Central-stringer stairs: prices and rules

The central stringer is the staircase that makes treads look as if they float. A single axial beam carries cantilevered treads on both sides, with no side stringers: the eye passes underneath, light comes through, the room feels larger.

How it works

The stringer is a square S235 steel tube from 100 × 100 to 120 × 120 mm, or a slimmer welded box for taller stairs. Each tread is clamped to a bracket welded to the stringer, then bolted or welded depending on the material. Solid oak treads are 40 to 50 mm thick; steel treads are folded and stiffened. The engineering is not decorative: a 120 mm stringer with 6 mm walls spans 2.85 m with no perceptible deflection; smaller and the stair bounces underfoot and starts to vibrate. We check deflection — around L/300 under load — and natural frequency, because a trembling stair does not just feel wrong, it fatigues its own fixings.

Pitch, treads, headroom

Four numbers govern comfort, and they are all linked. Pitch sits between 25° and 35°, with the sweet spot at 28° to 32°. Riser height must stay between 17 and 18 cm: above 19 cm the stair feels hard, below 16 cm it stretches out and costs a lot of floor opening. Going stays between 24 and 28 cm. Blondel’s rule links the two: 2h + g should land between 58 and 64 cm — 17.5 cm rise with a 26 cm going gives 61 cm, the middle of the comfortable range. Headroom, measured vertically from tread nosing to ceiling or floor opening, must be at least 190 cm, and we aim for 200 cm. A recent Saumur project: 2.85 m to climb, 16 risers of 17.8 cm, 26 cm goings, 90 cm width, 205 cm headroom.

Floor opening and footprint

Floor length is simple arithmetic: (number of treads − 1) × going. Sixteen 26 cm treads project 3.90 m, so the opening needs 3.60 to 4.00 m depending on the last tread position and slab thickness, and at least 90 cm clear plus the guardrail. A central stringer needs a longer opening than a side-stringer stair of the same height, because headroom cannot be gained by raising the treads. If the existing opening is too short: increase the pitch (at the cost of comfort), switch to a quarter turn with a winder, or move a beam to open the floor further. We never draw without the opening plan and the finished ceiling height.

Guardrails and safety

Any stair rising more than 60 cm needs protection. The guardrail fixes to the treads or the stringer and follows the pitch: 1.00 m high measured vertically from the nosing, maximum 11 cm gaps, a continuous graspable handrail. On a central stringer the rail is usually a simple side frame of bars or tensioned cables, fixed to the treads themselves. The floor opening needs protecting too, so that nobody falls from the landing.

Prices by configuration

A central-stringer staircase is entirely made to measure: price depends on height, number of treads, species, guardrail and installation complexity. Ranges below include tax, a steel stringer, oiled solid oak treads, installation and a simple guardrail unless stated.

Lead times and installation

Allow six to eight weeks from plan approval to installation. Design alone takes one to two weeks: survey, dimensioned drawing, deflection calculation, fixing choice, sign-off with you. Fabrication takes two to three weeks, finishing adds a week for oiled treads, and installation one to two days depending on height. We install after heavy building work and before painting, usually alongside electrics when step lights are planned. Clear access matters: a 3.90 m stringer does not fold around a twelve-tread stairwell.

More detail on the staircase page of the catalogue, and on the solid wood guide for species and maintenance.