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An introduction to external cladding

BSBG Architectural Technologist David Dignam (ACIAT) writes the first Technical Note of the year, in which he provides an in-depth study of external cladding. We begin with an answer to the most fundamental of questions...

What is cladding?

Cladding is a building system used to cover the external wall or roof of a building. Cladding is non-loadbearing, it does not have any structural function and is supported by the main structure. Most types of cladding are carried on sub-or intermediate support systems that span between the elements of the main structure. Cladding is mainly used for aesthetic reasons, and comes in a variety of materials, colours and textures. Depending upon the budget of the project and the client’s preference, different cladding systems are available with different anchor systems. These anchor systems can be visible or invisible. Costs vary depending on what anchor system is selected, but generally an invisible system will be more expensive.

What are the different types of cladding?

1. Metal

Different types of metal claddings include:

Steel – must have a protective coating to prevent corrosion. Can be either galvanized and/or plastic or powder coated.

Aluminium – can be used without a protective coating but is normally powder coated. More costly than steel but can be formed to curves, shapes and angles more easily. Also lighter in weight in comparison to steel, which reduces the load on the building envelope. Can be single flat sheet type, or form part of either a composite or built-up system.

Aluminium composite panels (ACP) are an example of a composite system that consists of two aluminium cover sheets bonded to a mineral filled core. ACP can have a variety of colours, textures and finishes, while some suppliers can provide stone or timber effect finishes. Every manufacturer of ACP will have different fixing systems.

A built-up system consists of liner sheets and liner trays. The liner sheet system consists of two sheets; an internal deck liner sheet, and an external cladding sheet. The inner liner sheet can be fixed to the main structure using cladding rails or purlins. The cladding sheets are separated by spacer rails and/or fixing clips, with insulation placed between the spacer rails or clips, and laid over a vapour control layer. The external sheet is then fixed to the spacer rail or clip depending on its profile.

When using a liner tray system, there is no need for the inner liner sheet, spacer rails or cladding rails as it is fixed directly to the structure. Insulation is placed within the liner trays and the external cladding sheet is fixed directly onto the outer flanges of the tray.

A big advantage of using a built up system, is that manufacturers of the system will have connection details for all architectural connection, ridges, parapets, sky lights and gutters.

Copper - Normally used as part of a built-up system over insulation and vapour barrier and provided in narrow strips, while installed to a particular set of specialist details. Copper offers a character and durability that no other metal roof can match. Some advantages of using copper are its aesthetic properties, durability and weight. Over time, copper turns from an orange brown colour to a bluish green colour. This colour change transforms the aesthetics of the building, giving it a whole new look. Copper can last for decades and can withstand severe weather. It is lightweight, which puts a lot less stress on the structure of the building.

2. Stone

Igneous, Sedimentary and Metamorphic rocks can all be used as cladding. Stone cladding can be fixed/applied in a variety of ways:

- Restrained against an inner leaf of blockwork or concrete using steel straps and dowels. Steel angles at each storey will carry the weight.

- Fixed to the floor slab.

- Galvanised or stainless steel frame support systems that carry and restrain stone cladding. Depending on where your building is located, different types of stainless steel must be used. If your building is in a marine environment, 316 stainless steel must be used.

- Veneer systems - thin (as little as 5mm) veneer of stone bonded onto a metal or concrete substrate which is then fixed back to the primary structure using a support system

Below is a table featuring stone typically used in the UAE/Dubai:

IGNEOUS	METAMORPHIC	SEDIMENTARY
NATURAL STONE TYPICALLY USED IN THE UAE
GRANITE	MARBLE	TRAVERTINE
GABBRO	GNEISS	SANDSTONE
BASALT	SLATE	LIMESTONE
SERPENTINE
PORPHYRY

3. GRC

GRC (Glass fibre reinforcement) is produced by adding glass fibres to the sand cement slurry mixture. Glass fibres are added for their high tensile strengths and impact resistance. GRC is mainly used in exterior building façade panels and as architectural features. GRC can be cast into many shapes using moulds.

