Studio Matrx Monthly · Volume 1 · Issue 2 · July 2026
Amogh N P
✦ In loving memory of Amogh N P — Architect · Designer · Visionary ✦
Drafting instruments, a scale rule and a pencil resting on a large unlettered technical drawing sheet showing only fine lines and hatching, in raking window light
Unit VInterior Services I — Plumbing & Water Supply

Services Studio

Drawing the layout, and the section that catches the clash

55 min readByAmogh N P· Architect & interior designer

Everything in the four units before this becomes, in the end, a drawing — and a drawing that someone must build from. This unit is about that translation: the five drawings and which of them you may scale, the symbols India never got round to standardising, the shaft with the one hard number worth memorising, the sunken slab that no code specifies, and the chase that a code you were told was silent turns out to describe in exhaustive detail. It closes on a single sentence from the National Building Code that is, quietly, the thesis of the whole course.

Learning objectives

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

1
CO2 · Apply

Prepare a plumbing layout and working drawings for fittings and fixtures, at the right scale.

2
CO6 · Understand

Use IS 962's abbreviations correctly — and know why every Indian plumbing drawing must carry its own legend.

3
CO4 · Analyse

Size and locate a pipe shaft, and sequence it correctly against plastering and first fix.

4
CO6 · Evaluate

State honestly where a code is silent (sunken slabs, chasing RCC) and where two codes contradict (test pressure).

5
CO4 · Analyse

Apply IS 1905's chasing limits, and explain why a half-brick wall cannot take a concealed cistern.

6
CO6 · Evaluate

Explain why coordination must be assessed in section, not only in plan.

IS 962:1989

The set, and the symbols India never standardised

Five drawings; only three are to scale. And the symbol standard you are looking for does not exist — which has a practical consequence for every sheet you will ever issue.[1, 5]

Five drawings. Only three are to scale. NOT to scale Single-line schematic topology — what connects to what Riser diagram vertical distribution across floors Students try to scale these. Don’t. Dimensioned, not scaled Isometric 3-D routing at 30°; verticals stay vertical IS 10990 (Pt 2):2023 = ISO 6412-2 What the fabricator actually works from. TRUE scale drawings Plan layout 1:100 general · 1:50 toilet or kitchen Sectional detail 1:20 · 1:10 · 1:5 IS 962 Table 4 gives the scale list. India has no national plumbing-symbol standard NBC Part 9 has no symbols table. The symbols live in IS 962:1989 cl. 11, Figs. 11–12. And IS 962’s own foreword records it was built on two ISO parts that are now WITHDRAWN. India’s live symbol reference descends from dead ISO parts. So every drawing must carry its own legend. Indian plumbing drawing is office-specific and legend-driven. Letter-coding (CW, HW, RWP) is, in practice, more reliable than line-styling — because there is no national line convention to fall back on. The CW/HW line conventions you find online are ASPE and CIBSE — American and British. Not Indian.
DiagramThe five plumbing drawings compared, showing which are to scale and which are not, with a note that India has no national plumbing symbol standard
Site vocabulary and the standard diverge In IS 962:1989 FT — floor trap GT — gully trap · GRT — grease trap ICH, IC — inspection chamber MH — manhole · IT — intercepting trap SP — soil pipe · WP — waste pipe S & VP — soil and vent pipe W&VP — waste and vent pipe RE — rodding eye RWP — rainwater pipe · FAI — fresh air inlet SV — stop valve · SV — sluice valve NOT in IS 962 SVP — ubiquitous on site. The standard writes S & VP. AVP · WVP — neither exists. CO · “cleanout” — imported vocabulary. The standard’s term is rodding eye (RE). “nahani” — site vernacular. The standard says floor trap (FT). Two collisions to warn your team about SV = stop valve AND sluice valve. IC = inspection chamber AND internal combustion. The nahani trap exists as a PRODUCT (IS 14735). It does not exist as a drawing symbol — and no IS standard dedicated to floor traps could be found. Do not invent one.
DiagramThe IS 962 abbreviations for plumbing drawings beside the site abbreviations that are not in the standard — SVP, AVP, WVP, CO and nahani

Five drawings, and only three are to scale

A plumbing package is not one drawing. The single-line schematic conveys topology — what connects to what — and is not to scale. The riser diagram shows vertical distribution across floors and the stack-to-branch relationships; also not to scale. The isometric shows three-dimensional routing at 30 degrees with verticals staying vertical, dimensioned but not to scale, and it is the drawing a fabricator actually works from. Only the plan layout (typically 1:100 general, 1:50 for a toilet or kitchen enlargement) and the sectional detail (1:20, 1:10, 1:5) are true scale drawings. Students routinely try to scale a riser diagram. It is not that kind of drawing, and it never was.[1, 4]

