The ‘clear width’ is the unobstructed width of a staircase. If the spiral staircase has only an outer handrail (as is common for most) then its clear width would be the distance from the inside face of the outer handrail to the face of the central pillar:
The clear width is used for all staircases, not just spirals, and is always measured in this way. The reason this measure is used is relatively simple, in some cases handrails can overhang the treads by quite some distance so even though the treads may be wide, the actual useable area of the tread is much smaller. The clear width removes this ambiguity and ensures there is always a safe amount of useable space on a staircase.
What is the minimum width for staircases?
The minimum clear width for a staircase depends on the building’s design and what it is being used for. The requirements here in the UK are outlined below (the minimum diameter applies only to spiral staircases):
| Stair category | Clear width (mm) | Minimum Staircase Diameter (mm) |
| Small private spiral staircase — used by a limited number of people e.g. a staircase in a home which doesn’t go to a communal room like a kitchen or an access stair to a small room not used by the public for a commercial property. | 600 | 1448 |
| Private spiral staircase — a main staircase within a home. | 800 (900 in Scotland) | 1830 (2083 in Scotland) |
| Small semi-public* spiral staircase — used by a limited number of people such as a communal staircase leading to more than one flat. | 800 (900 in Scotland) | 2100 (2300 in Scotland) |
| Semi-public* spiral staircase — used by a larger number of people such as a stair serving a large area in a factory, office or shop. | 900 | 2300 |
| Public spiral staircase — intended to be used by large numbers of people at the same time (meaning room is needed for passing). | 1000 | 2500 |
* defined as a space serving up to 50 people and serving a limited floor area
Approved Document K in England and Wales actually offers a little wriggle room here saying that where space is limited, the diameter is at the discretion of the building inspector. As such, if you have a tight space it is worth getting building control’s opinion to see if it would be possible to install a spiral staircase with a slimmer diameter. We offer spiral staircases with diameters down to 970 mm. Also if the staircase is secondary access to an area or the room is not classed as habitable then there is no width requirement but we would always recommend that this is agreed with building control before making any final decisions.
If you live outside the UK then local laws may apply but these widths can still provide a useful starting point for deciding how wide you would like your treads to be.
Once you are measured up, take a look at our gallery and get in touch or send a quote request. Our friendly team will help to design a made to measure staircase for your space.
