Why a Curved Staircase Needs a Structural Engineer as Well as a Staircase Designer

Clients sometimes ask why we insist on involving an engineering consultant when we've been making these things for two decades. It's a fair question. The honest answer is that our drawings describe a shape, and a shape is not a load path. Those are related documents, but they answer different questions, and only one of them satisfies building control.

There's a more interesting reason too, and it has to do with geometry rather than paperwork. A straight flight loaded from above mostly bends. Put the same weight on something that sweeps, and it bends and twists at once, because the load sits offset from the axis running through the supports. Torsion, in other words. Straight stringers barely encounter it. Curved ones live with it constantly.

That single difference reshapes almost everything downstream.

Quick Answer: Who Owns What

The staircase designer resolves geometry, proportion, materials and how the piece is made: sweep radius, riser count, tapered setting-out, string laminations, handrail templating, finishes.

The structural engineer resolves whether it stands up and stays comfortable: load paths, thrust at the supports, deflection, connection details, and how the surrounding building takes what the flight hands to it.

Neither role covers the other. A curved staircase requires both, and the awkward truth is that the overlap between them is where most problems hide.

How a Curve Loads Differently

Three effects come up on nearly every commission.

• Torsion: Because the walking surface is offset from the line joining top and bottom supports, weight applied anywhere along the sweep produces a twisting moment. The flight, in effect, tries to unwind itself. Laminated timber handles this reasonably well; slender steel spines handle it less forgivingly, and a spine that looks generous on a drawing can prove disappointingly flexible in torsion.

• Horizontal thrust: Helical and curved flights push sideways at their bearings. Somebody has to catch that force. If the top bearing sits on a timber trimmer that was sized for vertical staircase load alone, the trimmer will move, and movement at the head of a flight shows up as creaking within a year or two. Concrete and masonry surrounds absorb thrust easily. Timber floor structures frequently need strengthening, and that decision belongs to an engineer rather than to us.

• Vibration: This one gets overlooked most often. People descend stairs faster than they walk on level floors, with step frequencies reaching around 4 Hz, and descent generates larger forces than ascent. Where a flight is slender and lightly damped, those footfalls can excite its natural frequency and produce a bounce that nobody expected. SCI P354 treats staircases below roughly 12 Hz as low-frequency for assessment purposes, and published research on slender feature stairs recommends keeping the fundamental frequency above about 5 Hz to avoid first-harmonic excitation.

Nothing about a stair failing building control here. A springy staircase is usually perfectly safe. It just feels cheap, which on a piece costing what these cost is arguably worse.

Two Sets of Drawings, Two Different Jobs

Confusion tends to start with the word "drawings", since both parties produce them and they look superficially similar.

Question	Answered By	Typical Output
What shape is it, and how is it made?	Staircase designer	Setting-out geometry, tapered step layout, lamination build-ups, handrail templates
Will it carry imposed load safely?	Structural engineer	Calculations to BS EN 1991-1-1, member sizing, deflection checks
What happens at the top and bottom bearings?	Both, jointly	Connection details, trimmer sizing, fixing schedules
Can the guarding resist a sideways push?	Structural engineer	Balustrade fixing design to BS 6180
Does it satisfy Part K geometry?	Staircase designer	Riser, going, pitch and headroom verification
Does the surrounding structure cope?	Structural engineer	Floor and trimmer assessment, strengthening where needed

Two numbers worth knowing, since they anchor a lot of the arithmetic. Imposed loading for stairs comes from BS EN 1991-1-1 with its UK and Irish National Annexes. Barrier loading comes from BS 6180, which sets a horizontal line load of 0.36 kN/m for areas serving a single dwelling, rising to 0.74 kN/m for other residential situations and higher again in commercial settings where crowding is possible. Guidance for helical and spiral geometry sits in BS 5395-2, though note its scope covers flights circular on plan, so elliptical work runs slightly beyond it.

Geometry itself remains governed by Approved Document K in England and Wales and Technical Guidance Document K in Ireland. Compliance there is our responsibility. Structural adequacy is not.

Where the Two Disciplines Have to Meet

Curved stairs require tighter alignment between the parties than any other element we make.

Five moments matter more than the rest:

1. Fixing the opening. Trimmer positions and sizes get decided here, and they depend on thrust the engineer has not yet calculated. Circular, but unavoidable. Best resolved with an early assumed load and a later confirmation.

2. Choosing the primary material. Timber, steel and concrete produce entirely different structural design routes. Switching later restarts the calculations. Our comparison of materials for bespoke curved work is worth reading before this point arrives.

3. Setting balustrade fixings. Glass channels and baluster shoes impose local forces into the outer string. Where those fixings land changes the string section required.

4. Confirming site tolerances. Fabrication happens to millimetres; buildings arrive with rather less precision. Somebody must decide who absorbs the difference.

5. Signing off before manufacture. Once a laminated curve has been pressed, changes are expensive in a way that flat components never are.

I'd add a slightly contradictory note. On modest residential commissions with masonry surrounds and a conventional timber flight, the engineering input can be genuinely brief. Not every project needs a fortnight of analysis. What matters is that somebody qualified has looked, rather than everyone assuming somebody else did.

What Late Involvement Actually Costs

Cost is the reason people delay, and delay is the reason cost increases. There's a pattern to it.

When calculations arrive after the opening has been formed, three outcomes are common: the trimmer needs replacing, the flight needs a heavier section that changes its proportions, or the balustrade fixing detail has to be redesigned around structure already built. Any one of those is manageable. Together they can add weeks.

Sequencing helps more than money does:

• Bring the engineering side in during concept design, not after planning

• Share the setting-out geometry as soon as the sweep is broadly settled

• Agree who is responsible for site survey and who carries dimensional risk

• Get the structural drawings issued before ordering material

Programme expectations are covered in our piece on how long a bespoke flight takes to design and install, and the questions worth putting to any manufacturer are set out in what to ask before you commission.

For commercial and hospitality work the picture tightens further, since higher imposed loads, greater barrier forces and heavier footfall all push the analysis harder. A feature flight in a hotel lobby is a different problem from a domestic one, even when the shape looks similar.

Frequently Asked Questions

Does a timber curved staircase need calculations, or only steel and concrete ones?

Timber flights need them too. Material does not determine whether structural verification is required; span, support conditions and loading do. A laminated timber staircase carries the same imposed loads as a steel equivalent, generates similar thrust at its bearings, and must satisfy BS EN 1991-1-1 alongside the relevant timber design code. Building control in Ireland and the UK can request calculations for any bespoke flight. Assuming timber is exempt because it feels traditional is a common and occasionally expensive mistake.

Talk to Bavari Before the Opening Is Formed

Getting both disciplines around the table early costs very little. Discovering the mismatch after the floor has been trimmed costs a great deal more.

Bavari designs, manufactures and installs custom staircases across Ireland and the United Kingdom, working alongside architects, engineers and interior designers on residential and commercial projects. Take a look at our bespoke curved staircases service, or read what we need from you in planning your staircase.

Contact the Bavari sales team to talk through your project with someone who has resolved this coordination hundreds of times.