---
title: "RCC stair, 2.1 m by 4.5 m room"
subject: "Building Technology"
chapter: 5
key: "semester_6/building_technology:rcc-stair-room-2p1m"
source: https://www.bctengineer.com/q/semester_6/building_technology/rcc-stair-room-2p1m
site: BCT Engineering
---

# RCC stair, 2.1 m by 4.5 m room

Building Technology · Chapter 5: Stair, lifts and escalators

- Asked in: 2072 Magh, 4 marks
- Video: [Construction of RCC stairs...](https://www.youtube.com/watch?v=mjBz-cJW4R0) — Sabin Paudel
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/rcc-stair-room-2p1m
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-5

## Question

Inside dimension of a stair room is 2.1 m×4.5 m. Floor to floor height is 3.0 m. Design a proper layout of an RCC stair.

## Short answer

Dog-legged stair with 20 risers of 150 mm, treads 250 mm, two flights of 10 risers each, flight width 1 m.

## Answer

#### Given data and assumptions
- Stairwell (internal): 4.5 m x 2.1 m; floor-to-floor height $H = 3$ m; stair for a residential building, RCC.
- Assumed riser $R = 150$ mm and tread $T = 250$ mm; flight width $b = 1$ m.

#### Design
1. **Number of risers** $= H/R = 3/0.15 = 20$, adopt **20 risers**; exact riser $= 3/20 = 150$ mm.
2. **Flights:** a dog-legged stair has two equal flights, so each flight has 20/2 = 10 risers and 10-1 = 9 treads (the landing or upper floor acts as the last tread).
3. **Going of one flight** $= 9 \times 0.25 = 2.25$ m.
4. **Width:** two flights of 1 m side by side need $2b = 2$ m; the remaining $W - 2b = 2.1 - 2 = 0.1$ m is the gap between the flights.
5. **Landing:** a half-space landing 1 m x 2.1 m (full stairwell width) at height $= 10 \times 0.15 = 1.5$ m.
6. **Length check:** going + landing $= 2.25 + 1 = 3.25$ m $\le 4.5$ m, so it fits; the spare 1.25 m is kept as entry/exit space.
7. **Check:** $2R + T = 2 \times 150 + 250 = 550$ mm (within 550-650 mm); pitch $= \tan^{-1}(R/T) = 31^\circ$ (within 25-40 degrees).
8. **Waist slab:** effective span about 3.25 m, so depth $\approx$ span/20 $= 162$ mm; adopt a 175 mm thick waist slab (reinforcement to be designed separately). Provide a handrail 0.9 m high, 20-25 mm nosing and a non-slip finish.

#### Plan and section
```text
 +------------------------+---------+
 | FLIGHT 2  <<<<<<  UP   |         |
 | 9 treads               | LANDING |
 +------------------------+ (b x W) |
 | FLIGHT 1  >>>>>>  UP   |         |
 | 9 treads               |         |
 +------------------------+---------+
 <-------- going -------> <-- b -->
 (entry at left end of flight 1; arrows point up the stair)
```
```text
 SECTION (not to scale)
                          +---- upper floor 3 m
                    _____|
               ____|   flight 2
        ______|  landing
   ____|  flight 1
 _|
 ground floor +0.00
```

#### Summary
| Item | Adopted |
|---|---|
| Riser | 150 mm (20 nos.) |
| Tread | 250 mm |
| Flights | 2 flights x 10 risers |
| Width of flight | 1 m |
| Waist slab | 175 mm |

**Answer: 20 risers of 150 mm and treads of 250 mm; 2 flights x 10 risers; flight width 1 m.**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which stair is designed for the 2.1 m x 4.5 m room?
   A. Dog-legged with two 10-riser flights
   B. Straight flight
   C. Three-flight
   D. Spiral
   Answer: A. Two flights of 1 m side by side fit the 2.1 m width. (Key point: Stair type)

2. 2.1 m room with two 1 m flights leaves
   A. 0.1 m
   B. 1.1 m
   C. 0.5 m
   D. 0
   Answer: A. 2.1 - 2 = 0.1 m. (Key point: Well gap)

3. The half-space landing size is
   A. 1 m x 2.1 m
   B. 2 m x 1 m
   C. 1 m x 1 m
   D. 4.5 m x 1 m
   Answer: A. It is flight width by full stairwell width. (Key point: Landing)

4. Going + landing = 2.25 + 1 = 3.25 m shows
   A. It fits within 4.5 m
   B. It exceeds 4.5 m
   C. Slab depth
   D. Riser count
   Answer: A. 3.25 m is less than 4.5 m, leaving 1.25 m spare. (Key point: Length check)

## Related in Building Technology

- [Dog-legged RCC stair design for 3 m floor height](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-dog-3m-assume) — Building Technology, chapter 5, asked 3×
- [RCC stair for 6.0 m by 4.2 m lobby](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arcc-stair-public-lobby-6m) — Building Technology, chapter 5
- [Stair design, 1.8 m by 3.9 m](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-residential-1p8m-wide) — Building Technology, chapter 5
- [Dog-legged RCC stair, 4.6 m by 2.6 m](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adog-legged-stair-residential-4p6m) — Building Technology, chapter 5, asked 3×
- [Vertical circulation elements; RCC dog-legged stair](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-circulation-068ba) — Building Technology, chapter 5
- [RCC dog-legged stair for office building](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arcc-stair-office-building) — Building Technology, chapter 5

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