Code of practice for building drainage
1983 Edition

IS 1742 defines methods to estimate peak domestic sewage flows and pipe sizing based on fixture unit ratings. Key parameters include:

• Fixture unit assignments for sanitary appliances (Clause 4.5.1, Table 1), such as tank water closets rated at 6 units and flush-valve water closets at 8 units.

• Fixture unit allocations relative to drain or trap diameters (Clause 4.5.1.1, Table 2), e.g., 30 mm drains equate to 1 unit, while 100 mm drains correspond to 6 units.

• Maximum fixture units permissible for pipe diameters on branches and stacks (Clause 5.1.4, Table 3), for instance, 50 mm pipes can accommodate 6 units on horizontal branches.

• Limits on fixture units for building drains and sewers adjusted by pipe diameter and slope gradient (Clause 4.5.1.4, Table 4), such as a 150 mm pipe at a 1/100 slope supporting 700 units.

These tables establish the foundation for designing domestic sewage drainage per IS 1742.

Clause 2.0 of IS 1742 provides essential definitions to ensure uniform understanding of terms used throughout the standard. Important fixture unit values and sewage flow estimations are detailed in:

• Table 1 (Clause 4.5.1): Assigns fixture units to various sanitary devices, such as 6 units for tank water closets, 8 for flush-valve water closets, and 2 for domestic kitchen sinks.

• Table 2 (Clause 4.5.1.1): Provides fixture unit values based on fixture drain or trap diameter.

• Table 3 (Clause 4.5.1.4): Specifies maximum fixture units allowed on branches and stacks depending on pipe size.

• Table 4 (Clause 4.5.1.4): Defines maximum fixture units for building drains and sewers according to pipe diameter and gradient.

These tables are fundamental for accurate design and flow calculations.

According to Clause 3.1, all materials and fittings utilized in drainage systems must comply with the most recent applicable Indian Standards or, if unavailable, be of the highest quality. The standard includes:

• Fixture unit values for various sanitary appliances (Clause 4.5.1, Table 1).

• Fixture unit ratings based on drain or trap dimensions (Clause 4.5.1.1, Table 2).

• Maximum permissible fixture units connected to branches and stacks by pipe diameter (Clause 4.5.1.4, Table 3).

• Fixture unit limits for building drains and sewers considering gradients (Clause 4.5.1.4, Table 4).

These tables guide the selection of pipe sizes, materials, and fittings to ensure compliance and functionality.

Clause 4.2 emphasizes the significance of comprehensive preliminary information collection prior to designing drainage systems. This includes site layouts, building plans, usage descriptions, availability of municipal sewer connections, adherence to local bylaws, sewage characteristics, cover depths, soil conditions, location of other utilities, surface reinstatement plans, traffic management, wayleave permissions, and assessment of potential damages.

Key fixture unit tables (Clauses 4.5.1 and 4.5.1.4) assist in estimating sewage flows and determining pipe sizing.

Design must accommodate subsoil characteristics, traffic loading, and regulatory requirements to ensure efficient and durable drainage installations.

Installation practices in IS 1742 include:

• Use of materials and fittings conforming to latest Indian Standards or best available quality (Clause 3.1).

• Restrictions on maximum fixture units per pipe diameter on horizontal branches and stacks (Clause 4.5.1.4, Table 3).

• Limits on fixture units for building drains and sewers as a function of pipe size and slope (Clause 4.5.1.4, Table 4).

• Sizing criteria for rainwater pipes based on roof areas and rainfall intensities (Clause 4.10.3.9, Table 5).

• Proper discharge of stormwater into well-constructed open drains with masonry at changes in alignment or gradient (Clause 4.10.3.9).

These guidelines ensure functional and code-compliant drainage installations.

Clause 4.10.3.8 mandates sizing rainwater pipes for roof drainage based on roof area and rainfall intensity as outlined in Table 5. Recommended spacing between pipes is approximately 6 meters, considering architectural features such as windows and arches. A bell mouth inlet at the roof surface is advised to enhance drainage efficiency if adequate slopes are provided.

The strainer surface area should be between one and two times the cross-sectional area of the pipe.

Clause 6.2 requires stormwater to be conveyed through open drains constructed with durable masonry at points where direction or gradient changes.

These requirements promote effective rainwater management on buildings.

IS 1742 details backfilling and pipe protection measures:

• Bedding (Clause 5.9.1) must be rectangular, extending at least 150 mm beyond the pipe's lateral edges.

• Concrete bedding thickness below pipes should be at least 100 mm for pipes under 150 mm diameter, and 150 mm for larger diameters.

• The concrete cradle must support the pipe up to the invert level, preventing direct line contact.

• Backfill operations (Clauses 5.14.3 and 5.14.4) require hand-packing of fine selected material around the pipe's lower half, followed by careful watering and compaction up to 150 mm above pipe crown, ensuring even pressure and avoiding tipping during initial backfill.

• Haunching involves extending concrete bedding up to the horizontal diameter of the pipe with sloped sides meeting the pipe barrel tangentially.

These procedures safeguard pipe integrity during and after installation.

The standard requires all materials and fixtures to meet latest Indian Standards and undergo manufacturer inspection prior to dispatch (Clause 3.1). Testing results must be rounded according to IS 2-1960, preserving the specified number of significant digits (Clause 0.4).

