The span is the horizontal distance that a structural element bridges. For a staircase, the span is the horizontal distance from the first to the last tread (a straight run of treads without an intermediate landing). For bridges, cranes and windmills, span refers to the horizontal reach or clear distance between supports.
Span in different structures
| Structure | Meaning of span |
|---|---|
| Staircase | Horizontal distance from the first to the last tread of a straight flight |
| Bridge | The clear distance between the supports |
| Crane | The horizontal reach of the crane arm |
| Windmill | The distance from the axle to the sail tip (half the sail length) |
| Balcony | The horizontal cantilever beyond the facade |
Span in staircases
A stair flight is an uninterrupted run of treads:
- Single flight: a straight staircase without an intermediate landing
- Double flight: two stair runs with an intermediate landing
- Multiple flight: three or more stair runs (for example in a stairwell)
Guidelines for staircases
| Dimension | Guideline |
|---|---|
| Maximum flight | 16-18 treads without a landing |
| Optimal riser height | 170-180 mm |
| Optimal going | 220-250 mm |
| Step formula | 2x riser + going = 570-630 mm |
Tips for the DIYer
- Limit a stair flight to a maximum of 18 treads; then provide a landing
- Measure the span horizontally, not along the pitch line
- For bridges and balconies: the greater the span, the heavier the structure must be
- Check the step formula to create a comfortable staircase
Related terms
- Flight: uninterrupted run of treads (synonym for stair flight)
- Landing: horizontal platform between two stair runs
- Clear span: the distance between two supports of a beam or bridge
- Cantilever: the projecting part beyond a support point