Free-form Skylights

What is an atrium roof?

An atrium roof is the glazed covering over a multi-storey internal void — the element that makes an atrium an atrium rather than a light well. It differs from a domestic rooflight in three ways that matter to everyone specifying one:

• Scale. Spans often run to tens of metres, so the roof is a structure carrying a glazed skin, not a frame holding a pane.
• Consequence. Anything that falls from it falls several storeys into occupied space, which sets the glass make-up and the safety classification before anything else is decided.
• Interaction with the building. An atrium roof drives the smoke strategy, the daylight, the cooling load and often the acoustics of every space that opens onto the void.

Span range

8 m – Unlimited

Light transmission

40 – 70%

Solar factor g, with solar-control coating

25 – 50%

Laminated insulating units

28 – 44 mm

The terms overlap in use. Broadly: a rooflight is an opening in a roof, usually domestic in scale. A skylight is the same thing, more often used at commercial scale. A structural glass roof is one where the glass itself, or the minimal framing supporting it, is the structure. An atrium roof is defined by what it covers rather than by how it is built. Free-form describes the geometry underneath all of them. The smaller types — domes, pyramids, ridge lights, barrel vaults and walk-on rooflights — and where each one gives way to an atrium roof are set out in skylight types explained.

Four jobs, all at once

A glazed roof has to admit daylight, exclude water, control heat, and be reachable for cleaning and replacement. On a flat rooflight these barely interact. On a curved surface each one constrains the others, and the order in which you resolve them determines whether the result works.

The short version of the conflict:

• More light transmission usually means more solar gain.
• Controlling solar gain with coatings reduces light transmission and shifts the colour of the daylight.
• Falls steep enough to drain well raise the roof and increase the glazed area, which increases both gain and cost.
• Access provisions for cleaning add structure, which reduces light and interrupts the surface.

Resolving these in the wrong order is how an atrium ends up with a beautiful roof that overheats, or a well-controlled roof that is gloomy under it.

Daylight: model it, don't assume it

Daylight modelled for level and glare, not assumed.

Light transmission on this system runs 40 – 70% depending on the glass and coating. That is a wide range, and where a project lands in it changes the character of the space completely.

Two things architects consistently underestimate on curved roofs:

Distribution, not average. A doubly curved roof delivers light unevenly by definition — the surface faces a different direction at every point. Average daylight factor tells you almost nothing. What matters is the distribution across the floor at the times of day the space is used.

Glare from the surface itself. A curved glazed roof produces reflections and bright patches that a flat one does not, and they move through the day. In retail and transport buildings, glare on a floor surface or a display is a more common complaint than heat.

Coatings, fritting or switchable glass are specified against the modelled result rather than chosen from a catalogue. Fritting is worth particular attention on curved roofs, because a gradient density can compensate for the parts of the surface that face the sun most directly — controlling gain where it occurs instead of dimming the whole roof.

• The same sun, a different angle at every point The parts of the surface facing the sun most directly take the most light and heat, so daylight reaches the floor unevenly — and it moves through the day.

• Frit density follows the sun A gradient frit can be densest where the surface faces the sun, controlling gain where it occurs instead of dimming the whole roof.

Sections. Each segment is shaded by how directly it faces one sun position.

Solar gain and the glass trade-off

Three numbers on the specification move together, and you cannot optimise all three:

Triple glazing at Ug 0.6 – 0.9 is worth its weight and cost in cold climates, or wherever the atrium is heated and internal humidity is high. In a hot, dry climate the g-value usually decides the specification and the U-value follows.

Water: drainage on a surface with no plan slope

A curved surface sheds water along paths that are not obvious in plan. Falls, gutters and outlets are designed from the surface model.

This is the single most common failure point on curved glazed roofs, and it is a geometry problem before it is a detailing problem. On a doubly curved surface the fall direction rotates continuously. There is no single slope to design to, and a gutter run that looks correct on a plan drawing can be flat or reversed in three dimensions.

• The fall rotates Water runs straight down the surface at every point, which on a dome is a different direction everywhere.
• Valleys concentrate it Where two curved surfaces meet, the flow from both arrives at once and the fall often flattens.
• A straight gutter on plan is not straight in 3D Set out across a curved surface it climbs and then falls — flat or reversed along part of its run.

Plan of two joined domes, contoured. Schematic.

The system is two-stage drained and pressure-equalised. Worth understanding what that means, because it changes how the joints are specified:

• The outer seal sheds the bulk of the water but is not relied on to be perfect.
• A drained cavity behind it collects whatever gets past and takes it back out to the exterior.
• Pressure equalisation lets the cavity reach external air pressure, so wind does not drive water inward through the outer joint. Pressure difference is the main force pushing water through a joint — remove it and most of the water stays out.

1. 1 · The outer seal sheds most of it It is not relied on to be perfect.
2. 2 · The cavity catches the rest Whatever gets past is collected and drained back to the outside.
3. 3 · Pressure is equalised The cavity reaches outside air pressure, so wind has no pressure difference to push water through the outer joint.

A joint cut through, simplified. Real profiles vary by system.

The alternative — a single line of sealant relied on absolutely — fails eventually on every building, and on a warped surface it fails sooner because the frame twists and the gasket compression varies along its length.

Watertightness is Class RE to EN 12154, verified by mock-up. The second half of that sentence carries the weight. A curved roof assembly is project-specific, so performance is proven on a physical sample of the actual geometry and the actual joints, not inferred from a system's published test. Budget time for it in the programme — the mock-up usually needs to be built and tested before fabrication is released.

