StairGen Pro › U-Shaped Steel Stair Calculator (Switchback)
Guide & codes
This U-shaped steel stair calculator, also known as a switchback or half-turn stair calculator, covers two parallel flights of bracketed stringer joined by a landing twice the width of the stair. It is the answer when the opening is short and wide, and the most common steel stair between floors in homes, warehouses and industrial units. In the UK the same layout, with the flights side by side and no gap between them, is called a dog-leg stair.
From the total rise (Y), the length of the opening (X), the width of each flight (W), the landing depth (E) and the steps in each flight (P and Q), the calculator works out the steps, checks comfort and produces the stringers, landing frame, posts and railing as fabrication drawings in DXF.
When to choose this stair
The U-shaped steel stair with a landing, a switchback, makes the most of a short, wide opening: the two flights climb side by side and the landing spans twice the stair width. It is the usual choice in industrial units, mezzanines and public buildings, because you arrive right above where you started.
Compared with an L-shaped stair, it fills a rectangle two stair widths wide (1,000 mm per flight) instead of two sides of a corner. Compared with the sawtooth version, it uses straight stringers with brackets. Keep the landing depth (E = 1,000 mm) at least equal to the flight width: a shallow landing forces people to pivot on one foot halfway up.
Calculator parameters
Each dimension carries the letter used in the editor and on the drawings.
- YTotal rise
- default 2,800 mm · 500 to 8,000 mm
- Height from the lower floor to the finished upper floor.
- XTotal run
- default 3,250 mm · 500 to 12,000 mm
- Plan length available for the flight (on turning stairs, the upper flight).
- WStair width
- default 1,000 mm · 500 to 2,500 mm
- Clear width of the flight.
- ELanding depth
- default 1,000 mm · 500 to 3,000 mm
- Depth of the landing in the walking direction.
- ZTread thickness
- default 50 mm · 3 to 120 mm
- Thickness of the tread (board or plate).
- FNosing
- default 50 mm · 0 to 100 mm
- How far each tread overhangs the one below.
- PSteps, lower flight
- default 7 · 1 to 25
- Steps before the turn.
- QSteps, upper flight
- default 8 · 2 to 25
- Steps after the turn, up to the top.
- AStringer depth
- default 100 mm · 30 to 300 mm
- Depth of the stringer tube.
- GStringer width
- default 50 mm · 20 to 300 mm
- Width of the stringer tube.
- BBracket size
- default 100 mm · 20 to 200 mm
- Tube section of the bracket under each tread.
- HHeight over stringer
- default 60 mm · 0 to 400 mm
- Vertical gap between the stringer and the tread.
- DBracket overhang
- default 60 mm · 0 to 300 mm
- How far the tread projects beyond the bracket in the walking direction.
- UBracket angle
- default 10° · -30 to 45°
- Tilt of the bracket from vertical.
- TNumber of stringers
- default 1 · 1 to 3
- One centre stringer, two side stringers or three.
How it's calculated
The total rise Y = 2,800 mm over 16 risers, including the one onto the landing, gives 175.0 mm per step. On the upper flight, the length X = 3,250 mm minus the landing depth E = 1,000 mm leaves room for Q = 8 steps: a run of 281 mm and a tread depth of 331 mm.
2 × 175.0 + 281 = 631 mm and a pitch of 31.9°: “Comfortable stair”. The landing depth E has a direct effect: make it shorter and every step of the upper flight gets more run, so the pitch drops. Make it deeper and you gain a more generous resting place at the cost of a steeper flight.
The landing is two flights wide (2 × W) and E deep. It has a tube frame and two posts at the free corners, and the two stringers arrive at it at different heights, one riser apart, which the side elevation in the drawings shows clearly.
Result with the default values
Comfort verdict: Comfortable stair.
Complies with IRC / IBC
| Step formula 2R + G | 631 mm | 600 – 660 mm |
| Tread depth | 331 mm | ≥ 280 mm |
| Pitch | 31.9° | 30° – 40° |
| Riser | 175 mm | ≤ 197 mm |
| Going | 281 mm | ≥ 254 mm |
| Clear width | 1,000 mm | ≥ 914 mm |
| Guarding height | 900 mm | ≥ 864 mm |
Steps
| Risers | 16 pcs |
| Riser height | 175 mm |
| Run (going) | 281 mm |
| Tread depth (run + nosing) | 331 mm |
| Open riser gap | 125 mm |
| Pitch | 31.9° |
| Nosing-to-nosing distance | 331 mm |
Structure
| Tread area | 4.969 m² |
| Bracket spacing on stringer | 331 mm |
Building codes
Under the IRC, a landing must be at least as wide as the stair it serves, and at least 36 in (914 mm) deep where the stair runs straight on. Approved Document K asks for landings at least as wide and as long as the narrowest flight, so on a switchback keep the landing depth E at least equal to the flight width W. On commercial and industrial stairs in the US, the IBC applies instead: risers of 4–7 in and treads of 11 in or more.
The calculator checks comfort (2R + G, going and pitch) and the building code of the country you pick in the panel (here, IRC / IBC for a dwelling): only the limits that code sets for riser, going, 2R + G, width, pitch, risers per flight and guarding. It is for guidance only: always confirm the code in force and local rules.
Building codes by country, with sourcesParts with the default values
The DXF (AutoCAD 2007, in millimetres or inches) carries these parts with their shop marks: one dimensioned sheet per part, the general arrangement and the bill of materials with weights.
What's in the DXF- Z1StringerTube 100×50 × 1
- Z2StringerTube 100×50 × 1
- S1BracketTube 100×100 × 1
- S2BracketTube 100×100 × 14
- E1Treadt=50 × 15
- M1Landing frameTube 100×50 × 2
- M2Landing frameTube 100×50 × 2
- R1Landingt=50 × 1
- P1PostTube 80×80 × 2
- B1HandrailTube Ø42.4 × 1
- B2BalusterRound bar Ø16 × 15
- B3HandrailTube Ø42.4 × 1
Frequently asked questions
How do you calculate a U-shaped stair with a landing?
Like a straight stair with one extra riser for the landing: divide the total rise by all the risers and each flight's run by its treads. The difference is that the landing takes part of the opening's length (the depth E), so subtract it before sharing out the treads.
How much space does a switchback stair need?
In plan, the width of two flights by the length X. The Summary panel shows the footprint, and it updates as soon as you change W, E or the number of steps.
U-shaped stair with a landing or with winders?
With a landing, like this one, it is safer and more comfortable. Without a landing, the turn is made with winder treads: that is the U-shaped winder stair, which has its own calculator in wood.