Cable-supported Skylight

What is a cable-supported skylight?

A glazed roof in which pre-tensioned cables carry part of the load that would otherwise go into bending in the steel members. The cables are set below or within the roof plane, tied back to the perimeter, and tensioned so that the steel above them works axially rather than in bending.

The result is a roof of the same span with substantially slimmer members — which on a skylight is the entire point, because every millimetre of member depth is sky the occupant does not see.

Span range

10 – 45 m

Stainless spiral strand

12 – 36 mm

Laminated insulating units

28 – 48 mm

Deflection under snow

L/100 – L/200

It sits among three neighbouring systems:

A useful way to decide between 02 and 07: curvature and cables are two solutions to the same problem. Both get load out of bending. If the architecture will accept a curved roof, curvature does it without pre-tension or a tensioning sequence — though the supports still have to take a curved roof's thrust. If the roof must stay flat, cables are how you keep the members slim. That choice belongs at concept stage. Where both sit among every other kind of skylight, sorted by span, is in skylight types explained.

• 05 Cable structures The cables are the structure, carrying a covering over open space.

• 06 Cable-supported curtain wall Vertical cables replace mullions; glass is point-fixed to them.

• 07 Cable-supported skylight Cables reinforce a steel lattice so its members stay slim.

• 08 Cable net structures Cables in two directions form the surface itself.

Why cables let the steel get thinner

A flat member spanning 30 m carries its load in bending. Stress concentrates at the extreme fibres, most of the section does very little, and the only way to span further is to go deeper. Depth grows with span, and weight grows with depth, which is why a long flat rooflight built conventionally ends up as a roof with some glass in it rather than a skylight.

Introduce a cable below the member, tie it back, and tension it. The cable pulls up against the load. The steel above now works predominantly in compression with the cable in tension, and the pair behave as a structural depth far greater than the member alone. Sections come back down and the roof reads as a lattice.

• The member alone Load is carried in bending, so the only way to span further is to go deeper — and every millimetre of depth is sky the occupant does not see.

• With a cable below The cable, tied back and tensioned, pulls up against the load. The structural depth is the distance between steel and cable, so the member itself can be slim.

Schematic. Deflections and member sizes are not to scale.

Three things follow that matter at design stage:

• The structural depth is the distance between steel and cable, not the depth of the member. A slim member held off a cable a metre below it is structurally deeper than a heavy member on its own. That depth occupies space below the roof, which has to be coordinated with the soffit, the lighting and any services.
• Pre-tension is what makes it work. The cable must stay tight under every load case including uplift. A slack cable does nothing, and the member is back to carrying the load alone — at a size it was not designed for.
• The system is only as good as its anchorage. The cable tension has to be resisted at the perimeter, by a ring beam, an edge truss or the primary structure. As on every cable system, this is designed from the outset rather than added later.

Span range on this system: 10 – 45 m. Cable type is stainless spiral strand, 12 – 36 mm, with pre-tension derived from form finding.

Ponding: the feedback loop

Deflection under snow must not create a pond that increases the deflection. Falls are designed for the loaded, not the unloaded, shape.

That design note describes a mechanism that can run away, and it deserves expanding.

The loop. A flat roof deflects under load. The deflected area sits lower than the surrounding roof. Water — rain, or snowmelt — flows to the low point and collects there. That water is additional weight, concentrated exactly where the roof is already deflecting most. The roof deflects further. The depression deepens. More water collects.

On a stiff roof the loop converges: each increment of water produces less additional deflection than the last, and the roof reaches equilibrium. On a flexible roof, or a roof whose drainage cannot keep up, it may not converge. This is a recognised collapse mechanism, and it is the reason flat roofs are never designed to be truly flat.

Why it needs particular attention on a cable-reinforced roof. The system is deliberately more flexible than an equivalent roof in heavy steel — that flexibility is what buys the slim sections. Flexibility is an asset structurally and a risk hydraulically, so the two have to be designed together rather than by different people at different times.

• 1. 1 The roof deflects under load.
  2. 2 Water runs to the low point.
  3. 3 Its weight lands where the deflection is greatest.
  4. 4 The roof deflects further — and round again.

• Fall set on the drawn shape A roof drawn with a 1:60 fall that deflects 150 mm at mid-span can lose its fall where it matters. Water collects below the outlet level.

• Fall set on the loaded shape The roof is built with more camber than the finished look suggests, so under snow it still falls to an outlet placed where the water will be.

Deflections exaggerated so they can be seen.

