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1.2 m wide dog-legged stair for residence

Short answer

Dividing 3 m floor height by 150 mm riser yields 20 risers. Two flights of 10 risers each with 9 treads of 250 mm; flight width 1.2 m, going 2.25 m, landing 1.2 m, waist slab 175 mm.

Design and draw plan of a 1.2 m wide doglegged stair for a residential building with 3 m floor height.

Flight width b=1.2b = 1.2 m. Stairwell width =2×1.2+0.1=2.5= 2 \times 1.2 + 0.1 = 2.5 m, and the length is assumed to be 4.5 m.

Given data and assumptions

  • Stairwell (internal): 4.5 m x 2.5 m; floor-to-floor height H=3H = 3 m; stair for a residential building, RCC.
  • Assumed riser R=150R = 150 mm and tread T=250T = 250 mm; flight width b=1.2b = 1.2 m.

Design

  1. Number of risers =H/R=3/0.15=20= H/R = 3/0.15 = 20, adopt 20 risers; exact riser =3/20=150= 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 =9×0.25=2.25= 9 \times 0.25 = 2.25 m.
  4. Width: two flights of 1.2 m side by side need 2b=2.42b = 2.4 m; the remaining W−2b=2.5−2.4=0.1W - 2b = 2.5 - 2.4 = 0.1 m is the gap between the flights.
  5. Landing: a half-space landing 1.2 m x 2.5 m (full stairwell width) at height =10×0.15=1.5= 10 \times 0.15 = 1.5 m.
  6. Length check: going + landing =2.25+1.2=3.45= 2.25 + 1.2 = 3.45 m ≤4.5\le 4.5 m, so it fits; the spare 1.05 m is kept as entry/exit space.
  7. Check: 2R+T=2×150+250=5502R + T = 2 \times 150 + 250 = 550 mm (within 550-650 mm); pitch =tan⁡−1(R/T)=31∘= \tan^{-1}(R/T) = 31^\circ (within 25-40 degrees).
  8. Waist slab: effective span about 3.45 m, so depth ≈\approx span/20 =173= 173 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

 +------------------------+---------+
 | 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)

 SECTION (not to scale)
                          +---- upper floor 3 m
                    _____|
               ____|   flight 2
        ______|  landing
   ____|  flight 1
 _|
 ground floor +0.00

Summary

ItemAdopted
Riser150 mm (20 nos.)
Tread250 mm
Flights2 flights x 10 risers
Width of flight1.2 m
Waist slab175 mm

Answer: 20 risers of 150 mm and treads of 250 mm; 2 flights x 10 risers; flight width 1.2 m.

✓ Answer checked by Claude Opus 5.5 (high effort)

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