This article summarises publicly-available content of NBC 2016. The full code (BIS publication) is the authoritative reference; verify specific values with your State Building Bye-Laws before relying on them in design submissions.

NBC 2016 Part 9 Explained — Plumbing Fixtures, Water Supply and Drainage by Occupancy

A building without adequate plumbing is a building that fails inspection and loses its occupancy certificate. National Building Code of India 2016, Part 9 — Plumbing Services is the reference document that tells architects, MEP consultants and site engineers how much water a building needs per day, how many WCs / urinals / washbasins are required per occupant, how drainage stacks are sized, how septic tanks are dimensioned, and how rainwater harvesting integrates with the broader plumbing layout. Part 9 is split into sub-sections — Section 1 (Water Supply), Section 2 (Drainage and Sanitation), and Section 3 (Solid-Waste Management) — but is commonly referenced as one document.

This article walks through the Part 9 provisions an MEP or architectural team uses on every project: water demand in LPCD by occupancy, fixture-count tables by occupancy and gender, drainage stack and vent sizing, septic tank design, and rainwater-harvesting mandates. The planning context for all of this — FAR, building height, parking — lives in NBC 2016 Part 3; Part 9 builds on it. Where clause numbers would require verbatim reproduction of NBC tables, descriptive language replaces citations.

1. What Part 9 covers (and what it doesn't)

Part 9 covers the plumbing services of a building — all water-carrying systems from source to disposal. Its scope includes:

• Water demand calculation in LPCD (litres per capita per day) by occupancy
• Source of water — municipal, ground, rainwater, recycled
• Underground and overhead storage tank sizing
• Pumping, pressure-reducing, and distribution design
• Fixture counts by occupancy (WC, urinal, washbasin, drinking fountain)
• Hot-water supply and circulation
• Drainage — soil, waste, vent stacks; horizontal run sizing
• Trap seals, cleanouts, interceptors
• On-site sewage treatment — septic tanks, soak pits, STPs
• Rainwater harvesting — collection, filtration, recharge or storage
• Greywater and recycled-water systems
• Cross-connection control
• Solid-waste storage and collection (Part 9 Section 3)

Part 9 does not cover the external sewerage network outside the building plot (that is a municipal responsibility), nor does it cover the design of municipal water-treatment plants (CPHEEO manuals are the reference there).

2. Water demand by occupancy — LPCD tables

The first number in any plumbing design is the design water demand per capita per day. NBC 2016 Part 9 Section 1 prescribes indicative values:

For detailed calculation with population forecasting and peak-factor, see Water Demand LPCD Calculator. Note these are source-side design values. Actual fixture flow rates at the tap are governed by the pressure head and aerator selection.

3. Storage tank sizing — underground and overhead

Storage is split between an underground (raw / municipal feed) tank and an overhead (gravity / distribution) tank. Typical approach:

• Underground (UG) tank: 1 day of total demand — sized as 1 × (daily demand). Some schemes go to 1.5–2 days for supply reliability.
• Overhead (OH) tank: 1/3 to 1/2 of daily demand — sized for peak-hour draw with the pump off.
• Separate tanks (or compartments) for domestic and flushing water where a dual-plumbing system is used — flushing fed from treated STP water or rainwater.
• Separate fire-water storage per NBC 2016 Part 4 — 2–3 hours of fire-demand, non-interchangeable with domestic draw except via a controlled make-up.
• Minimum two compartments in every tank so one side can be cleaned while the other is in service.

Indicative: a 200-flat high-rise (say 800 persons, 180 LPCD) has a daily demand of about 144 kL. UG tank around 150 kL, OH tank around 50–75 kL, plus a fire tank sized from the fire-pump demand curve.

4. Fixture counts by occupancy — the headline tables

Part 9 fixture counts are tabulated by occupancy and gender. Indicative counts:

Residential fixture count is straightforward — flats are self-contained. Staff toilets in commercial buildings are counted separately from customer toilets. At least one WC per occupancy type must be disabled-access (unisex is acceptable for small counts), with 1500 × 1750 mm internal dimensions, 900 mm door opening, grab bars and emergency call.

5. Drainage — soil, waste, vent stacks

Drainage is split between soil (WC + urinal), waste (basin + bath + kitchen sink) and vent (air-balance) stacks. A two-pipe system separates soil and waste; a single-stack system combines them and is now standard for most building types. Pipe sizing is by fixture units (FU) with each fixture rated for its peak flow contribution:

Working stack sizes:

A vent stack of one pipe-size smaller than the soil stack runs alongside and is connected to the main stack at every third or fourth floor to prevent trap-seal siphonage. Horizontal branches are kept at a minimum slope of 1:100 (100 mm pipe) to 1:150 (150 mm) with adequate cleanout access.