What "falls designed for the loaded shape" means in practice:

• The fall is set on the deflected geometry under the relevant load case, not on the geometry as drawn. A roof drawn with a 1:60 fall that deflects 150 mm at mid-span may have no fall at all — or a reverse fall — where it matters.
• The roof is therefore built with more camber or fall than the finished appearance suggests, so that it still drains when loaded.
• Outlets go where the water will actually be, which is not necessarily where a plan drawing puts them.
• Deflection limit: L/100 – L/200 under snow. The limit is defined against snow rather than wind — a sign that on this system snow is the governing serviceability case.

Drainage on a roof that moves

A moving roof plane needs seals that move with it. Gutters and outlets are sized for the deflected geometry.

Three consequences beyond the ponding question:

Gutters have to work in both shapes. A gutter set into a roof that deflects 100 mm has a different gradient loaded and unloaded. Size and set it out for the loaded case; check it still drains in the unloaded one.

Overflow is not optional. If the primary outlets block and the roof is flexible, the failure mode is not a leak — it is accumulating load on a roof that gets more receptive to water as it deflects. Overflow provision on this system is a structural safeguard, not just good practice.

Seals move with the plane. Every joint in the glazing line opens and closes as the roof deflects, and the perimeter joints take the most. Watertightness is Class RE to EN 12154, verified by mock-up — and on a moving roof the mock-up is doing real work, because it tests the actual geometry and the actual joints rather than a system's published performance. Programme it before fabrication is released.

Snow, and why it governs here

Snow and live load are project-specific, to EN 1991-1-3 or the local code.

Wind governs a vertical cable wall. On a flat glazed roof, snow usually governs, and it behaves less predictably than the uniform figure suggests:

• Drifting. Snow accumulates against upstands, in valleys and wherever the roof form creates shelter. Local depth can be several times the uniform design figure — and drifts collect in low points, which is where deflection is greatest, which is where ponding starts.
• Asymmetric loading. A pre-stressed system loaded unevenly behaves differently from one loaded uniformly. Half a roof loaded with snow while the other half is clear is frequently a worse case than the whole roof loaded, because it distorts the geometry the pre-tension was set for.
• Melt and refreeze. Meltwater running to a low point and refreezing blocks the outlet that the ponding calculation assumed was working.
• Sliding. Where the roof is steep enough to shed, what it sheds onto needs designing for.

• Uniform The uniform figure to EN 1991-1-3 or the local code. Snow on a real roof behaves less predictably.

• Drifting Snow builds against upstands and in sheltered low points, several times the uniform depth — at the places ponding starts.

• Asymmetric Half the roof loaded, half clear, often governs: it distorts the geometry the pre-tension was set for.

Depths and deflections exaggerated. Melt and refreeze adds a fourth case: ice blocking the outlet the ponding calculation assumed was working.

In the Caucasus, northern Iran, Turkey and continental Europe this is routinely the governing case on a long-span glazed roof. In milder climates it is often dismissed early in design and rediscovered in the analysis.

Glass and access class

Whether the glazing is non-fragile, restricted-access or walk-on changes the build-up entirely. Decide before the glass is specified.

Glazing build-up: laminated insulating units, 28 – 48 mm overall. Access and fragility: non-fragile to ACR[M]001; walk-on optional.

The access class is the decision that most changes the cost, and it is made too late on most projects.

• Non-fragile to ACR[M]001 means the complete assembly — glass, frame and fixings together — has passed a drop test establishing that a person falling onto it will not go through. It is an assembly property, not a glass property, which is why it cannot be assumed from the glass specification alone. This is the default: a fall during maintenance does not become a fall through the roof.
• Restricted access assumes nobody goes onto the roof except under a controlled system of work, and relies on that being enforced for the life of the building.
• Walk-on is a different build-up entirely — designed for deliberate, repeated foot traffic, with a sacrificial top leaf. Substantially heavier and more expensive, and the added dead load feeds back into the cable sizing.

Laminated always, because the roof is over occupied space and fragments must be retained in the frame if a pane breaks. The general behaviour of glass over people — breakage, heat soaking, condensation — is set out on free-form skylights.

Fixings: spider or rotule

Point-fixed spider or clamped rotule.

A spider is the four-armed articulated bracket familiar from glazed walls, picking up the corners of adjacent panes.

A rotule is the fitting at the glass itself — a spherical bearing seated in the drilled hole, allowing the pane to rotate freely in every direction relative to its support. A clamped rotule passes through a plain cylindrical hole and grips the glass between plates on either face, rather than sitting flush in a countersunk hole.

