Stone, terracotta, GFRC & composites (ACP)
When the architecture wants mass, texture or a sheer flat plane, the facade reaches past glass and aluminium - to stone, fired clay, glass-fibre concrete and the composite panel whose core decides whether it is safe.
An ACP panel and its near-identical twin look the same on the wall - but one has a plastic core that burns like petrol, and that single choice has cost lives.
Not every facade wants to be glass. Architecture often calls for the weight and permanence of **stone**, the warm fired texture of **terracotta**, the mouldable sculptural plane of **glass-fibre-reinforced concrete (GFRC)**, or the dead-flat, any-colour sheen of an **aluminium composite panel (ACP)**. Each of these heavy or panel claddings carries its own engineering: how much it weighs (and so what it costs to hang), how it is fixed (because you cannot rely on adhesive alone on a tall building), and how it survives decades of sun, rain and pollution. And one of them - ACP - hides the single most consequential material choice in modern facade history inside its core, where the difference between a fire-rated mineral core and a cheap polyethylene one is the difference between a safe building and a Grenfell.
Mass, fired clay, cast concrete and the layered panel
Heavy, durable, mechanically fixed - the cladding hangs on anchors, not glue
Natural stone (granite, limestone, sandstone, marble) is the oldest cladding and still the most prestigious - dense, durable, ageing with dignity. But it is heavy (granite is around 2,700 kg/m3, so a 30 mm slab weighs ~80 kg/m2) and brittle, so it is never glued on alone above low level. It is mechanically anchored back to the structure with stainless-steel kerf anchors, dowels or undercut anchors that grip the slab edge, each panel independently supported and restrained so a single failure cannot cascade. Stone selection must check flexural strength, water absorption and weathering - a soft, porous limestone that drinks water will spall and stain in a monsoon climate.
Terracotta - fired clay, the material of Indian roof tiles for millennia - has returned as a high-end rainscreen cladding: extruded baguettes (solid or hollow rods, used as brise-soleil and screens) and tiles (flat plates clipped onto rails). It is light (~30-45 kg/m2), through-coloured so it never needs repainting, naturally UV-stable, and breathable - a natural fit for a drained, ventilated rainscreen. It chips if struck and the clips must allow thermal movement, but as a durable, low-maintenance, warm-textured skin it is hard to beat.
Above low level, heavy cladding is held by mechanical anchors. Adhesive alone on a tall stone facade is a falling-panel waiting to happen.
GFRC casts concrete thin; ACP sandwiches a core between two aluminium skins
GFRC (glass-fibre-reinforced concrete) is concrete made castable and thin by replacing steel rebar with alkali-resistant glass fibres mixed through the matrix. The result is a strong panel only 10-20 mm thick - a fraction of the weight of precast concrete (~40-60 kg/m2 versus hundreds) - that can be cast into almost any shape, texture or colour from a mould: jali screens, deep reveals, sculptural facades, stone-like finishes at a fraction of stone's weight. It is durable and non-combustible, but needs a steel sub-frame and careful detailing of its (relatively heavy, stiff) connections.
ACP (aluminium composite panel) is the flat-sheet workhorse of cheap, colourful, fast cladding: two thin (~0.5 mm) aluminium skins bonded to a core, giving a rigid, dead-flat, lightweight (~5-8 kg/m2) panel that routs and folds into crisp cassettes. Everything turns on the core. A PE (polyethylene) core is cheap plastic - and highly combustible: it is the material that turned Grenfell Tower into a chimney. An FR (fire-retardant) core mixes mineral filler into the polymer to slow burning. An A2 core is mostly non-combustible mineral, classified A2-s1,d0 to EN 13501 - the only core appropriate for tall or high-risk buildings. The lesson of the last decade, codified into facade practice worldwide and into India's NBC fire provisions, is brutal and simple: on anything but low-rise, the core must be fire-classified - PE-core ACP has no place on a tall facade.
Read every cladding by three numbers: how heavy, how fixed, how long it lasts
These four materials span an enormous range, and a facade engineer reads each one through the same three questions. Weight drives the support: a 30 mm granite slab at ~80 kg/m2 needs a serious anchored sub-frame and big brackets; an ACP cassette at ~6 kg/m2 hangs off light rails. Fixing must suit the weight and allow movement: heavy stone is independently mechanically anchored (never adhesive-only above low level); terracotta and ACP clip or hook onto rails that let each panel expand; GFRC bolts to a steel frame. Durability is climate-specific: stone must resist your rainfall and pollution, terracotta and GFRC are inherently weather-stable, ACP's aluminium skin lasts but its bond and core define its real life - and its fire risk.