6. Trap seals and cleanouts

Every fixture discharges through a water-sealed trap — minimum seal depth 50 mm, typical 75 mm for floor traps. The seal prevents sewer gas from entering occupied spaces. Cleanouts are required:

• At the base of every soil / waste stack
• At every change of direction greater than 45° in horizontal runs
• At the start of long horizontal branches
• Every 15 m on long horizontal runs
• At each junction between a horizontal branch and the stack (where space allows)

Cleanouts are kept accessible — no walling-up behind permanent finishes. Floor traps in kitchens and bathrooms need a trap-primer or frequent-use guarantee to maintain their seal; dry traps are the single biggest source of drainage-odour complaints.

7. Septic tank sizing — on-site sewage disposal

For plots not connected to a municipal sewer, a septic tank plus soak pit or a packaged STP is the sewage solution. NBC 2016 Part 9 Section 2 prescribes septic tank sizing based on number of users and retention time:

Two-chamber design is standard for larger tanks — inlet chamber holds 2/3 of volume, outlet chamber 1/3, with a scum baffle and an inspection cover over each. Minimum liquid depth 1.0 m, free board 300 mm. Effluent is discharged into a soak pit sized from soil percolation test (typical 10–30 L per m² wall area per day).

For projects above a threshold size (commonly >100 persons or mandated by State Pollution Control Board), an on-site sewage treatment plant (STP) replaces the septic tank — MBR, SBR or MBBR technologies with treated effluent reused for flushing / landscaping.

8. Rainwater harvesting — mandatory for plots above threshold

Part 9 Section 1 and many State bye-laws mandate rainwater harvesting (RWH) for plots above a prescribed area (commonly >100 m² for residential, >200 m² for other, but strictly State-dependent). The system has two modes:

• Recharge mode — collected rainwater filtered through a pebble / sand / charcoal bed and led to a recharge pit or borewell to top up the local aquifer. Standard in Delhi, Bangalore, Hyderabad, Chennai.
• Storage mode — collected rainwater stored in a tank for non-potable reuse (flushing, landscaping, washing). More common where municipal supply is unreliable.

Sizing thumb rule: harvestable volume = roof area × annual rainfall × runoff coefficient (0.8 for RCC / metal roof, 0.6 for tiled, 0.3 for green roof). For a 500 m² terrace in Bangalore with 900 mm annual rainfall, harvestable volume is about 360 kL / year. Minimum first-flush diverter of 2–5 mm to discard the first runoff, then filtration, then recharge / storage.

9. Greywater recycling and dual plumbing

A dual-plumbing system delivers potable water to washbasins, showers and kitchens, and non-potable (treated or recycled) water to flushing cisterns, landscape irrigation and cooling-tower make-up. This pushes the treated-STP output to productive reuse and reduces fresh-water draw by 30–40% in a typical office / residential building.

Key design precautions:

• Non-potable pipework coded distinctly (purple / magenta wrapping is the global convention).
• Separate headers, separate pumps, separate tanks — never tapped from a potable main.
• Signage at every outlet: “Non-potable water — not for drinking.”
• Cross-connection control — air gaps or backflow preventers on every potable-feed point that could be contaminated.
• Periodic microbiological testing of the non-potable loop — quarterly or as required by State Pollution Control Board.

10. Cross-connection control — the safety-critical item

A cross-connection is any arrangement where non-potable water could flow back into the potable supply — a hose submerged in a tank, a sprinkler system with no backflow preventer, a boiler make-up line without an air gap. Part 9 prescribes:

• Air gap of at least 2 × pipe diameter (minimum 25 mm) at every fill point to a tank or fixture.
• Reduced-pressure zone (RPZ) backflow preventer on fire-system feeds, irrigation feeds, industrial process feeds, and any feed into a non-potable loop.
• Double-check valves on medium-hazard feeds.
• Vacuum breakers on hose-bibs and flush-valve fixtures.

Annual inspection and test-report filing is mandated by many water-supply utilities as a condition of the water connection.

11. Differences between NBC Part 9 and international plumbing codes

12. State-level variations — rainwater harvesting in particular

• Karnataka: RWH mandatory for all plots ≥2,400 ft² (~225 m²) in Bangalore; penalties for non-compliance tied to water-board connection.
• Tamil Nadu: RWH mandatory statewide for all buildings since 2003; enforced strictly in Chennai Metropolitan Area.
• Delhi: RWH mandatory for plots >100 m² per DDA and MCD rules; recharge configuration preferred.
• Maharashtra: RWH mandatory for plots above 300 m² in municipal areas.
• Telangana / Andhra: RWH mandatory for all plots; tied to building-permit sanction.
• STP thresholds: CPCB and State PCBs prescribe mandatory on-site STP for residential projects above 20 kLD / 100 units / 5,000 m² built-up — thresholds vary by State.