Where the drawing meets the building

Shafts, slabs and chases

Three subjects, and each carries a surprise: one hard number, one total silence, and one code that is far more explicit than you were told.[2, 3, 7]

One number worth memorising Plan 1 m minimum 1 m “In no case shall the cross-section area of the shaft be less than a square of 1 m side.” — NBC 2016 Section access at EVERY level if anything serviceable is inside access door at ground level shaft ventilation The sequencing trap “All pipe shafts shall be plastered BEFORE any pipes are installed in the shaft.” A first-fix lesson in one line. Once the pipes are in, the shaft wall cannot be finished — and a shaft you cannot finish is one you cannot fire-stop or clean. In a continuous shaft, provide a concrete slab or similar flooring for maintenance at each floor.
DiagramA pipe shaft in plan and section showing the minimum one metre square, access at every level, ventilation, and the instruction to plaster before any pipe is installed
The code is not silent. It is fully explicit. IS 1905:1987 cl. 6.5 — Chases, Recesses and Holes Vertical — up to 1/3 1/3 wall thickness Not closer than 2 m apart. Not within 34.5 cm of an opening or 23 cm of a cross wall. Horizontal — up to 1/6 1/6 Width max 6 cm. Length max 1 m. Max two per wall. Total lengths not over 1/4 of the wall length. Half-brick — NOTHING cl. 6.5.3.9 “No chase, recess or hole shall be provided in half-brick load bearing wall” — except minimum scaffolding holes. 115 mm And that is exactly the partition an interior designer reaches for. cl. 6.5.3.8: leave chases AT THE TIME OF CONSTRUCTION by inserting sleeves. Where cutting is unavoidable, use tools depending on ROTARY and not heavy impact action. cl. 6.5.3.10: no cutting into hollow units after they are built in.
DiagramChasing limits from IS 1905 — vertical to one third and horizontal to one sixth of wall thickness, and nothing at all in a half-brick load bearing wall

One hard number, and a sequencing trap

The National Building Code gives an interior designer one figure to hold on to here, and it is worth memorising: in no case shall the cross-sectional area of a pipe shaft be less than a square of one metre side. The shaft must have an access door at ground level and provision for ventilation, and access at every level if anything serviceable sits inside it. Then the sequencing instruction that catches teams out: all pipe shafts shall be PLASTERED BEFORE any pipes are installed in the shaft. Read that as a first-fix lesson in one line. Once the pipes are in, the shaft wall cannot be finished, and a shaft you cannot finish is a shaft you cannot fire-stop or clean.[2]

Where India differs

Two honest notes on vocabulary and testing

“First fix” and “second fix” are not Indian terms. Search CPWD Specifications and IS 2065 and you will find them zero times — they are British vocabulary, and North America says “rough-in” and “trim-out” instead. You will still meet them constantly on hotel, retail and international-consultant fit-outs, so learn them. But know that Indian documents encode the same two-stage split differently: through measurement and payment. CPWD’s internal pipework rate explicitly excludes concealed work, “in which case cutting of chase and making good shall be paid separately” — and the second-fix items are enumerated as their own separate clauses. The split is entirely real in India. It just does not have that name.[8]

And the codes contradict each other on test pressure. For a main after laying, NBC asks for 0.5 N/mm² or double the maximum working pressure, whichever is greater. IS 2065 cl. 14.2 asks for 0.5 N/mm² or working pressure plus fifty percent. Two Indian codes, one question, two answers — which is almost certainly why “1.5 times” circulates as folklore. Worse, CPWD gives no number at all for CPVC, the most-used domestic water material in Indian interiors: it says only that the tests “are similar to the test pressure used for other plastic/metal pipes.” Which is exactly why site practice defaults to the manufacturer’s manual — and why that is the correct answer to give.[2]

The thesis

Plans lie about clashes. Sections do not.

A plan can tell you that a drain and a duct occupy the same footprint. It cannot tell you whether that matters — whether one passes cleanly below the other, or whether the drain, having fallen at its mandatory gradient across the room, has arrived exactly where the duct needs to be. Nor can it tell you the answer that catches most fit-outs: that both services clear each other and neither fits above the ceiling you promised the client.

