A common request we hear from clients is ‘I would like the smallest, tightest spiral possible’.  Obviously, people have restricted space and do not want the spiral to take up too much room.   The trouble is that there is not a definitive answer to this.

It is physically possible to manufacture a spiral to virtually any diameter but there are two fundamental aspects to consider, and these are the building regulations and usability.

Usability

Whatever the diameter, the spiral staircase must be practically usable.

To see how usable a spiral is we need to look at the actual unobstructed usable width of the spiral staircase which is the distance between centre column and inside of handrail. This is called the Clear tread width.  This is less than half the diameter (radius) as the centre column and handrail must be factored in – so might be smaller than you think.  Keep in mind the smaller the diameter the closer you are to the narrow part of the step near the centre column.

Of course, what is considered usable, comfortable, and safe is subjective and varies depending on experience, expectation and the users’ age, height, and weight.   Expectations of usability will also be influenced by the application of the spiral.  For example, you would not be as concerned about a smaller occasional use spiral as you would a flight in constant demand.

Personally the smallest spiral I have walked up is a 1000mm diameter.  I am a male of average height and build and found it extremely tight with hips brushing the handrail and the centre column.   I would certainly not recommend a spiral of this diameter –  although of course it could be considered a safer alternative to a ladder.

For larger refurbishments, new builds, and any work subject of building control approval in the UK, building regulations will have a role in dictating the diameter (clear tread width).

Building Regulations

Building regulations in the UK divide spiral staircases into two categories for domestic applications.  These are outlined in BS – 5395 and are simply –

• Category A – secondary stair – clear tread width 600mm
• Category B – primary stair – clear tread with 800mm (900mm in Scotland)

Click to view full spiral category regulations

So, what diameter do you need whilst adhering to this regulation and the clear tread widths?  This varies depending on the diameter of the centre column and handrails and this will be slightly different from model to model and manufacturer to manufacturer.

Our off the shelf Klan spiral stair model for example would need to be 1500mm to comply as a secondary stair whilst a model from our custom-made spiral staircase range would be 1400mm.  This is because the column on the custom models is smaller.

The smallest diameter to comply with a category B spiral (primary spiral) would be between 1800-1900mm.

The below is an approximate guide with precise figures varying on spiral type.