Crucially, most of these heavy and panel claddings are rainscreens - they are a screen that breaks the rain, with the real waterproof line on a barrier behind a drained, ventilated cavity (the principle from Module 0). The cladding does not have to be perfectly watertight; the cavity and barrier behind it are. That is what lets stone, terracotta and ACP have open or baffled joints and still keep the building dry.
Choose the cladding for the architecture, but specify it for the engineering. Stone gives prestige and permanence at a real weight and cost penalty; terracotta gives warm, through-coloured, maintenance-free texture ideal for a ventilated rainscreen; GFRC gives sculptural freedom and stone-like surfaces at a fraction of the weight; ACP gives flat, colourful, economical planes - **but only ever with a fire-classified (A2 or at least FR) core on anything above low-rise.** Never let a contractor substitute a PE-core panel to save money; that substitution is how the worst facade tragedies happened.
Own three things per cladding: the **weight** (it sizes the sub-frame and the slab anchors), the **fixing strategy** (mechanical anchoring for heavy stone, independent support so no panel cascades, movement-tolerant clips for tiles and cassettes), and the **fire classification** of any composite core (demand A2-s1,d0 / EN 13501 certification on tall buildings; verify it, do not trust a label). Check stone's flexural strength and water absorption against the climate, detail every heavy cladding as a drained rainscreen, and never let adhesive be the sole support of a heavy panel at height.
For heavy cladding, the anchors are everything - install every kerf clip, dowel and undercut anchor exactly as detailed, in stainless steel, and never substitute a 'temporary' adhesive dab for a missing mechanical fixing. For ACP, **read the core marking on every panel** and confirm it matches the specified fire class - a PE-core panel slipped into an A2 order is a life-safety defect, not a paperwork slip. Keep movement gaps open as drawn (claddings expand in the sun), and protect porous stone from cement and mortar staining, which is permanent.
EN 13501-1 (reaction to fire)
Cladding/core fire class
Euroclass reaction-to-fire (A1, A2-s1,d0, B...) - the classification that separates an A2 mineral core from a combustible PE one. It rates the material, not the whole assembled facade's fire spread.
NBC 2016 Part 4 (India)
Fire & life safety
India's National Building Code fire-safety provisions, tightened post-Grenfell for external cladding combustibility on high-rises. State bye-laws and approval bodies enforce it locally with varying rigour.
ASTM C1242 / EN 1469
Stone cladding anchorage
Guidance and specs for mechanically anchoring dimension-stone cladding (kerf, dowel, undercut anchors). They cover the anchoring; stone selection (strength, absorption) is a separate check.
EN 1170 / IS 13000-series
GFRC / fibre concrete
Test methods and specifications for glass-fibre-reinforced concrete and fibre-reinforced products. They define material performance, not the panel's connection design - that stays an engineering job.
“All aluminium composite panels (ACP) are basically the same - they are all aluminium, so they are all fire-safe.”
The aluminium skins are nearly inert; the **core** between them decides everything. A polyethylene (PE) core is essentially solid plastic fuel and burns ferociously - it is what drove the vertical fire spread at Grenfell Tower. A fire-retardant (FR) core slows burning, and an A2 core is mostly non-combustible mineral (A2-s1,d0 to EN 13501). On any tall or high-risk building, only a fire-classified core is acceptable, and modern codes (including India's NBC fire provisions) reflect this. Two visually identical ACP cassettes can have completely opposite fire behaviour.
Worked example - weigh three claddings and size the support decision
The first number you need for any cladding is its area weight, because weight decides the sub-frame, the brackets and the slab anchors. Let's weigh three very different claddings for the same 1.2 m x 2.4 m facade panel and see how differently each must be carried.
A calculator and the material densities (granite ~2,700 kg/m3, terracotta ~30 kg/m2, ACP ~6 kg/m2).
GIVEN one facade bay, panel size 1.2 m x 2.4 m (area A = 2.88 m2): OPTION A : 30 mm natural granite slab (rho ~ 2700 kg/m3) OPTION B : terracotta rainscreen tile (~ 30 kg/m2) OPTION C : 4 mm A2-core ACP cassette (~ 6 kg/m2) Area weight of stone = thickness (m) x density (kg/m3) Panel weight = area weight (kg/m2) x A (m2)
- 1Granite area weight = 0.030 m x 2700 kg/m3 = 81 kg/m2. Panel weight = 81 x 2.88 = 233 kg - a quarter-tonne slab per bay. This demands a stainless mechanically anchored sub-frame and substantial slab brackets; adhesive-only is out of the question.
- 2Terracotta at ~30 kg/m2: panel weight = 30 x 2.88 = 86 kg - a third of the stone. It hooks onto horizontal rails with movement-tolerant clips as a ventilated rainscreen; the support is light steel, not heavy brackets.
