What is the minimum required slope for a 100 mm horizontal drain pipe?

A gradient of 1 in 100 is recommended, although 1 in 120 may be permissible in specific cases (Table 3).

What is the minimum diameter for a soil pipe that serves a water closet (WC)?

The minimum diameter is 100 mm (Clause 6.1.1).

What is the purpose of a trap in a plumbing fixture?

A trap holds a water seal (minimum 50 mm) to prevent foul air and gases from the drainage system from entering the building (Clause 4).

How are drainage pipes sized according to this code?

Pipes are sized based on the total number of 'Fixture Units' (FU) they serve. Each sanitary appliance is assigned an FU value (Table 1), and the cumulative FU determines the required pipe diameter (Table 4).

IS 11729 Part 4

: 1993

Code of practice for plumbing services in buildings, Part 4: Drainage and sanitation

CurrentFrequently UsedCode of PracticeBIMMEP · Environmental and Sanitation

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This code provides guidelines for the design and installation of drainage and sanitation systems within buildings. It covers pipe sizing using fixture units, material selection, trap requirements to block sewer gases, and ventilation principles to prevent siphonage. The standard also outlines procedures for testing the integrity of installed drainage systems.

Provides guidelines for the design, installation, and testing of drainage and sanitation systems within buildings.

Quick Reference Values

Minimum water seal in traps50 mm

Minimum soil pipe diameter for WC100 mm

Minimum waste pipe diameter for wash basin32 mm

Minimum gradient for 100mm drain1 in 100

Minimum ventilating pipe diameter50 mm

Fixture unit value for a Water Closet (15L)6 FU

Practical Notes

While this standard is still in circulation, its provisions are largely incorporated and updated within the National Building Code (NBC) 2016, Part 9. It is critical to refer to the latest NBC for current projects.

Ensuring correct gradients during installation is crucial for the long-term performance of the drainage system and to prevent blockages. A 1 in 100 slope is a common benchmark for 100mm pipes.

Proper installation of anti-siphonage pipes is essential in multi-story buildings to protect the water seals of traps on lower floors.