rainfall intensity (50-150 mm/h short-duration design storm).
Spacing of downpipes: - Maximum 12 m between downpipes for typical roof - Closer spacing for heavy-rainfall areas or large roofs
Gutter cross-section (semi-circular GI):
| Roof area (m²) | Gutter dia (mm) | |---|---| | Up to 50 | 100 | | 50-100 | 125 | | 100-200 | 150 | | 200-400 | 200 |
Rainfall intensity (design storm — varies by region): - Heavy-rainfall (Mumbai, Kolkata, Chennai): 100-150 mm/h - Medium (Delhi, Bangalore, Hyderabad): 50-75 mm/h - Low (Rajasthan, Gujarat dry zones): 30-50 mm/h - Use IMD rainfall data + return period (typically 25-year for residential, 50-year for commercial)
Surface drain sizing:
| Catchment area (m²) | Drain cross-section (mm) | |---|---| | Up to 100 | 150 × 150 (V or rectangular) | | 100-300 | 200 × 200 | | 300-1000 | 300 × 300 | | 1000-5000 | 400 × 400 | | > 5000 | per hydraulic design (Manning's equation) |
Slope of surface drain: - Minimum: 1:100 (for self-cleaning) - Maximum: 1:30 (steeper causes erosion in unlined drains) - Typical: 1:80 to 1:100
Soakaway sizing: - Volume = (peak rainfall × roof area) / soil infiltration rate - Soil infiltration rate (per IS 2470 Part 2 percolation test): - Sandy: > 50 mm/hr (good) - Loamy: 25-50 mm/hr (moderate) - Clay: < 25 mm/hr (poor; soakaway not recommended) - Typical residential soakaway: 1-3 m³ (for ~150 m² roof, 50 mm/hr infiltration)
Slopes and gradients: - Surface drain longitudinal: 1:100 minimum - Sub-surface pipe drain: 1:60 to 1:100 - Roof gutter: 1:200 minimum - Downpipe: vertical (with foot bend at base)
Setback from buildings: - Soakaway: ≥ 5 m from building foundation (avoid foundation moisture) - Drains: along plot boundary or paved area; avoid passing under building
1. Undersized downpipe. Roof overflows during heavy rain; water enters building, damages walls. Size per roof area + local rainfall intensity (use IMD data). 2. Inadequate slope on gutter / drain. Water doesn't flow; pools, breeds mosquitoes, leaks. Minimum 1:100 to 1:200 for self-clearing. 3. No periodic cleaning. Leaves, debris block downpipes, gully traps; system fails when needed. Mandatory pre-monsoon cleaning. 4. Direct discharge of roof water onto plinth. Erodes plinth, dampens building wall, damages foundation. Provide rainwater shoe + carry water away. 5. Soakaway too close to foundation. Foundation moisture rise; basement walls dampen. Maintain ≥ 5 m setback. 6. No connection to municipal storm sewer where available. Discharging to street drain causes street flooding; municipal violation. Connect properly. 7. PVC pipe outdoors without UV protection. Brittle within 1-2 years; cracks. Specify UV-stabilised grade or paint over. 8. Inadequate manhole spacing. Cleaning + inspection becomes impossible; system blocks. Manholes at every 30-50 m + every junction. 9. No provision for rainwater harvesting. Statutory requirement in many states (Maharashtra, Tamil Nadu, Karnataka, Kerala) — penalty + occupancy certificate refused without RWH. 10. Wrong material for application. PVC drain pipe adequate for residential; not for high-load commercial / heavy traffic where RCC (IS 458) is needed. 11. No drainage for paved area / parking. Water pools, ice in winter (north India), unsafe. Plan surface drainage for all impervious areas. 12. Overflow from one roof onto another. Water cascades, damages second roof, increases load. Each roof must drain independently. 13. Drainage discharging onto neighbour's plot. Civil dispute; municipal violation; mandatory connection to street sewer or own soakaway.
Storm-water drainage design cascade for a building / plot:
1. Site analysis: - Plot area + roof area - Existing topography (high / low points) - Existing municipal sewer / storm drain availability - Soil percolation test (per IS 2470 Part 2) for soakaway sizing - Water table depth 2. Rainfall data: - Local IMD records - Design storm intensity (mm/hr) for return period (typically 25-year residential, 50-year commercial) 3. Roof drainage design (this code, IS 1742 Part 1): - Calculate peak runoff per roof segment - Size gutters + downpipes per roof area - Layout downpipe locations 4. Surface drainage on plot: - Layout open channels / drains for paved areas, green areas, runoff from neighbours - Size per catchment area 5. Sub-surface drainage: - Pipe drains (PVC small, RCC large) - Slope, manhole spacing, inspection chambers 6. Disposal strategy: - Connection to municipal storm sewer (if available) - Soakaway pit (with sufficient capacity) - Rainwater harvesting tank (statutory in many states) - Combined approach (RWH for a portion, soakaway / sewer for excess) 7. Anti-mosquito features: - Mosquito-proof gully traps - Mesh on inlet / overflow openings - Avoid stagnant pools 8. Construction: - Per detailed drawings - Slope verification by spirit level / theodolite - Pressure / flow test on completion 9. Operations + maintenance: - Pre-monsoon cleaning of gutters, drains, manholes - Periodic flushing of sub-surface drains - Inspection of soakaway pit (annual) - Repair of any cracks / leaks
IS 1742 Part 1 is the foundational code; modern integrated urban stormwater management adds: street-level surface treatment (permeable pavement, rain gardens), centralised stormwater treatment (SUDS — Sustainable Urban Drainage Systems), and integration with city-wide storm-water master plans.
| Parameter | IS Value | International | Source |
|---|---|---|---|
| Minimum Self-Cleansing Velocity (Pipes) | 0.6 m/s (for drains up to 300 mm diameter) | 0.75 m/s (for storm sewers at design flow) | BS EN 752 |
| Minimum Gradient Approach | Prescribes minimum gradients based on pipe size (e.g., 1 in 120 for 150mm pipe). | Performance-based: Gradient is calculated to achieve the required self-cleansing velocity. | ASCE MOP 77 / BS EN 752 |
| Minimum Pipe Diameter (Main Line) | 150 mm | 200 mm (8 inches) often recommended for public storm sewers to minimize blockages. | ASCE MOP 77 |
| Manning's 'n' (Smooth Concrete Pipe) | 0.015 | 0.013 (for modern precast concrete) | ASCE MOP 77 |
| Maximum Manhole Spacing (e.g., 200-300mm pipe) | 30 metres | Up to 90-120 metres, depending on access for modern cleaning equipment (jetting/CCTV). | BS EN 752 |
| Design Storm Return Period (Urban) | Not explicitly defined, often taken as 2-5 years in practice. | Typically 1-in-20 to 1-in-30 years for no surcharge, with flood routing checks for 1-in-100 years. | BS EN 752 |