Water testing (Clause 8.2.3) is emphasized to verify system integrity.

For French drain construction (Clause 7.1.2), excavation and graded filling materials must be used to ensure effective drainage.

These provisions establish rigorous quality assurance and compliance verification processes.

Clause 9.2.2 outlines maintenance steps:

• Remove inspection chamber and manhole covers; clean side benchings and channels.

• Flush and clean intercepting traps with clean water, ensuring proper replacement of rodding arm stoppers.

• Access gully traps by removing covers and plates; plunge and flush traps without pushing deposits downstream.

• Repair identified defects during inspection or testing.

• Replace covers, bedding them with appropriate grease or materials.

• Paint ladders, rings, and external manhole covers with approved coatings.

Clause 9.4 requires periodic examination of subsoil drains for blockages such as root intrusion.

Fixture unit values (Clause 4.5.1) assist in maintaining adequate drainage capacity.

Safety protocols for sewer maintenance workers (Clause 9.2.1) include:

• Prohibition of smoking or open flames; precautions against sparks.

• Erection of warning signs and use of gas-proof electric lighting only.

• Atmospheric testing for toxic gases and oxygen deficiency before entry.

• Entry with safety belts and at least two personnel; repeated gas testing for prolonged work.

• Use of breathing apparatus, non-sparking tools, and protective footwear when ventilation is inadequate.

• Careful inspection of ladders and presence of emergency lifting personnel.

• Use of weatherproof, flameproof portable air blowers positioned on the leeward side at minimum 2 meters from openings; allowance for diesel engines without sparking plugs.

Additionally, all materials and fittings must meet relevant standards or be of superior quality (Clause 3.1).

These guidelines ensure worker safety and regulatory compliance.

Clause 4.10.5 describes multiple surface water disposal methods, prioritizing the approach in Clause 4.10.5.1. Disposal strategies must consider local site characteristics including flood elevations and requirements set by authorities (Clause 4.2.4). Natural watercourses and soakaways are acceptable if permitted.

Other sewage disposal methods must comply with local water supply authority regulations to prevent contamination, especially when using subsoil dispersion (Clauses 4.2.3 and 4.2.3.3).

While specific formulas and tables are not included here, adherence to effluent tolerance limits and appropriate sampling/testing protocols is stressed.

Consult the relevant sub-clauses for detailed disposal design guidance.

Clauses 7.1 and 4.11 focus on subsoil drainage, aiming to remove groundwater to protect structures from moisture damage.

• Subsoil drains (Clause 7.1) collect and divert soil water beneath buildings.

• Drainage principles (Clause 4.11) emphasize prevention of hydrostatic pressure buildup.

• Typical designs involve perforated pipes embedded in gravel trenches (Clause 4.11.4).

• Drainage systems are broadly defined in Clause 2.41, encompassing subsoil drainage.

Design considerations include minimum pipe diameters (commonly ≥100 mm), gravel surrounds to facilitate flow, and gradients between 1:100 and 1:200.

Refer to the specified clauses for comprehensive design details.

Key design and construction requirements include:

• Minimum dimensions: circular chambers require at least 1.4 m diameter; rectangular chambers must be at least 1.2 m by 0.9 m internally (Clause 1.4), aligned with standard brick sizes to minimize material waste.

• Chamber sizes should accommodate inspection and clearance needs and be enlarged if multiple pipe entries exist (Clause 4.9.2).

• Manholes or inspection chambers must be installed at every change in pipe direction, slope, or diameter. Junctions and bends should be grouped into manholes where feasible (Clause 4.9).

• For manholes deeper than 1.5 m, detailed sectional and drop manhole designs are provided (Clause 1.5, Figures 4 and 5).

These ensure ease of maintenance, structural soundness, and accessibility.

IS 1742 provides comprehensive data for estimating sewage flow:

• Fixture unit values assigned by fixture type (Clause 4.5.1, Table 1) range from 1 unit for drinking fountains to 8 units for flush-valve water closets.

• Fixture unit values based on drain or trap sizes (Clause 4.5.1.1, Table 2) range from 1 unit for 30 mm drains to 6 units for 100 mm drains.

• Converting fixture units to peak flow rates (litres/min) uses graphical curves (Clause 4.5.1.3), differentiating between flush valve and flush tank dominated systems.

• Maximum fixture units permissible per pipe diameter for horizontal branches and stacks (Clause 4.5.1.4, Table 3) and for building drains and sewers at various gradients (Clause 4.5.1.4, Table 4) guide pipe sizing.

These tables and methods enable accurate sizing of drainage systems to handle anticipated sewage loads.

The standard specifies corrosion protection measures:

• Cast iron pipes are vulnerable to external corrosion in soils containing peat or significant sulphates; local knowledge should inform their application and protective measures (Clause 4.7.3.1).

• Protective coatings on cast iron pipes must be inspected; minor damages are repaired with hot tar or bitumen, while pipes with extensive damage require recoating before installation (Clause 8.1.3).

• Joints must be water-tight, achieved by riveting, bolting, or soldering, with at least 50 mm overlap on gutters to prevent ingress (Clause 6.1.2).

• Rainwater pipe sizing (Clause 4.10.3.9) also contributes to corrosion prevention by ensuring adequate drainage and avoiding water stagnation.

These measures enhance longevity and reliability of drainage components.