How to measure a stairwell for a spiral staircase

Three figures provide an initial feasibility check for a spiral staircase: finished floor to finished floor, the clear opening above and the obstruction envelope around it. They are not a complete quotation pack. A fabricated stair also needs exit direction, the usable lower-floor footprint, proposed column centre, intended use and delivery limits.

Finished floor to finished floor

This is the total rise, and the figure most often got wrong. Measure vertically from the surface you will walk on downstairs to the one you will walk on upstairs. A domestic rise is often around 2600 mm (about 8 ft 6 in), but yours is whatever you measure. The stair divides that height into equal risers, so an error lands in one step instead of spreading across the flight.

The classic mistake is measuring to the subfloor: bare chipboard, the joists, a screed still waiting for its tiles. If the finish is not laid, add its full build up, tile plus adhesive bed or board plus underlay. Fifteen millimetres of unlaid tile sounds trivial, but once it is down the bottom riser is 15 mm shorter than all the others, the odd step people trip on.

1. Fix the finished level at the bottom and at the top, and note what makes each of them up, layer by layer.
2. Measure between them with a laser or a tape, not by adding up figures off a drawing.
3. Repeat at three or four points around the opening, because timber floors dip and screeds are rarely flat.
4. Report every reading from a fixed datum, with the floor build-ups. The designer defines the controlling level, landing interface and any approved packing detail.

The opening in the floor above

Measure the clear structural opening between the finished faces, not the bare sides of the trimmer joists if those are still to be lined. A 12.5 mm plasterboard lining on two sides takes 25 mm out of the opening, enough to stop a stair going in. Take both directions, then both diagonals: a square opening is rarely square, and unequal diagonals mean a parallelogram, which the workshop has to know about.

The diameter of the stair is not the size of the hole. A spiral turns inside the well and needs clearance to the outer edge of the tread and to the handrail, which stands proud of the treads, so the opening has to be larger than the nominal diameter. How much larger is set by the design, so send the opening and let the workshop confirm the largest stair that passes through it.

Nominal stair diameter is not usable walking width. The centre column, tread shape, handrail and outer clearances reduce the walking zone. State whether this is primary or secondary access and let the responsible designer calculate the usable width under the applicable national rules.

Where you walk off at the top

The exit direction drives the number of treads and the total rotation, because every tread turns through the same angle and they must add up to the turn you need. The top tread has to land square to the landing edge and open into the room, not into a wall or the side of a door. Send a plan sketch showing where you step off, and say whether you turn clockwise or anticlockwise going up.

Survey the headroom obstruction profile

You cannot measure vertically from a future tread nosing before the stair geometry exists. Instead, locate opening edges, trimmers, beams, ceilings, ducts and roof slopes in plan and elevation from fixed datums. Record the underside level and horizontal position of every obstruction.

The stair designer then checks headroom along the proposed walking line against the applicable national rule. Do not choose one worst-looking dimension or invent the future tread position on site.

Map the lower-floor envelope and column support

Draw the usable footprint below the opening, the proposed column centre, doors, circulation routes, radiators, furniture and other obstacles. Record the clear area in plan from fixed walls. This determines whether the stair can occupy the room and where its base can land.

A spiral concentrates load at the column base. Describe the floor build-up and supporting structure before the base plate is designed. A suspended floor, floating screed or unknown slab needs project-specific structural review. Any new trimming or support frame belongs with properly designed steel structures.

The rules are national

There is no single European figure for riser height, going, handrail height or clear width. They are set nationally: DIN 18065 with the relevant Landesbauordnung in Germany, the warunki techniczne in Poland, NF P01-012 for the garde-corps in France, Approved Document K in England and Wales, DM 236/89 with local norms in Italy, CTE DB-SUA in Spain. Industrial access stairs follow EN ISO 14122 instead.

One distinction matters more than any figure: in most national rules a spiral that is the only access to a habitable storey is judged far more strictly than a secondary stair to a loft or a gallery, and an external stair is different again. The barrier around the new opening answers to the same rules, including the national limits on gaps in a guard: the options are set out under railings and balustrades.

Put interface tolerances on the approved drawing

Site fit is controlled by the drawing-specific functional and interface tolerances at floors, opening, landing, base and connections. EN ISO 13920 may govern general welded-construction tolerances only when the commission actually invokes it; it is not a substitute for the stated fit requirements.

Check the as-built shell, finished levels and lined opening before release. Any permitted packing, adjustment or planned site connection must be shown and assigned, not improvised after delivery.

What to send the workshop

Next

With this survey pack, the workshop can assess geometry and quotation scope. Use stairs and access for the stair and railings and staircases for the guard around the opening.