Always verify the State rule for the binding threshold on RWH and STP.

13. What changed from earlier editions

• Stronger treatment of rainwater harvesting and greywater recycling with sizing guidance.
• Dual-plumbing recognised and encouraged for new construction.
• Aligned hot-water and solar water-heating provisions with ECBC.
• Updated water-demand LPCD values for hospitals and five-star hotels.
• Sharper cross-connection control requirements (RPZ, vacuum breakers).
• Expanded solid-waste storage section (Part 9 Section 3) aligned with SWM Rules 2016.
• Post-2016 amendments have pulled in updated uPVC / CPVC / SWR pipe specifications.

14. Cross-references — IS codes and other NBC parts

• IS 1172 — code of basic requirements for water supply, drainage and sanitation
• IS 2064 — selection, installation and maintenance of sanitary appliances
• IS 2065 — code of practice for water supply in buildings
• IS 1742 — code of practice for building drainage
• IS 2470 — design and construction of septic tanks
• IS 774 — flushing cisterns
• IS 2556 — sanitary ware specifications
• IS 5329 — sanitary pipe work above and below ground (cast iron)
• IS 13592 / IS 14735 — uPVC / CPVC pipe specifications
• NBC 2016 Part 3 — planning context (FAR, occupancy, habitable-room sizes drive fixture counts)
• NBC 2016 Part 4 — fire-water storage and fire-pump discharge cross-references
• NBC 2016 Part 11 — approach to sustainability (ECBC linkage, water efficiency)

15. Practical compliance tips

• Use the LPCD calculator early. Water Demand Calculator takes occupancy and population and gives the daily demand. Build UG + OH tank sizes from there.
• Two compartments in every tank. Single-compartment UG tanks leave the building dry when you clean them.
• Separate flushing main from domestic. Day-one dual plumbing is cheap; retrofitting later is expensive.
• Fixture count includes staff. Don't forget cleaning / security / kitchen staff when sizing office or hotel toilets.
• Disabled-access WC on every floor. At least one unisex accessible toilet per public-occupancy floor.
• Vent stack one size smaller than soil. A common site shortcut is to use the same size — it looks tidy on drawing but over-spends on pipe.
• Cleanouts at every directional change. Easy to miss in tight service-shaft layouts; retrofitting a cleanout after commissioning is a nightmare.
• First-flush diverter on every downtake. Rainwater harvesting without a first-flush diverter contaminates the storage with roof dust within a single rain.
• Treated STP effluent has a BOD / COD target. Typical CPCB discharge norms: BOD ≤ 10 mg/L, COD ≤ 50 mg/L for reuse; verify local PCB limits.
• RPZ backflow preventer on the fire-system feed. Non-negotiable — fire water stagnates, and its backflow into the domestic main is the single largest cross-contamination risk.

16. FAQ — NBC 2016 Part 9

What's the design water demand for a residential flat?

Per NBC 2016 Part 9, 135 LPCD for domestic consumption + 45 LPCD for flushing = 180 LPCD total for a standard-quality residential flat. Economy residential uses 135 LPCD combined. Peak-hour demand factor of 2.0–2.5 is applied for pipe sizing.

How many WCs are required in an office building?

One WC per 25 males and one per 15 females (fewer females than males is the code assumption; actual count should meet the higher-gender staff headcount). Plus one urinal per 25 males and one washbasin per 25 persons. Add one disabled-access unisex WC per floor.

Is rainwater harvesting mandatory under NBC?

NBC Part 9 recommends RWH for all new plots above a prescribed area. The legal mandate comes from State bye-laws — Karnataka, Tamil Nadu, Delhi, Maharashtra and others have made it mandatory with triggers in the 100–300 m² range. Verify against the local rule.

What's the minimum septic tank size for a 10-user household?

Approximately 1,400 L net working capacity, typically 2.0 × 0.9 × 1.0 m (length × width × liquid depth), plus 300 mm free board. Two-chamber design is preferred even for small tanks to improve settling.

What's the difference between a septic tank and an STP?

A septic tank is a passive two-chamber settler with anaerobic digestion — it reduces BOD by 30–50% but produces an effluent still unfit for direct reuse, typically led to a soak pit. An STP is an active biological treatment system (MBBR, SBR, MBR) that produces reusable effluent for flushing and landscaping. STPs are mandatory for projects above the CPCB / State-PCB threshold.

How is a drainage stack sized?

By summing Discharge Units (DU) for all fixtures on the stack and selecting a pipe size from the NBC Part 9 Section 2 table. As a working reference: 100 mm stack serves most residential blocks (~240 DU); 150 mm is used for high-rise commercial and mall soil stacks (~960 DU); 200 mm for very tall blocks. A vent stack one nominal size smaller runs alongside.