Unit IInterior Services I — Plumbing & Water Supply

Water Supply in Buildings

Potability, demand, storage and the chain from main to tap

50 min readByAmogh N P· Architect & interior designer

Water is the only service a building cannot fake. The chain from the municipal main to a tap is shorter than students expect and more code-governed than site practice suggests — and the two disagree openly about the most basic question of all, which tank should be bigger. This unit walks that chain: what makes water potable, how much a building actually needs, how it is stored, and what heats it. Along the way it flags four beliefs that dissolve on contact with the code — including one about pipe diameter that BIS itself calls deprecated.

Learning objectives

By the end of this lesson, you will be able to — mapped to the course outcomes for Interior Services I:

CO1 · Understand

State what makes water potable under IS 10500:2012 — and why the standard sets no minimum for TDS.

CO1 · Understand

Read IS 1172's demand rates correctly, including the flushing allowance that is carved out, not added.

CO2 · Apply

Size an overhead tank and sump for a real building, and apply the code cross-checks that bite.

CO2 · Apply

Calculate rainwater harvesting potential and identify the constraint that actually binds.

CO3 · Evaluate

Judge RO, harvesting and solar claims against what they deliver in India, not what they promise.

CO3 · Analyse

Explain why pressure is a function of head, not pipe diameter — and cite the code that deprecates the opposite belief.

IS 10500:2012

What makes water potable

Two columns, a handful of no-relaxation parameters — and one consequential absence that decides the whole RO argument.^{[1, 12]}

What IS 10500:2012 actually requires

The standard gives two columns: an acceptable limit, and a permissible limit in the absence of an alternate source. pH 6.5–8.5 with no relaxation. Turbidity 1 NTU acceptable, 5 permissible. Total dissolved solids 500 and 2000. Total hardness as calcium carbonate 200 and 600. Chloride 250 and 1000. Fluoride 1.0 and 1.5. Iron 0.3 with no relaxation. Nitrate 45 with no relaxation. Arsenic 0.01 and 0.05. Lead 0.01 with no relaxation. And for E. coli the wording is absolute: it shall not be detectable in any 100 ml sample. Learn the shape of the table rather than every row — the pattern is that the parameters which cause irreversible harm get no relaxation at all.^[1]

IS 1172 · IS 2065

Demand, and the split everyone misreads

The flushing allowance is carved out of the total, never added to it. Get this wrong and every number downstream is wrong.^{[2, 3]}

DiagramBar diagram showing 155 litres of domestic use plus 45 litres of flushing making 200 litres in total, beside the crossed-out myth of 135 plus 45 making 180

DiagramThe Indian water supply chain from municipal main to underground sump, pumped up to an overhead tank and gravity-fed down to taps, with the head marked

IS 1172:1993 — how much water a building needs

For residences the band is 150 to 200 litres per head per day for communities over 100,000, 100 to 150 for towns of 20,000 to 100,000, and 135 for LIG and EWS housing. Non-residential rates run far lower: offices 45 litres per head per day, day schools 45, hostels and boarding schools 135, hotels 180 per bed, restaurants 70 per seat, cinemas 15 per seat. Hospitals are the outlier at 340 litres per bed for up to 100 beds and 450 per bed above that — note that larger hospitals demand MORE per bed, not less, because theatres, laundry and sterilising scale faster than beds do.^[2]

Try the tank & sump sizer

Size it yourself

Set an occupancy and watch the chain resolve. Then look at the two storage panels side by side: the code makes the sump half the overhead tank, and site practice makes it double. Both are defensible. Only one is a code rule — and knowing which is which is the difference between quoting IS 2065 and misquoting it.

Tank & sump sizer · what the code asks, and what site does

Occupancy typeNumber

IS 2065 cl. 11.11.1 assures a minimum of 200 lphd, of which about 45 is flushing and the remaining 155 is other domestic use.

Daily demand

16,000 litres/day

12,400 domestic

3,600 flush

The flushing water is a SLICE of the total, not an addition to it. 200 = 155 + 45.

Code minimum

Overhead tank: 8,000 L
Sump: 4,000 L

The sump is half the OHT. IS 2065 sets a ratio between the tanks — not against daily demand.

Common Indian practice

Overhead tank: 8,000 L
Sump: 16,000 L

The sump is twice the OHT — the code’s ratio inverted. Defensible against unreliable supply, but not a code rule.

Code cross-checks that bite

▲Over 5,000 L — IS 2065 cl. 11.7 wants two interconnected compartments, so one can be cleaned while the other supplies.
▲That OHT is 8.0 tonnes ≈ 78.5 kN on the slab. IS 2065 cl. 11.6.1 wants a structural check — and it is a permanent load.
●Leave ≥ 60 cm clear above the tank (cl. 11.6), and put the outlet 50–75 mm above the bottom, opposite the inlet.

