Fibre Reinforced Facade Encyclopedia: Design and detailing
A cladding substructure is the secondary framing that carries facade panels back to the primary structure: the brackets anchored to the slab edge, column or backing wall, the horizontal or vertical rails they support, the thermal break pads between them, and the anchors tying the assembly into concrete or steel. It is a separate design problem from the way the panel attaches to that framing, which is covered under panel fixing systems. The substructure is everything behind the panel; the fixing system is the connection to the panel itself.
A typical aluminium rainscreen substructure weighs 6 to 12 kg per square metre of facade and adds 60 to 120 mm to the wall zone. A galvanised steel top hat and bracket system weighs 10 to 18 kg per square metre.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Bracket types | L brackets and T brackets, fixed and sliding | One fixed point per rail, the rest slotted for movement |
| Bracket spacing | 600 to 1,200 mm horizontally | Set by panel weight, rail span and zoned wind pressure |
| Rail materials | Aluminium 6060 or 6063 extrusion; galvanised or stainless steel | Aluminium for light panels, steel where spans or loads are high |
| Corrosion protection | Hot dip galvanised to AS/NZS 4680, or grade 304 and 316 stainless | 316 within a few kilometres of a surf coast |
| Thermal break | Polyamide, glass fibre reinforced plastic or dense EPDM pads | Cuts the point thermal bridge at every bracket |
| Adjustment | ±10 to ±25 mm in three axes, typical | Coarse at the bracket, fine at the rail |
| Thermal movement of aluminium | about 23 × 10⁻⁶ per °C | A 3 m rail over a 60 °C range moves about 4 mm |
| Deflection head allowance | commonly 20 to 25 mm | Accommodates slab deflection, creep and inter-storey drift |
| Design responsibility | Facade or panel engineer, to the project wind load | Primary structure capacity and tolerances come from the project engineer |
What the substructure has to do
The substructure has four jobs and they conflict. It carries load, taking panel dead load and zoned wind pressure back to the structure through a checkable path. It absorbs tolerance, reconciling a panel made to a few millimetres with a structure built to tens. It allows movement, letting facade, rails and structure expand, contract and deflect independently. And it does all that while crossing a continuous insulation layer as few times and as thinly as possible.
Those requirements pull against each other. Stiffness helps the load path and hurts the movement requirement. Adjustability helps tolerance and reduces capacity. A deeper bracket buys cavity depth and costs thermal performance. The substructure design is where those trades are settled, and it is worth settling them before the panel geometry is frozen rather than after.
Brackets, rails and anchorage
The bracket is the adjustable component. An L bracket takes load on one side and suits a slab edge; a T bracket is symmetrical and suits a rail centred between floors. Slotted holes give in and out adjustment against the structure, and further slots at the rail connection give vertical and horizontal adjustment. Serrated washers or toothed plates are used where the slot must not creep under load; a plain washer in a slot is a friction connection and has to be assessed as one.
The anchorage into the structure is the part that most often fails review. The options are cast-in channels, the most reliable and the most demanding on coordination; cast-in ferrules; post-installed screw or expansion anchors; and chemical anchors. Each has edge distance, spacing and embedment requirements, each is sensitive to hole cleaning and installation torque, and each has reduced capacity in cracked concrete, which is the usual condition of a slab edge. Post-installed anchors should be specified with a recognised assessment, installed by trained operatives and proof tested on site to an agreed sample rate.
Rails are aluminium extrusion for most rainscreens and steel where loads, spans or fire requirements demand it. Aluminium is light, corrosion resistant and easy to extrude with adjustment built in, but it has a low modulus, softens in fire and moves roughly twice as much as steel with temperature. Mixing the two is normal; putting them in direct contact is not.
Thermal breaks and thermal bridging
Every bracket is a metal path from a heated interior structure, through the insulation, to a ventilated cavity. Aluminium conducts at roughly 160 to 200 W per metre kelvin and steel at about 50, so a grid of unbroken brackets at 600 by 1,200 mm centres can cut the effective performance of a well insulated wall substantially. The effect is expressed as a point thermal transmittance in watts per kelvin per bracket and added to the plane element U value.
The mitigations, in order of effect, are: fewer and smaller brackets, achieved by longer rail spans and a substructure designed for actual loads rather than a default; a thermal break pad of polyamide, glass fibre reinforced plastic or dense EPDM at the structure connection; brackets with a reduced section where they cross the insulation; and insulation cut tight around each bracket.
