Fibre Reinforced Facade Encyclopedia: Design and detailing
A spandrel is the opaque zone of an external wall between the head of one window and the sill of the window above it on the next storey. It covers the floor slab edge, the slab edge fixings, the perimeter fire stopping and usually a band of insulation, and it is the part of a glazed facade that is not glass. In a fire it is also the vertical separation between one storey’s openings and the next, which is why the spandrel carries requirements that the glass above and below it does not.
On a typical glazed tower the spandrel runs from about 900 mm below the floor level to about 900 mm above it. Everything in that band is wall, not window, whatever it looks like from the street.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Function | Opaque wall zone between vertically stacked openings | Covers slab edge, perimeter fire stopping and insulation |
| Typical height | 900 to 1,800 mm | Set by floor to floor height, sill height and head height |
| NCC vertical separation | Spandrel dimension between openings in successive storeys | |
| NCC alternative | Horizontal projection instead of a spandrel | |
| Fire resistance | Spandrel required to be non-combustible and fire resisting | |
| Common construction | Glass spandrel with shadow box or back pan; metal, GFRC or FRP panel | Material choice constrained by building type and height |
Why the spandrel exists
Structurally the spandrel is the band where a facade meets the building. The slab edge lands in it, the brackets that carry the facade bolt into it, the perimeter fire stopping seals the gap behind it, and the insulation that is missing across the vision glass has to be made up here. Architecturally it is the part of the elevation the design tries to hide, which is why spandrel glass is back painted so that it reads as continuous glazing.
The fire separation function
Fire spreads up the outside of a building by coming out of a window on one storey and entering the window above. The oldest and simplest control on that path is to put enough fire resisting wall between the two openings, which is the spandrel. The National Construction Code (check the edition adopted in your jurisdiction) sets a minimum vertical dimension between openings in successive storeys, together with a required fire resistance level and a requirement that the spandrel be of non-combustible construction, and offers a horizontal projection from the face of the wall as an alternative to the vertical dimension.
The exact dimensions and the fire resistance level are edition specific and have changed between editions, so read them from the edition applying to your project rather than from a detail carried over from a previous job. The fire resistance level page explains what an FRL means and how the three numbers are read. The requirement is also frequently traded away in a Performance Solution where the building is sprinkler protected, which is a fire engineer’s decision, not a detailing decision.
How spandrels are built
- Spandrel glass: heat treated glass, back painted or with a ceramic frit, over an insulated back pan. Visually seamless with the vision glass, but the glass itself does nothing for fire and the wall behind it does all the work
- Shadow box: a spandrel glass with a visible depth behind it, giving the facade a sense of thickness. The cavity runs hot under solar load and needs venting and a thermal stress assessment
- Metal panel: an insulated aluminium or steel panel, light and fast, with the composite core question to answer if it is a sandwich panel
- Moulded panels: glass fibre reinforced concrete (GRC, GFRC) or fibre reinforced polymer spandrels where the design wants relief, a profile or a deep reveal rather than a flat band
Where the spandrel is expressed rather than hidden, it becomes the element that gives a repetitive glazed facade its character: a projecting band, a folded panel, a deep shadow line repeated at every floor. Moulded panels suit that because the geometry is in the mould and repeats at no extra cost across hundreds of units.
Detailing the spandrel zone
The spandrel is the busiest 1.5 metres of a facade. The perimeter fire stopping between the slab edge and the back of the facade has to be installed as tested, with the right backing, the right depth and the right movement allowance, and it fails when it is installed as a generic seal. The facade brackets pass through it and bridge the insulation, so the thermal bridging calculation is done here. The insulation has to be continuous and its reaction to fire class has to suit the building. Drainage from the cavity above is collected on a flashing at this level and taken out through the spandrel.
Standards and references
- NCC 2022 Volume One, Part C, spandrel and vertical separation provisions for openings in external walls (check the edition adopted in your jurisdiction)
- AS 1530.4 Fire resistance tests for elements of construction; AS 1530.1 combustibility
- AS 4284 Testing of building facades; AS/NZS 1170.2 wind actions
- ASTM E2307 perimeter fire barrier systems using the intermediate scale multi-storey test apparatus (United States)
- Approved Document B (England) and BS EN 13501-2 for the European fire resistance equivalent
- NCC Verification Method C1V3 and AS 5113 where the facade is assessed as a system
Related terms
Questions specifiers ask
What is a spandrel panel?
The opaque panel filling the wall zone between the head of one window and the sill of the window above. It covers the slab edge and the perimeter fire stopping.
How tall does a spandrel have to be?
The National Construction Code sets a minimum vertical dimension between openings in successive storeys, with an alternative horizontal projection. The figures are edition specific, so read them from the edition adopted in your jurisdiction.
Does spandrel glass provide fire separation?
No. The glass is an appearance layer. The fire separation is provided by the wall construction behind it, which must achieve the required fire resistance level and be non-combustible where the building type requires it.
Can a spandrel be a moulded panel?
Yes. Glass fibre reinforced concrete and fibre reinforced polymer spandrels are used where the design wants profile or relief rather than a flat band. The material has to suit the building type, and on Type A and B buildings the Deemed-to-Satisfy route requires non-combustible construction.
Why do shadow box spandrels overheat?
The cavity behind the glass absorbs solar radiation and has little ventilation, so temperatures rise well above ambient. That drives thermal stress in the glass and needs to be assessed with the glass supplier.
Designing an expressed spandrel band? ShapeShell-RC is a non-combustible GFRC panel system tested Not Deemed Combustible to AS 1530.1 by Ignis Labs, moulded to whatever profile the elevation needs. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.