And firefighting storage is ADDITIONAL

Underground

Terrace

25,000 L

Apartment house · The terrace tank PEAKS in this band. It is typically three times the domestic OHT below it — and it, not the domestic tank, governs the terrace structure.

Compare the two numbers. In a 15–35 m apartment block the 25,000 L terrace tank is roughly three times the domestic OHT beneath it — and it, not the domestic tank, governs the terrace structure. Fire storage from NBC 2016 Part 4 Table 7; confirm against a licensed BIS copy.

IS 15797

Harvesting, and the coefficient that surprises everyone

A flat concrete roof harvests less than a sloping tiled one — it ponds and evaporates while the slope sheds fast. And once you plot the monthly arrival against demand, you discover that storage, not roof area, is what binds.^{[4, 11]}

DiagramA flat RCC roof with water ponding and evaporating at a runoff coefficient of 0.80, beside a sloping tiled roof shedding fast at 0.95

Head, not diameter

The pressure myth, deprecated by BIS itself

Static pressure is ρgh — height of water, full stop. Diameter never appears. A bigger pipe cuts friction loss while water is moving, so it holds pressure better under flow; at zero flow it changes nothing. If the pressure is poor with every tap shut, no pipe size will save you. Raise the tank.

Rare and quotable: IS 1742’s own note under cl. 4.6.1.3 addresses the drainage version of this belief head-on — using a larger pipe than the flow requires, in order to justify a flatter gradient, “does not result in increasing the velocity of flow but reduces the depth of flow and for this reason is to be deprecated.” A code-level statement that a bigger pipe does not help.

DiagramTwo tanks at different heights feeding identical pipes, showing pressure follows head, beside the crossed-out myth that a bigger pipe gives better pressure

Code vs practice, and myth vs reality

At a glance

Aspect	One side	The other
What sizes the sump	CODE: 50 percent of the overhead tank (IS 2065 cl. 11.12.2)	PRACTICE: one full day of demand — making it 2–3× the OHT
Which tank is larger	CODE: the overhead tank	PRACTICE: the sump. The ratio is inverted
The 45 litres of flushing	CARVED OUT of the 150–200 band (IS 2065 cl. 11.11.1: 155 + 45 = 200)	MYTH: added on top, giving 135 + 45 = 180
What 135 lphd means	The reduced TOTAL for LIG/EWS housing	MYTH: the domestic (non-flushing) portion of a larger figure
Roof runoff coefficient	Flat RCC ≈ 0.80 — it ponds and evaporates	Sloping tiled or GI ≈ 0.95 — it sheds fast. The opposite of intuition
Static pressure at a tap	Set by HEAD — raise the tank to raise the pressure	MYTH: set by pipe diameter — a bigger pipe cuts friction, not pressure
Firefighting storage	ADDITIONAL to domestic, and often much larger	MYTH: part of, or interchangeable with, the domestic tank

Vocabulary

Key terms

Potable water

Water fit for human consumption, defined in India by IS 10500:2012 — a specification of water QUALITY, not of pipe materials.

lpcd / lphd

Litres per capita (or head) per day. The demand unit for every sizing calculation. Always check the BASIS — some IS 1172 rows are per seat or per bed, not per head.

Plumbosolvency

The capacity of water to dissolve lead from pipework. IS 2065 states that most of the waters in India are plumbo solvent — which is precisely why lead supply pipe is forbidden.

Overhead tank (OHT)

The storage that provides pressure by height. IS 2065 asks for a minimum of half a day where supply is intermittent.

Sump / underground tank

The buffer against supply failure. The code sizes it as a ratio of the OHT; practice sizes it against daily demand. The two disagree.

Head

The height of water above a point, and the sole determinant of static pressure. One bar is roughly 10.2 metres of head. Diameter does not appear in this relationship.

Runoff coefficient (K)

The fraction of rainfall on a surface that reaches the harvesting system. Counter-intuitively, a flat RCC roof performs WORSE than a sloping tiled one.

Solar fraction

The share of a year's water-heating energy actually delivered by the sun. In India, typically 55–80 percent — so 20–45 percent still comes from backup.

Free residual chlorine

The only MINIMUM in IS 10500 (0.2 mg/l) — the disinfectant left in the water to protect it downstream of treatment.

Firefighting storage

A reserve required by NBC Part 4, ADDITIONAL to domestic storage. In a mid-rise apartment it typically dwarfs the domestic tank.

Apply it

Study task

Take a real building you know — your own block, your college hostel — and size its storage twice: once to the code minimum, once to common practice. Draw both tanks to scale on a section through the building, and mark the firefighting reserve as a separate volume. Then write a short paragraph answering honestly: which of your two answers would you actually build, and on what grounds? If you would build the practice number, say plainly that it is not what IS 2065 asks for and explain why the code’s assumption about supply reliability does not hold at your site. That paragraph — not the arithmetic — is the professional skill.