Three details to resolve early:

• Valleys. Where two curved surfaces meet, water volume concentrates and the fall often flattens. Valleys need calculating, not assuming.
• Overflow. What happens when the primary outlets block. On an atrium roof the consequence of not answering this is water entering the building at its most public point.
• Interfaces. Where the glazed roof meets the parapet, the abutment or the primary structure. Almost every leak is at an interface rather than in the field of the glazing.

Snow, and why it often governs

Snow and live load are project-specific, to EN 1991-1-3 or the local code. On curved roofs snow rarely behaves the way the plan suggests:

• Drifting. Snow accumulates in the low points and against upstands. A curved roof creates its own sheltered zones, and the accumulated depth there can be several times the uniform figure.
• Asymmetric loading. A shell loaded unevenly behaves very differently from one loaded uniformly, and for shallow curvature the asymmetric case frequently governs the design.
• Sliding. A steep, smooth glazed surface sheds snow suddenly, onto whatever is below. If that is an entrance, a canopy or a pavement, it has to be designed for — snow guards, or a geometry that prevents the slide.

• Drifting Snow gathers in the low points and sheltered zones a curved roof creates — several times the uniform depth.

• Asymmetric loading A shell loaded on one side behaves very differently from one loaded evenly. On shallow curvature this case frequently governs.

• Sliding A steep, smooth glazed surface sheds snow suddenly onto whatever is below — an entrance, a canopy, a pavement.

Depths and deflections exaggerated.

In mild climates snow is often dismissed early in design. In the Caucasus, northern Iran, Turkey and continental Europe it is regularly the governing load case on a large glazed roof.

Safety: fragility, and what happens if a pane breaks

Two lines on the specification do most of the safety work, and both are worth explaining because architects often inherit them without being told why.

Heat-soaked toughened outer. Toughened glass can contain microscopic nickel sulphide inclusions that expand slowly and cause spontaneous breakage years after installation, with no impact and no warning. Heat soaking holds the glass at elevated temperature to force most of those inclusions to fail in the factory instead of over an atrium floor. It reduces the risk rather than removing it, which is one more reason the inner pane is laminated.

Laminated inner. If the inner pane breaks, the interlayer holds the fragments in the frame. Glass above occupied space is always laminated on the inner leaf, for exactly this reason.

1. Outer pane: heat-soaked toughened Heat soaking forces most nickel sulphide inclusions to fail in the factory rather than over the atrium floor.
2. Sealed cavity With a low-E coating in the unit, for thermal transmittance and solar control.
3. Inner pane: laminated Two plies and an interlayer. If it breaks, the fragments stay in the frame.

Section; layers not to scale. Laminated insulating units, 28 – 44 mm overall.

Non-fragile to ACR[M]001, with a walk-on build-up available on request. The non-fragility classification is a drop test on the complete assembly — glass, frame and fixings together — establishing that someone falling onto the roof during maintenance will not go through it. It is an assembly property, not a glass property, which is why it cannot be assumed from the glass specification alone.

Condensation

Overhead glazing has a condensation problem that vertical glazing does not: what forms on the inside surface drips on people.

The risk is a function of the internal surface temperature, the internal humidity and the external temperature. Higher-performance glass raises the internal surface temperature, which is why Ug matters on an atrium roof for comfort reasons even in climates where heat loss is not a concern.

Where the atrium contains planting, water features or high occupancy, internal humidity rises and the risk with it. The detail response is a frame designed to collect and drain condensate back to the drainage system rather than letting it run to the low point of the surface and fall.

This is worth raising at concept stage with the services engineer, because the answer is sometimes ventilation rather than glass.

Glass, polycarbonate or ETFE

Chosen against span, climate, snow load and the access strategy for cleaning.

• Glass gives the best optical quality, the longest service life and the most predictable appearance. It is also the heaviest, and the structure is sized for it.
• Polycarbonate is lighter and impact-resistant, with lower optical quality and a finite service life before yellowing. Useful where weight or impact governs.
• ETFE weighs a small fraction of glass. On long spans it changes the structure fundamentally, because the glazing is a large part of the dead load a glazed roof structure carries. It admits more light across a broader spectrum, and it is repairable in place. The trade is acoustics — rain noise is real — and a different maintenance model.

Where dead load rather than snow or wind is driving the structure, ETFE is worth pricing as an alternative before the steel is fixed. The saving compounds: lighter infill, lighter structure, smaller foundations. ETFE is also one of the infills a cable net can carry.

Building an atrium roof over an occupied building

Many atrium roofs are either retrofits or the last element of a fit-out, which means the space below is in use or nearly so.

The Auto Mall skylight — 2,400 m² of free-form roof glazing — was installed above a live city block. That constraint shapes everything:

• Craneage and lift zones are decided before panel sizes are, not after. Maximum panel size is often set by what can be lifted from the available position rather than by what can be fabricated.
• Protection to the space below during erection and glazing, and the programme allowance for installing and removing it.
• Working hours are often restricted, which changes the sequence more than it changes the total labour.
• Prefabrication earns its cost here. Panels assembled and digitally set out in the factory reduce the time spent working above an occupied space, which is where the risk and the cost sit.

What to decide, and when

1. 01

   Concept

   The surface, the span, and what the roof is for — daylight level, appearance, or covering something that was previously outside. Also the cooling strategy, because the g-value follows from it.

2. 02

   Developed design

   Glass make-up and performance targets, the drainage strategy resolved against the surface model, snow and wind figures for the site, and the access and cleaning strategy. Cleaning and glass replacement routes resolved at design stage — retrofitting access to a curved roof is expensive, and it is a common change after handover.

3. 03

   Technical design

   Node and framing details, movement joints, the interface with the primary structure, and the mock-up scope and programme.

4. 04

   Before tender

   A defined panel set, a resolved drainage layout and a mock-up allowance in the programme. Tendering a curved roof without these produces prices padded for risk.