Amogh N P
✦ In loving memory of Amogh N P — Architect · Designer · Visionary ✦
A reinforced-concrete dog-legged staircase under construction with formwork and reinforcement bent to the flight and mid-landing — stair geometry and detailing built into shape.
Unit VArchitectural Detailing and Working Drawing

Staircase, Toilet & Services

The 2R+T rule, the sunk slab that drains, and coordinating the trades.

≈ 45 min + studio task

A comfortable stair obeys the 2R+T rule (2×riser + tread ≈ 600–640 mm) with headroom of at least 2.1 m — get this wrong and the stair is tiring or unsafe. Learn to set out a flight (treads = risers − 1), the dog-legged vs open-well stair, and how to represent waist-slab reinforcement. Then the wet areas: the sunk slab that conceals plumbing, the floor fall to the nahani trap, the waterproofing turn-up and ledge wall — and how to coordinate plumbing and electrical service runs across the whole drawing set.

Learning objectives

By the end of this lesson, you will be able to — mapped to the course outcomes for Architectural Detailing and Working Drawing:

1
CO5 · Apply

Set out a staircase by the 2R+T rule with correct riser, going, headroom and width.

2
CO5 · Apply

Detail dog-legged and open-well stairs and represent waist-slab reinforcement.

3
CO5 · Apply

Detail a wet area — sunk slab, floor fall, nahani trap, waterproofing and ledge wall.

4
CO6 · Analyse

Coordinate plumbing and electrical service runs across the drawing set.

2R+T, setting out, the waist slab

Staircase detailing

Set the riser and going to satisfy 2R+T with ≥2.1 m headroom; set out with treads = risers − 1; and anchor the waist-slab bars correctly at the re-entrant flight/landing corner.[1, 3]

Stair geometry — the 2R + T rule going (T) 250–300 riser (R) 150–190 headroom ≥ 2.1 m 2R + T ≈ 600–640 mm (Blondel's stride rule) 300 going + 175 riser = 650 is too steep-and-long — recompute into the 600–640 range.
DiagramStair geometry — going and riser bound by the 2R plus T comfort rule, with headroom of at least 2.1 metres

The governing rules

A comfortable stair obeys 2R + T ≈ 600–640 mm (Blondel): going/tread 250–300 mm, riser 150–190 mm — a steeper riser pairs with a shorter going. Headroom (nosing line to the flight above) must be ≥2.1 m. Number of risers = floor-to-floor ÷ riser. A stair of 300 going + 175 riser (= 650) is too long-and-steep — recompute into range.[1]

Dog-legged stair — section mid-landing waist-slab main bars (along slope) anchor bars correctly at the re-entrant corner (or they straighten & spall) Treads = risers − 1 per flight; the landing is at least the stair width.
DiagramA dog-legged stair section — two flights with a mid-landing on an inclined waist slab, reinforcement anchored at the re-entrant corner
Sunk slab, fall, trap, coordination

Toilet, wet areas & services

Drop the slab in wet areas, slope the floor to a sealed nahani trap, turn the waterproofing up the walls with a ledge wall, and coordinate plumbing and electrical runs across the set.[2, 3, 5]

Wet area — sunk slab, fall, trap sunk slab — dropped 200–300 mm floor fall ~1:80–1:100 → nahani trap · ~50 mm seal turn-up ~300 ledge wall No sunk slab / no turn-up / no fall → pipes exposed, water creeps to dry rooms, floor ponds.
DiagramA toilet wet-area section — a sunk slab below floor level, the floor sloping to a nahani trap, waterproofing turned up the walls and a ledge wall

Drop the slab

Draw the enlarged toilet plan at 1:25/1:20 with fixtures (WC, basin, shower, floor trap) and clearance/tile-setting-out dimensions (WC needs ~600 mm clear width, ~600 mm in front). Wet areas need a SUNK SLAB — the structural slab dropped ~200–300 mm below the surrounding FFL — so plumbing and the fall fit and the finished floor matches the adjacent rooms.[2, 3]

Dog-legged vs open-well

At a glance

AspectOneThe other
Well spaceDog-legged: none (flights back-to-back)Open-well: central well between flights
Area neededDog-legged: compactOpen-well: larger footprint
HandrailDog-legged: returns at landingsOpen-well: can run continuously
Stair widthResidential: ≥ 1.0 mPublic egress: ≥ 1.2 m (NBC)
Wet-area floorMyth: same level, flatReality: sunk slab + fall 1:80–1:100 to trap
Vocabulary

Key terms

Going / tread

The horizontal depth of a step walked on (250–300 mm).

Riser

The vertical height between treads (150–190 mm).

2R + T rule

Blondel's comfort relation: 2×riser + tread ≈ 600–640 mm.

Waist slab

The inclined RCC slab forming the structural stair flight (~150–200 mm).

Nahani / floor trap

A floor drain with a ~50 mm water seal preventing sewer-gas entry.

Sunk slab

A slab lowered ~200–300 mm in wet areas to conceal plumbing and provide the fall.

Apply it

Studio task

For a floor-to-floor height of 3.0 m, design a dog-legged stair: choose a riser and going that satisfy 2R+T (≈600–640), compute the risers and treads, and draw the plan and section with the mid-landing, headroom and waist slab. Separately, draw an enlarged toilet plan and section showing the sunk slab, the fall (~1:80) to a nahani trap, the waterproofing turn-up and the ledge wall.

Check your understanding

Self-assessment

1. Which combination satisfies the 2R+T comfort rule (≈ 600–640)?

2. The minimum clear headroom on a staircase is —

3. A toilet floor is sloped at ~1:80–1:100 in order to —

In a nutshell

Recap

A stair obeys 2R+T ≈ 600–640 with riser 150–190, going 250–300, headroom ≥2.1 m, width ≥1.0–1.2 m.
Set out with treads = risers − 1 per flight, a mid-landing ≥ stair width, drawn in plan and section.
Dog-legged is compact; open-well gives a continuous handrail and light; anchor waist-slab bars correctly at the corner.
Wet areas need a sunk slab (200–300 mm), a fall (1:80–1:100) to a sealed nahani trap, and a 300 mm waterproofing turn-up + ledge wall.
Coordinate plumbing and electrical service runs with structure on layered sheets — a set is complete only when reconciled.
The evidence

References & further reading

  1. [1]National Building Code of India 2016, Part 4 (stairs — going/riser, headroom, width, railings).
  2. [2]National Building Code of India 2016, Part 9 (Plumbing Services); IS 1742 / IS 2064 / IS 2065 (sanitation).
  3. [3]Francis D.K. Ching, Building Construction Illustrated (stairs, baths, plumbing details).
  4. [4]W.B. McKay, Building Construction (stair geometry and construction).
  5. [5]S.C. Rangwala, Building Construction & Water Supply and Sanitary Engineering (Indian toilet/plumbing detailing).

Further reading

  • Francis D.K. Ching — Building Construction Illustrated.
  • W.B. McKay — Building Construction.
  • S.C. Rangwala — Building Construction; Water Supply and Sanitary Engineering.

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.