Check your understanding

Self-assessment

1. Per IS 10500:2012, what is the minimum total dissolved solids (TDS) permitted in drinking water?

2. IS 2065 cl. 11.11.1 assures a minimum of 200 litres per head per day. How does it split?

3. Per IS 2065, how should an underground sump be sized relative to the overhead tank?

4. Two roofs of equal area harvest rainwater in the same city — one flat RCC, one sloping tiled. Which yields more, and why?

In a nutshell

Recap

IS 10500:2012 sets no minimum TDS, calcium or magnesium — every parameter is a maximum, and the only minimum is free residual chlorine at 0.2 mg/l.

IS 2065 assures 200 lphd = 155 domestic + 45 flushing. The flushing allowance is carved OUT, never added. 135 is the reduced LIG/EWS TOTAL.

The code sizes the sump as 50 percent of the OHT; practice sizes it at a full day, making it 2–3× the OHT. Both are teachable; only one is a code rule.

Firefighting storage is additional to domestic storage, and in a 15–35 m apartment block the 25,000 L terrace tank dwarfs the domestic OHT above it.

Rainwater: a flat RCC roof (K ≈ 0.80) harvests LESS than a sloping tiled one (K ≈ 0.95). Storage, not roof area, is the binding constraint.

Static pressure is a function of head, not diameter. IS 1742's own note deprecates the practice of upsizing a pipe to justify a flatter gradient.

The evidence

References & further reading

[1]IS 10500:2012, Drinking Water — Specification (Second Revision). Bureau of Indian Standards, FAD 25. https://law.resource.org/pub/in/bis/S06/is.10500.2012.pdf
[2]IS 1172:1993, Code of Basic Requirements for Water Supply, Drainage and Sanitation (Fourth Revision). Bureau of Indian Standards, CED 24. https://law.resource.org/pub/in/bis/S03/is.1172.1993.html
[3]IS 2065:1983, Code of Practice for Water Supply in Buildings (Second Revision). Bureau of Indian Standards, CED 24. https://law.resource.org/pub/in/bis/S03/is.2065.1983.pdf
[4]IS 15797:2008, Roof Top Rainwater Harvesting — Guidelines. Bureau of Indian Standards.
[5]IS 774:2004, Flushing Cistern for Water Closets and Urinals (Other than Plastic Cistern) — Specification (Fifth Revision). Bureau of Indian Standards.
[6]IS 12976:1990, Solar Water Heating Systems — Code of Practice. Bureau of Indian Standards.
[7]National Building Code of India 2016, Part 4 (Fire and Life Safety), Table 7 — fire-fighting water storage by occupancy and height. Bureau of Indian Standards. (Clause and table numbers to be confirmed against a licensed BIS copy.) ⚠️ WITHDRAWN. BIS's own standards catalogue lists SP 7:2016 (National Building Code of India 2016, Vol I and II), committee CED 46, with status “withdrawn”, superseded by SP 7:2026 — NATIONAL BUILDING CONSTRUCTION STANDARDS 2026 (Volume 1 & 2), same committee, status “Active” (verified against BIS's catalogue, July 2026). The provision cited here is from the WITHDRAWN 2016 edition, which is what current Indian teaching and site practice still quote. Whether the provision survives unchanged into SP 7:2026, and how the Parts are renumbered, has NOT been verified — check SP 7:2026 before relying on this.
[8]Kumpel, E. & Nelson, K. L. (2013). Comparing microbial water quality in an intermittent and continuous piped water supply. Water Research, 47(14), 5176–5188. https://doi.org/10.1016/j.watres.2013.05.058
[9]Kumpel, E. & Nelson, K. L. (2016). Intermittent water supply: prevalence, practice, and microbial water quality. Environmental Science & Technology, 50(2), 542–553. https://doi.org/10.1021/acs.est.5b03973
[10]Ercumen, A. et al. (2015). Upgrading a piped water supply from intermittent to continuous delivery and association with waterborne illness. PLOS Medicine, 12(10), e1001892. https://doi.org/10.1371/journal.pmed.1001892
[11]Srinivasan, V., Gorelick, S. M. & Goulder, L. (2010). Sustainable urban water supply in south India: desalination, efficiency improvement, or rainwater harvesting? Water Resources Research, 46(10), W10504. https://doi.org/10.1029/2009wr008698
[12]Kozisek, F. (2005). Health risks from drinking demineralised water. In: Nutrients in Drinking Water, Ch. 12. World Health Organization, Geneva. (Note: a 1980 expert team's recommendations published BY the WHO — not adopted WHO Guideline values. WHO proposes no health-based TDS guideline.)