The break pad carries the full bracket load in bearing and shear, so it is a structural component, not a gasket. It is also a polymer inside the external wall build-up and has to be assessed against the fire provisions. Get the pad, its thickness and its fire status into the schedule rather than leaving it as a site decision.
Movement, fixed points and deflection heads
A facade substructure has to be determinate in movement terms. Each rail gets one fixed point, restrained vertically and horizontally, and every other connection is slotted so the rail grows and shrinks around it. A 3 metre aluminium rail over a 60 °C service range moves about 4 mm and a 6 metre rail about 8. Fix both ends and that movement becomes a compressive force that bows the rail and pushes the panels out of line.
The structure moves more. Concrete slabs deflect under live load, creep, shorten with shrinkage and, in a tall building, shorten under axial load; steel frames drift. Where a substructure crosses a floor level or a structural joint it needs a deflection head: a connection that carries horizontal wind load but lets the structure move vertically past it. An allowance of 20 to 25 mm is common, but the number belongs to the project engineer, and inter-storey drift under wind and earthquake actions counts alongside gravity deflection. Panel joints then have to accept whatever the substructure passes on.
Tolerance take-up
Structural and facade tolerances are an order of magnitude apart. A cast in situ slab edge is routinely out by 20 to 40 mm over a floor, while a factory-made GRC or FRP panel is made to a few millimetres and a joint has to read consistent to within one or two.
The substructure absorbs that difference, and only if the adjustment is designed in and surveyed before anything is fixed. The sequence is coarse adjustment at the bracket to structure connection, medium at the bracket to rail connection, and none at the panel, which is set out to a surveyed facade grid rather than to the concrete. Where the survey shows deviation beyond the bracket range, the answer is a packer, a deeper bracket or a local redesign agreed in advance, never a panel forced into place.
- Survey the as-built structure before the substructure is ordered, not after
- State the assumed structural tolerance in the specification and the adjustment range that covers it
- Set out the facade grid independently of the structure and record the deviation
- Keep a documented allowance for out-of-tolerance zones, and agree who pays for it
Standards and references
- AS/NZS 1170.2 Wind actions and AS/NZS 1170.0 General principles
- AS 4100 Steel structures and AS/NZS 4600 Cold-formed steel structures
- AS/NZS 1664 Aluminium structures, parts 1 and 2
- AS 5216 Design of post-installed and cast-in fastenings in concrete; AS 3600 Concrete structures
- AS/NZS 4680 Hot dip galvanized (zinc) coatings on fabricated ferrous articles
- AS 4284 Testing of building facades
- NCC 2022 Volume One, C2D10 and the external wall provisions applying to components within the wall build-up (check the edition adopted in your jurisdiction)
- CWCT Standard for Systemised Building Envelopes (UK); European Assessment Documents for kits for external wall claddings
Related terms
Questions specifiers ask
What is the difference between substructure and panel fixing systems?
The substructure is the secondary framing: brackets, rails, thermal breaks and the anchors into the primary structure. The panel fixing system is the connection between that framing and the panel, such as an undercut anchor, a cast-in socket, a hook or a face fixing.
Should a facade substructure be aluminium or steel?
Aluminium for most lightweight rainscreens: light, corrosion resistant and easy to extrude with adjustment built in. Steel where spans, loads or fire requirements demand extra stiffness. Either way, isolate dissimilar metals.
How much adjustment should a bracket have?
Enough to cover the specified structural tolerance with margin, commonly ±10 to ±25 mm in each of three axes, reconciled against a survey of the as-built structure before anything is ordered.
Why does each rail need a fixed point?
So thermal movement has somewhere to go. One fixed point per rail with slotted connections elsewhere lets the rail expand and contract freely. Fixing both ends converts movement into force and bows the rail.
How much does a bracket affect the wall U value?
Enough to matter. An unbroken aluminium bracket grid adds a significant point thermal transmittance. Fewer and smaller brackets, a structural thermal break pad and tightly cut insulation are the standard mitigations.
Who designs the substructure?
The facade or panel engineer, working to the zoned wind pressures, tolerances, movement allowances and fire requirements issued by the project team. The project structural engineer confirms the primary structure can take the reactions.
Need a substructure designed with the panels rather than after them? ShapeShell panels are supplied with an engineered bracket and rail system coordinated to the project tolerances. Talk to the engineering team
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.