Plans lie about clashes. Sections do not. In plan — they cross drain duct clash? The plan says they occupy the same footprint. It cannot say whether that matters. In section — the truth slab soffit drain falling at 1 in 50 duct 300 deep ceiling line they miss but the VOID fails They pass. But now the drain has fallen into the duct’s space, and neither fits above the ceiling you promised. NBC, on coordination: “...shall be assessed in plans AND SECTIONAL DETAILS.” That requirement is this whole unit, in one sentence.
DiagramThe same drain and duct shown in plan appearing to clash and in section revealing they pass at different levels while the ceiling void still fails

The National Building Code puts it in one line: coordination between architectural, structural and services drawings “shall be assessed in plans and sectional details.” Not or. And. That sentence is this unit, and arguably this whole course, compressed into eleven words.[2]

Myth vs reality

At a glance

AspectOne sideThe other
Plumbing symbolsIS 962:1989 cl. 11, Figs. 11–12 — the closest India hasMYTH: an NBC Part 9 symbols annex. It does not exist
Soil and vent pipeIS 962 writes “S & VP”MYTH: “SVP” — ubiquitous on site, absent from the standard
Drain access pointIS 962: rodding eye (RE)NOT in IS 962: “cleanout” or “CO”
Pipe shaft minimumNBC: in no case less than a square of 1 m sideAnd plastered BEFORE any pipe is installed — a sequencing trap
Sunken slab depthCONVENTION: ~150–200 toilet block, 200–250 EWC, 400–450 Orissa panNBC 2016 contains NO sunken-slab provision. There is no code figure
Chasing a load-bearing wallIS 1905: vertical ≤ ⅓, horizontal ≤ ⅙ of wall thicknessHalf-brick load-bearing wall: NO chase at all (cl. 6.5.3.9)
Chasing RCCIS 456 has NO prohibition clause — argue from cl. 26.4 cover insteadMYTH: a specific clause bans it. Report the silence honestly
IS 456 coverTable 16 is by EXPOSURE: mild 20, moderate 30, severe 45, very severe 50, extreme 75MYTH: “slab 20 / beam 25 / column 40 / footing 50”. “Beam 25” is not in the code at all
Test pressure for a mainNBC: 0.5 N/mm² or DOUBLE working pressure, whichever is greaterIS 2065: 0.5 N/mm² or working pressure + 50 PERCENT. The codes contradict each other
Vocabulary

Key terms

S & VP

IS 962's abbreviation for a soil and vent pipe. Note it is NOT “SVP” — the ubiquitous site abbreviation is not in the standard.

Rodding eye (RE)

IS 962's term for the drain access point. The standard has no “cleanout” and no “CO” — those are imported vocabulary.

Floor trap (FT)

IS 962's term for what site calls a nahani trap. “Nahani” appears nowhere in the standard.

Pipe shaft

A vertical service enclosure. NBC: in no case less than a square of 1 m side; access door at ground level; ventilated; plastered BEFORE any pipe is installed.

Chase

A groove cut in masonry for services. IS 1905 cl. 6.5.3.2: vertical no more than ⅓ and horizontal no more than ⅙ of wall thickness. None at all in a half-brick load-bearing wall (cl. 6.5.3.9).

Nominal cover

The concrete cover to reinforcement required by IS 456 cl. 26.4. Table 16 is organised by EXPOSURE, not by member. Chasing RCC destroys it.

Sunken slab

The dropped floor slab housing a bathroom's drainage. NBC specifies NO depth — the 150–450 mm range is convention, driven by the WC outlet type.

First fix / second fix

British programming vocabulary you will meet on hotel and international fit-outs. It appears ZERO times in CPWD Specifications or IS 2065 — Indian documents say “concealed pipework” and “fixing” instead.

Isometric

A three-dimensional pipe routing drawing at 30°, verticals staying vertical. Dimensioned but not to scale. India's live standard is IS 10990 (Part 2):2023 = ISO 6412-2.

Riser diagram

A not-to-scale drawing of vertical distribution across floors, showing stack and branch relationships. Do not try to scale it.

Apply it

Study task

Prepare a small plumbing package for one toilet: a plan at 1:50, a section at 1:20, and a single-line riser diagram. Three disciplines apply. First, draw your own legend — and use IS 962’s abbreviations exactly, which means S & VP rather than SVP, rodding eye rather than cleanout, and floor trap rather than nahani. Second, mark on the section every dimension that is governed and every dimension that is chosen, and put a clause number against the first group; you will find the gradient and the seal are governed, and the sunken slab depth — the number your section is built around — is not, because no Indian code specifies one. Third, find one clash that your plan cannot show and your section can. Then write a sentence explaining why the code asks for both.

