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IS 11721:1995 is the Indian Standard (BIS) for plumbing. This code provides guidelines for the design, installation, and maintenance of plumbing systems in buildings. It covers water supply distribution, drainage systems (soil, waste, and vent pipes), and the testing of these systems to ensure functionality, hygiene, and safety.
Provides guidelines for general plumbing installations including water supply, drainage, and sanitation in buildings.
Overview
Status
Superseded — superseded by IS 17429:2020 (Part 1 & 2)
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! The concept of 'Fixture Units' (both for water supply and drainage) is central to sizing pipes; designers must carefully sum the units for all connected fixtures.
! Proper ventilation of the drainage system is critical to prevent trap siphonage and back pressure, which can allow foul sewer gases into the building.
! Always cross-reference with the latest National Building Code of India (Part 9 - Plumbing Services), which provides more comprehensive and often updated requirements.
Comprehensive code for the design, installation, and inspection of plumbing systems, fixtures, water supply, and drainage.
AS/NZS 3500.2:2021Standards Australia / Standards New Zealand, Australia/New Zealand
HighCurrent
Plumbing and drainage - Part 2: Sanitary plumbing and drainage
Specifies requirements for the design and installation of sanitary plumbing and drainage systems within buildings.
BS EN 12056-2:2000British Standards Institution (BSI) / European Committee for Standardization (CEN), UK/Europe
MediumCurrent
Gravity drainage systems inside buildings - Part 2: Sanitary pipework, layout and calculation
Focuses specifically on the design, calculation, and layout of sanitary gravity drainage pipework, a subset of a full plumbing code.
UPC 2021International Association of Plumbing and Mechanical Officials (IAPMO), USA
HighCurrent
Uniform Plumbing Code
A widely adopted model code covering all aspects of plumbing installation, water efficiency, and system resilience.
Key Differences
≠IS 11721:1995 is based on older British practice, primarily advocating for single stack or modified single stack systems. Modern international codes like the IPC mandate more complex, fully vented systems (individual, common, wet, and circuit venting) for multi-story buildings to better protect trap seals.
≠IS 11721:1995 does not explicitly address the use of Air Admittance Valves (AAVs). In contrast, the IPC and BS EN 12056 provide detailed requirements and limitations for the application of AAVs as an alternative to traditional atmospheric vents.
≠The method for sizing drainage pipes differs. The IPC uses Drainage Fixture Units (DFUs) with specific values per fixture type. BS EN 12056 uses Discharge Units (DUs). While IS 11721 also uses a fixture unit concept, the assigned values and resulting pipe sizes can differ significantly from modern international calculations.
≠Modern international codes have much more stringent and detailed requirements for backflow prevention, specifying the exact type of device (e.g., RPZ, DCVA, PVB) required based on the degree of hazard, which is less detailed in the 1995 Indian standard.
Key Similarities
≈All codes mandate the use of a water trap (e.g., P-trap) at every plumbing fixture to create a water seal that prevents sewer gases from entering the building. This is a fundamental principle of sanitary plumbing.
≈All standards require horizontal drainage pipes to be installed with a continuous slope or gradient to ensure self-scouring velocity, preventing the deposition of solids. The typical minimum gradients (e.g., 1% to 2%) are broadly similar.
≈The provision of access points (cleanouts, inspection chambers) for maintenance and clearing blockages in the drainage system is a common requirement across IS 11721 and its international counterparts.
≈All codes require mandatory testing of the completed drainage and water supply systems (e.g., water test, air test) to verify integrity and ensure they are free from leaks before being concealed or commissioned.
Parameter Comparison
Parameter
IS Value
International
Source
Minimum Trap Seal Depth
50 mm for most fixtures (75 mm for some larger fixtures like WCs)
51 mm (2 inches) minimum; 102 mm (4 inches) maximum
IPC 1002.4
Minimum Slope of Horizontal Drain (100mm / 4")
Typically 1:80 to 1:100 (1.25% to 1%)
1/8 inch per foot (approx. 1% or 1:96)
IPC Table 704.1
Cleanout Spacing (Horizontal Drains ≤ 100mm)
At every change of direction and at intervals not exceeding 15 m
Not more than 100 feet (approx. 30.5 m) apart
IPC 708.1.5
Minimum Size of Vent Pipe
50 mm, or not less than half the diameter of the drain pipe served
Not less than 1 1/4 inches (32 mm) and not less than one-half the diameter of the drain served
IPC 904.2
Drainage Fixture Unit (DFU) for a standard WC (>6L flush tank)
4
4
IPC Table 709.1
System Test Pressure (Drainage, Water Test)
Head of water equal to the height of the building or 3 m, whichever is more, for 30 minutes
A head of water not less than 10 feet (3 m) or a pressure equivalent to a 1-story height, for 15 minutes
IPC 712.2
⚠ Verify details from original standards before use
Key Values6
Quick Reference Values
Minimum trap seal depth50 mm
Minimum diameter for a soil pipe100 mm
Minimum slope for horizontal drainage pipes (4-inch and larger)1:100 or 1%
Minimum slope for horizontal drainage pipes (3-inch and smaller)1:50 or 2%
Water supply system test pressure1.5 times the working pressure
Minimum air gap for water supply outlets20 mm
Tables & Referenced Sections
Key Tables
Table 1 - Water Supply Fixture Units (WSFU)
Table 2 - Drainage Fixture Units (DFU)
Table 4 - Sizing of Stacks
Table 5 - Sizing of Building Drains and Building Sewers