- 3A2-core ACP at ~6 kg/m2: panel weight = 6 x 2.88 = 17 kg - a one-person lift. It clips into a light cassette rail system; the engineering effort shifts from weight to fire class and bond integrity, not bracket size.
- 4Compare the three: 233 kg vs 86 kg vs 17 kg for the identical bay. The sub-frame, the bracket size and the slab-edge anchor design scale directly with this number - the cladding choice is also a structural-support choice made in Module 4.
- 5Add the non-weight rule for each: stone - check flexural strength and water absorption for the monsoon; terracotta - keep movement gaps open; ACP - verify the A2 core certification on every panel. Weight sizes the support; durability and fire decide whether it survives and is safe.
You’ll walk away with
Three area weights (granite 81, terracotta 30, ACP 6 kg/m2) and the support logic each forces - the calculation that turns 'which cladding?' into a quantified decision about sub-frame, fixing and fire class.
Two quick observations to anchor heavy cladding.
- 01Find a stone-clad building and look closely at the joints - can you spot the open or baffled joints of a drained rainscreen, and the small stainless clips or pins at the slab edges that mechanically hold each panel?
- 02Look at an ACP-clad building and consider: do you know what its core is? After the post-Grenfell re-cladding programmes worldwide, that question - PE, FR or A2 - is now the first one a facade engineer asks about any composite panel.
Heavy and panel claddings - stone, terracotta, GFRC, ACP - are read through three numbers: weight (it sizes the support and anchors), fixing (mechanical for heavy stone, movement-tolerant clips for tiles and cassettes, never adhesive-only at height), and durability (climate-specific, and for ACP, fire-class-critical). Most are rainscreens, keeping the building dry by a drained cavity behind the screen - and on any tall building, an ACP core must be fire-classified.
Stone is heavy (~80 kg/m2 for 30 mm granite), brittle and mechanically anchored - never adhesive-only above low level. Terracotta is a light (~30-45 kg/m2), through-coloured, UV-stable rainscreen of baguettes and clip-on tiles. GFRC casts concrete thin (~40-60 kg/m2) and sculptural with AR glass fibres. ACP sandwiches a core between aluminium skins - and the core (PE combustible, FR retardant, A2 non-combustible) decides its fire safety. Read every cladding by weight, fixing and durability.
What is the difference between PE, FR and A2 core ACP panels?
All three are aluminium composite panels - two thin aluminium skins bonded to a core - but the core determines fire behaviour. A PE (polyethylene) core is essentially solid plastic and highly combustible; it is the material that drove the fire spread at Grenfell Tower and is unacceptable on tall buildings. An FR (fire-retardant) core adds mineral filler to slow burning. An A2 core is mostly non-combustible mineral, classified A2-s1,d0 to EN 13501-1, and is the appropriate choice for high-rise and high-risk facades. Two visually identical panels can have opposite fire performance, so the core class must be verified, not assumed.
How is natural stone cladding fixed to a building?
On anything above low level, stone cladding is held by **mechanical anchors**, not adhesive - stainless-steel kerf anchors, dowels or undercut anchors that grip the slab edge and tie each panel back to the structure or a sub-frame. Every panel is independently supported and restrained so that one failure cannot make panels cascade. Because stone is heavy (a 30 mm granite slab is around 80 kg/m2) and brittle, the anchorage and sub-frame are engineered for the weight, the wind and thermal movement, and the assembly is usually detailed as a drained, ventilated rainscreen.
What is GFRC and why use it on a facade?
GFRC (glass-fibre-reinforced concrete) is concrete reinforced with alkali-resistant glass fibres instead of steel bar, which lets it be cast thin - typically 10-20 mm - and into almost any shape, texture or colour from a mould. It weighs a fraction of precast concrete (around 40-60 kg/m2), is durable and non-combustible, and lets architects achieve sculptural facades, deep reveals, jali screens and stone-like finishes without stone's weight or cost. It needs a steel sub-frame and carefully engineered connections, but offers great formal freedom for a moderate weight.
Peer-reviewed journals & authoritative standards
- 01Ventilated facade system: A review (rainscreen cladding families, stone/terracotta/composite and fixing). — ScienceDirect (Elsevier), 2025.
- 02McKenna, S.T. et al. Fire behaviour of modern facade materials - understanding the Grenfell Tower fire. Journal of Hazardous Materials, 368. — Journal of Hazardous Materials (Elsevier), 2019.
- 03Yuen, A.C.Y. et al. Evaluating the fire risk associated with cladding panels: an overview of fire incidents, policies, and future perspective in fire standards. Fire and Materials, 45(5). — Fire and Materials (Wiley), 2021.
_Whatever the cladding - glass, metal, stone or panel - it is held, sealed and cushioned by a quiet family of components most people never notice: the gaskets, sealants, tapes and fixings of Lesson 2.4._