• Countersunk rotule Seated in a countersunk hole, flush with the outer face. The spherical bearing lets the pane rotate freely in every direction relative to its support.

• Clamped rotule Passes through a plain cylindrical hole and grips the glass between plates on either face, rather than sitting flush in a countersunk hole.

The dashed outline is the same pane turned about the ball. Tilt exaggerated.

The property that matters on a roof is the same as on a wall, only more so: rotation. The glass stays flat; the structure beneath it deflects and changes angle under snow. If the fitting cannot rotate, that angular difference becomes bending in the glass at the hole — the one place glass is least able to take it. On a roof where deflection is measured in L/100 and the load is snow rather than a gust, the fitting is rotating under sustained load rather than momentarily.

The tensioning sequence

Cables are stressed in a defined order. The sequence is part of the design, not a site decision.

A cable-reinforced roof is not stable until it is fully tensioned. Until then it is a lattice of slender members carrying load in bending — the condition they were specifically sized not to carry. So:

• The sequence is a set of design cases. Each stage is checked, because the partially tensioned structure is a different structure from the finished one.
• Stressing out of order overloads members. Tensioning one cable fully before its neighbours draws the geometry to one side and puts force through members at angles the design never considered.
• It is staged and surveyed. Tension is introduced in increments across the whole roof, with geometry checked against the model between stages, rather than cable by cable to final load.
• Temporary support is part of it. What holds the lattice while it is being tensioned is designed, not improvised.

• Staged across the whole roof Tension goes in by increments across the whole roof, with the geometry checked against the model between stages. Each stage is a design case of its own.

• One cable first Stressing a cable to its final load before its neighbours pulls the lattice out of shape and puts force through members at angles the design never considered.

For a contractor, the practical implication is that the tensioning sequence is issued as a document with the shop drawings, and the site is not free to depart from it.

On the Fanap Campus Complex — three flat roofs, one of 20 × 20 m and two of 26 × 17 m, and Best Special Steel Structure in Iran, 2022 — around eleven thousand turned components were fabricated to complete the cable systems.

Thermal and condensation

Thermal transmittance Ug 1.0 – 1.6 W/m²K.

With minimal framing, the glass and its edge seal carry essentially the whole thermal load. On overhead glazing the U-value matters for comfort as much as for energy, because it sets the internal surface temperature and therefore whether condensation forms above people's heads.

The general behaviour of overhead glazing — condensation risk, solar gain, daylight distribution — is covered on free-form skylights, and applies equally here. The point specific to this system is that condensate has to reach the same drainage that the rainwater uses, on a plane that moves. The frame is detailed to collect it and carry it to the gutter rather than letting it run to the low point of the deflected roof and fall.

What drives the cost

In order of leverage:

1. The access class. Non-fragile, restricted or walk-on is the largest single swing on this page. Walk-on build-ups are heavier, which resizes the glass, the fittings and then the cables beneath them. Decide it before the glass is specified, not after.
2. Span, and the perimeter that resists it. 10 – 45 m. The cable tension has to be anchored into a ring beam, edge truss or the primary structure, and if that element was not designed for it, stiffening it can cost more than the skylight.
3. Glass thickness, which the fixing and the span set together. Point fixings concentrate stress locally, so the glass is thicker than span alone suggests — the same relationship as on cable-supported curtain wall.
4. How many fitting and node types are unique. The same rationalisation argument that governs panel sets on free-form structures. One fitting type used two hundred times costs a fraction of eight types used across the same area.
5. Mock-up and tensioning. Both are real programme items on this system rather than overheads. A mock-up that has to be built and tested before fabrication is released sits on the critical path.

What to decide, and when

1. 01

   Concept

   The span, and whether the roof must be flat — because if it can curve, curvature may do the structural work instead. What is above and below it structurally. The snow figures for the site, or just the location.

2. 02

   Developed design

   Access class, which sets the glass. The fall, set on the deflected shape rather than the drawn one. Drainage layout and outlet positions against the loaded geometry. Indicative cable reactions issued to the structural engineer before the perimeter structure is fixed.

3. 03

   Technical design

   Cable schedule and fittings, the structural depth between steel and cable and its coordination with the soffit, the tensioning sequence, movement joints at the perimeter, and the mock-up scope.

4. 04

   Before tender

   A confirmed access class, a defined fall and drainage strategy, and a perimeter that is known to take the reactions. Tendering without these produces a price with a risk margin in it, because all three change the scope materially.