Check your understanding

Self-assessment

1. What is the minimum cross-sectional size of a pipe shaft under NBC 2016?

2. Per IS 1905:1987, what is the rule for chasing a half-brick load-bearing wall?

3. Which Indian code specifies the depth of a sunken slab in a toilet?

4. NBC requires coordination between architectural, structural and services drawings to be assessed how?

In a nutshell

Recap

Five drawings, three to scale: schematic, riser and isometric are NOT scale drawings; plan (1:100 / 1:50) and section (1:20 / 1:10 / 1:5) are.
India has no national plumbing-symbol standard. Symbols live in IS 962:1989, whose foreword descends from two withdrawn ISO parts. Every drawing must carry its own legend.
IS 962 writes S & VP, not SVP; rodding eye, not cleanout; floor trap, not nahani. And SV means both stop valve and sluice valve.
A pipe shaft is never less than a square of 1 m side, has an access door and ventilation, and is plastered BEFORE any pipe is installed.
No Indian code specifies a sunken-slab depth. The 150–450 mm range is convention, and the WC outlet type drives it.
IS 1905 cl. 6.5 is fully explicit on chasing: vertical ≤ ⅓, horizontal ≤ ⅙, rotary tools not hammers, and NOTHING in a half-brick load-bearing wall.
IS 456 has no clause prohibiting chasing RCC — argue from cl. 26.4 cover. And Table 16 is by exposure, not by member.
NBC says test at DOUBLE working pressure; IS 2065 says working pressure plus 50 percent. Two Indian codes, one question, two answers.
The evidence

References & further reading

  1. [1]IS 962:1989, Code of Practice for Architectural and Building Drawings (Second Revision). Bureau of Indian Standards, CED 51. Clause 11 and Figs. 11–12 carry the sanitary and drainage symbols. https://archive.org/stream/gov.in.is.962.1989/is.962.1989_djvu.txt
  2. [2]National Building Code of India 2016, Part 9 (Plumbing Services), Sections 1 and 2. Bureau of Indian Standards. (Clause and page 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.
  3. [3]IS 1905:1987, Code of Practice for Structural Use of Unreinforced Masonry (Third Revision), cl. 6.5: Chases, Recesses and Holes. Bureau of Indian Standards. https://archive.org/details/gov.in.is.1905.1987
  4. [4]IS 10713 / IS 7973:1976, Architectural and Building Working Drawings. Bureau of Indian Standards, CED 51.
  5. [5]IS 10990 (Parts 1, 2 and 3):2023, Technical Product Documentation — Simplified Representation of Pipelines (identical adoption of ISO 6412-1/-2/-3:2017). Bureau of Indian Standards. Part 2 covers isometric projection of pipelines.
  6. [6]IS 2064:1993, Selection, Installation and Maintenance of Sanitary Appliances — Code of Practice (Second Revision). Bureau of Indian Standards. https://law.resource.org/pub/in/bis/S03/is.2064.1993.pdf
  7. [7]IS 456:2000, Plain and Reinforced Concrete — Code of Practice (Fourth Revision), cl. 26.4 (nominal cover) and cl. 31.8 (openings in flat slabs). Bureau of Indian Standards.
  8. [8]CPWD Specifications 2019, Volume 2 — Chapter 18 (Water Supply) and cl. 18.22 (Cutting Chases in Masonry Walls). Central Public Works Department, Government of India. https://cpwd.gov.in/Publication/Specs2019V2.pdf
  9. [9]IS 5329:1983, Code of Practice for Sanitary Pipe Work Above Ground for Buildings, cl. 6.2.2 (horizontal branch gradient: not flatter than 1 in 50, not steeper than 1 in 10) and cl. 7.2.2.1 (air test). Bureau of Indian Standards. https://archive.org/details/gov.in.is.5329.1983
  10. [10]IS 1239 (Part 1):2004, Steel Tubes, Tubulars and Other Wrought Steel Fittings, Part 1: Steel Tubes. Bureau of Indian Standards.
  11. [11]Aryan, Y. et al. (2024). A review of drinking water quality and treatment in India. Journal of Water and Health, 22(2), 235–267. https://doi.org/10.2166/wh.2023.082

Further reading

  • SP 46:2003 — Engineering Drawing Practice for Schools and Colleges. Bureau of Indian Standards.
  • SP 35:1987 — Handbook on Water Supply and Drainage (with Special Emphasis on Plumbing). Bureau of Indian Standards.
  • CPWD — Specifications 2019, Volumes 1 and 2. Central Public Works Department, Government of India.
  • Uniform Illustrated Plumbing Code — India (UIPC-I 2022). Indian Plumbing Association and IAPMO.
  • Ching, F. D. K. — Building Construction Illustrated. John Wiley & Sons.
  • Grondzik, W. T. & Kwok, A. G. — Mechanical and Electrical Equipment for Buildings. John Wiley & Sons.

Sources gathered and fact-checked June 2026. Published values vary by source, sample and method — treat as indicative and confirm against the cited standard before structural use.

A

The author

Amogh N P

Architect, interior designer, and creative polymath. Studio Matrx began in his notebooks — his vision of design made honest, useful, and open to everyone. Its Academy is written and taught in his memory, and free, forever.

More about Amogh →