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Making buildings: Mike Tonkin on the Water Tower's spiral stair

Words:
Jan-Carlos Kucharek

Introducing a new series on architects’ details and the decisions that informed their thinking, the Tonkin Liu director explains what drove the design for the spiral stair at the practice's Stephen Lawrence Prize-winning Norfolk house

The sculpted soffit celebrates the fact that each tread is comprised of two mechanically connected CLT pieces.
The sculpted soffit celebrates the fact that each tread is comprised of two mechanically connected CLT pieces. Credit: Dennis Pedersen

What ideas drove your initial design thinking for the staircase?

The decision to run the stair tower alongside the building was driven by all the water tank’s supporting structure at the top as well as the need to put all the accommodation in the volume below it. Floor to ceiling heights were tall in the original structure so a pure spiral stair would have been challenging for users, while a dogleg stair, with a landing at both ends, would have made the runs too steep.

We were initially inspired by traditional Georgian stairs – I grew up in Bath – and the way they run straight and then fan around at their ends; but more so by their French equivalents, which fan all the way along the run. Their form might have originated from right-handed knights defending Norman keeps but it seemed to apply very well here.

How does it meet building regs guidance?

Building regs say you’re supposed to have a landing every 16 stairs, but given that we were dealing with an existing condition, we explained to the building control officer that the stairs were going to be unconventional. The fans on the turn are quite big, which allows you to stand in the widest part while someone passes. There is also a regs requirement for a minimum of 50mm width at the tightest part of the turn but we exceeded that throughout. A lot of the design criteria fell between the regs for a spiral staircase and a straight stair. In the end, building control was okay with its hybrid nature.

It looks expensive but the irony is that it’s not

So how is the stair engineered?

Along the lines of the way Robert Adam and Wren did their cantilivered stairs: supported at three points and delivering load down the outer unsupported edge– it’s how a compression spiral works. 

The stair tower is four storeys high, yet our engineer Mervyn Rodrigues managed to get their structural thickness down to 80mm. The reason why they’re so thin is that the compression spiral is transferring most of the load down through the outer edge and so the strip foundations for the timber walls at ground level step in to take account of this. 

The design is extremely simple. Small cut-outs for the treads were cut from the timber wall before they came to site. Each tread is made up of two layers of 4mm plywood almost invisibly making up the tread rise. Every second piece of CLT slides into and engages with the wall, a shoulder on each one to ensure perfect alignment. The interstitial piece of CLT slides in and is glued and mechanically bolted through to the next wall-engaged piece. With only every other CLT piece interfacing, we didn’t compromise too much of the wall’s structural integrity.

With each tread made up of two pieces, I suppose we thought to express that on its soffit where it steps. But it’s part of a tradition. Robert Adam created very sculpted undersides to his stairs that gave them real elegance. When you look up the well from below, it looks a bit like an optical illusion.

The timber stair tower not only supports itself but brings lateral stability to the steel tank structure adjacent.
The timber stair tower not only supports itself but brings lateral stability to the steel tank structure adjacent.

How was it built?

Like pretty much everything else on this building: locally. The glulam timber walls and treads were prefabricated by Binderholz and brought to site but it was assembled by two carpenters from the village who built the whole thing in five days, working from the bottom up. The client acted as the main contractor to make savings on the project and initially he felt we were being too sculptural with our design. But a steel stair would have been £15,000 per flight and a stone one £12,000. This timber one got built for under £7,000 including labour. It looks expensive but the irony is that it’s not.

What about Part B regs?

Regs have changed since Grenfell, and building control is understandably more risk averse. The fire officer looked at everything. Being the sole means of escape, this had to be a protected stair. There are mains-fed sprinklers on the ‘bridge’ to the water tower and in all the rooms, adding £40,000 to the project.

What’s the story with the balustrade?

Well, we got it for nothing! All the balusters are made from the many rusty tension rods that were removed from the tank structure, cut to length, rubbed down and just grit filled into drilled holes in the treads. On the turns, we could have used one baluster to meet regs but it looked odd, so we decided to have two balusters per tread regardless. The effect is a visual tightening and expansion, which is satisfyingly rhythmic and sculptural.

What do you like most about the design?

I suppose it’s the effect the spiral design has on the integrity of the whole system: equalising forces and helping keep everything stiff. How many buildings with 100mm walls are four storeys tall? If you took the stair out, the walls would have to be twice as thick. The stair tower also takes lateral loads, helping hold the original tank structure in place. In effect, wood is stabilising steel, which we think is a lovely inversion. 

  • The staircase’s continuous fan allowed the stair design to deal with the existing floor-to-floor flights.  Grd-2nd FL= 80mm stair tread + 80mm wedge tread. 2nd-3rd FL= 120mm stair tread + 90mm wedge tread
    The staircase’s continuous fan allowed the stair design to deal with the existing floor-to-floor flights. Grd-2nd FL= 80mm stair tread + 80mm wedge tread. 2nd-3rd FL= 120mm stair tread + 90mm wedge tread
  • Inspired by traditional French staircases, the CLT stair design is strikingly simple and modern.
    Inspired by traditional French staircases, the CLT stair design is strikingly simple and modern. Credit: Tara Wilkhu
  • The prefabricated stair and wall elements were brought to site and assembled by two carpenters, who constructed it, tread by tread, in five days.
    The prefabricated stair and wall elements were brought to site and assembled by two carpenters, who constructed it, tread by tread, in five days. Credit: Tonkin Liu
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