Drainage System Design for Indian Buildings — IS 1742 Guide

Plumbing & Drainage

Drainage System Design — IS 1742 Guide for Indian Buildings

The drainage system is one of the least glamorous but most consequential parts of MEP design. Poor drainage causes water logging, foul odours, pest infestation, and in severe cases, structural damage. IS 1742 (Code of Practice for Building Drainage) is the primary Indian standard, and this guide covers the key design requirements for above-ground and below-ground drainage systems.

1. IS 1742 — Drainage System Overview

IS 1742 distinguishes between soil systems (carrying faecal matter from WCs) and waste systems (carrying waste from sinks, basins, baths) — or combined systems that carry both. Indian buildings typically use a single-stack or separate-stack system depending on building height and type.

System Type	Description	Application	IS 1742 Ref
Single-stack	One stack carries both soil and waste — no separate vent	Low-rise residential (up to 5 floors)	Cl. 4.3
Separate stack	Separate soil and waste stacks — each vented	Mid to high-rise, hotels, hospitals	Cl. 4.4
Ventilated system	All branches vented — maximum protection against trap seal loss	High-rise, critical facilities	Cl. 4.5
MUPVC (soil/waste)	Most common material in India	All building types	IS 4985

2. Pipe Sizing — Discharge Unit Method

Similar to water supply, IS 1742 uses discharge units (DU) per appliance to determine simultaneous discharge load on each pipe section.

Appliance	Discharge Units (DU)	Min Trap Size	Min Branch Pipe
WC (S-trap)	8	75mm deep seal	100mm
Washbasin	1	32mm	32mm
Bath	3	40mm	40mm
Kitchen sink	3	40mm	40mm
Shower tray	2	40mm	40mm
Floor trap (bathroom)	2	75mm	40mm
Dishwasher	2	—	40mm
Washing machine	3	—	40mm
Urinal (stall)	0.3	—	40mm

Stack Sizing by Total DU

Total DUs on Stack	Stack Diameter	Max Floors (Single Stack)
Up to 10	75mm	2
11–50	100mm	5
51–200	150mm	10
201–500	200mm	20
>500	250mm	30+

3. Gradient Requirements

Pipe Dia	Min Gradient	Max Gradient	Notes
32mm (basin waste)	1:60 (17mm/m)	1:12	Steep ok — self-cleaning
40mm (bath/sink waste)	1:80 (12mm/m)	1:18	Standard
50mm	1:80	1:20
100mm (soil branch)	1:80	1:18	Minimum for self-cleansing velocity
150mm (main drain)	1:150 (7mm/m)	1:40	Under-slab or external
225mm (underground drain)	1:225 (4mm/m)	1:80	External below ground

4. Vent Pipe Design

Primary vent: extend stack 1m above roof level — open to atmosphere or use air admittance valve (AAV) where permitted
Re-vent (anti-siphon): connect from downstream of trap to vent stack — prevents trap seal loss under discharge
Branch vent: for long branches (>1.5m to soil stack) on WCs — prevents compression
All vents: minimum 50mm diameter — 100mm where vent serves WC branches
Vent termination: minimum 3m from any opening into the building (window, air intake)

5. Below-Ground Drainage — Inspection Chambers

Inspection chambers at every change of direction, every junction, and at maximum 12m intervals on straight runs
Chamber depth < 600mm: half-round channel in concrete base
Chamber depth 600mm–1.2m: benched chamber — 450mm × 450mm clear opening
Chamber depth > 1.2m: manhole — 600mm × 600mm minimum clear opening
Cover: ductile iron or RCC — rated for traffic load if in vehicle zone
Invert levels: all inlets must be above the outlet invert — step to maintain gradient

6. Common Drainage Failures in Indian Buildings

No or inadequate slope on branch pipes — slow drain, blockage, backfall
Missing or dry trap seals — foul smell in occupied spaces
Incorrect AAV placement — AAV not accessible, or used where not permitted (below ground)
WC branch too long before joining stack — siphonage of trap seal
External drainage too flat — pooling in inspection chambers, mosquito breeding
No cleanout access — first blockage requires breaking floor or ceiling to clear