Geoff Wilkinson provides a guide to specifying the correct spiral staircase
I was recently contacted by an architect who was struggling to fit a spiral staircase into an office and wanted to know if, as building inspector, we could consider a relaxation of the regulations to accept a reduced size. After a quick conversation we discovered that they had, in fact, selected the wrong type of spiral staircase and could install a smaller size without the need for a relaxation of the rules. So the first thing to check is that you are using the correct type of spiral stair. This is set out in British Standards Code of Practice BS 5395. Here’s a guide:
Category A Small private spiral staircase used by a limited number of people generally familiar with the staircase, for example an internal staircase in a dwelling serving one room that is not a living room or kitchen; spiral staircase to a small room or plant in an office, shop, or factory and not used by the public; or fire escape for a small number of people. Rise per tread 170-220mm; min. clear width 600mm; min. going centre 145mm.
Category B Private spiral staircase similar to category A, but also providing main access to upper floor of a private dwelling. Rise per tread 170-220mm; min. clear width 800mm; min. going centre 190mm.
Category C Small semi-public spiral staircase intended to be used by a limited number of people, some of whom may be unfamiliar with the staircase, for example a spiral staircase in a factory, office, shop or common stair serving more than one dwelling. Rise per tread 170-220mm; min. clear width 800mm; min. going centre 230mm.
Category D Semi-public spiral staircase intended to be used by larger numbers of people, some of whom may be unfamiliar with the staircase, for example a spiral staircase in a factory, office, shop or common stair serving more than one dwelling. Rise per tread 150-190mm; min. clear width 900mm; min. going centre 250mm.
Category E Public spiral staircase intended to be used by large numbers of people at one time, for example a spiral staircase in a place of public assembly. Rise per tread 150-190mm; min. clear width 1000mm; min. going centre 250mm.
Other common mistakes we see on applications include:
• Picking the wrong combination of treads and risers – the minimum tread size is 220mm from the nosing to the nosing of the next tread and the riser must be between 190mm and 220mm. But not all combinations can be used in all cases and the pitch must never exceed 42 degrees.
• Picking the wrong width – in a dwelling there are no recommendations for minimum staircase widths, so you can choose anything you like – but remember anything less than 600mm is impractical for moving furniture and won’t comply with the BS Code of Practice.
• If the flight comprises both tapered and straight treads, the tapered treads must maintain the same going as the straight section. Use the diagram below to ensure you measure this correctly:
• Remember that a staircase with open risers cannot be used where children under five are likely to be present, so these require a cross-rail or similar so that a 100mm sphere cannot pass through the open risers.
• Landings are required at the top and bottom of each flight and should be the same size as the stair width – for example: 600 x 600mm for a 600mm-wide stair, 800 x 800mm for a 800mm-wide stair and so on.
• The landing must be clear of any door swing by 400mm, so a 600mm-wide stair could require a 1000mm-deep landing.
• Clear headroom of 2000mm should be provided over the whole width of your staircase, but this can be reduced to 1900mm in some situations, for example for loft conversions. Clever positioning of an appropriate rooflight can help increase the available headroom where floor-to-ceiling dimensions are extremely tight.
Geoff Wilkinson is managing director of Wilkinson Construction Consultants www.thebuildinginspector.org