4. Terracotta

Terracotta is a 100% natural material, it is recyclable, sustainable and eco-friendly. Terracotta is composed of clay, made using fire and water. Terracotta panels can be lightweight due to their hollow profile. This profile is obtained during the production process. In its raw form, terracotta is pushed through a mould. By doing this, it guarantees the uniformity of the profile. Terracotta cladding uses a support system where the tile or terracotta is held in place or hung off a galvanized or stainless steel secondary support grid with a vapour barrier, insulation and inner wall behind. This allows for a fast and easy application. Terracotta is a non-flammable material and has a very good fire resistance. It is durable and resistant to ultraviolet radiation, meaning it does not fade in sunlight. Terracotta’s acoustic properties help reduce the level of sound absorption into any building. Depending on what system you use, terracotta panels can naturally provide thermal benefits to boost the thermal efficiency of buildings. If installed correctly, terracotta needs little maintenance.

Important Design Considerations

1. Fire Saftey

Over the last few years, certain events around the world such as Grenfell Tower, London or the Torch Tower and Address hotel in Dubai have caused the construction industry to review its fire safety regulations, especially when it comes to external cladding. Certain types of cladding materials, such as stone, have a very low combustibility rating whereas other materials like aluminium composite panels could have a very high rating. Building material companies are now having to test all of their products by approved testing facilities to see how they withstand to fire and fire spread. Large demonstration walls are built with the complete cladding system. This system is then subjected to a fire that replicates a severe fire. These tests such as NFPA 285, BS8414 look at whether the system resists fire spread up a wall. The cladding that was used in Grenfell Tower was never fire tested. More information on these tests can be found at the end of this document.

When specifying any façade material, it should comply with the local authorities’ standards. In Dubai, only materials and façade systems which are tested and listed with the Dubai Civil Defence are allowed on exterior facades, based on the building height and occupancy types.

Façade installers must also have company registration with the Dubai Civil Defence before any installation work can take place.

Except for natural stones and concrete, the materials listed below should be tested under the following sections.

Metal Composite Materials (MCM, ACP) complying with Section 4.6.
Exterior Insulation and Finish Systems (EIFS) and External Insulation Composite System (ETICS) complying with Section 4.7.
Polycarbonate External Wall and Façade System (PEWFS) complying with Section 4.8.
Sandwich Panels complying with Section 4.9.
GRC /GRFC and GRP Systems to Section 4.10.
Glazing Systems complying with Section 5.

For more information, please see the UAE Fire and Life Safety Code of Practice (September 2018 Edition).

2. Weathering/Climate

The weather can dictate the type of cladding system that is most suitable for the building in any given environment. Cladding systems often have sealed joints between the panels, to stop rain or moisture in the air from entering directly into the cavity. This type of system creates a pressure difference between the outside air, and the air within the cavity, creating a constant airflow known as the ‘chimney effect’. The continuous airflow eliminates humidity and prevents condensation build up within the cavity as it is constantly drying out any moisture that forms.

However with regards to fire safety, the chimney effect can also pose a huge risk, as the ventilated airway can provide a clear path for fire to spread very quickly up the vertical face of the façade.

Many cladding systems are provided with an ‘open jointed’ design, which allows the cavity void to breathe naturally but will not prevent wind driven rain from entering the cavity, while also reducing wind suction on the panels. However with a suitable damp proofing system on the structure and a seamless insulation system installed, the risk of water ingress is minimal. An open jointed system also prevents the risk of the aforementioned ‘chimney effect’ in a facade fire situation, as the open joints prevent the constant airflow, thus reducing the risk of vertical fire spread.

All rainscreen cladding systems in the UAE will require a breather membrane, which acts as a barrier to prevent condensation forming on the layer between the external humid air and the mechanically cooled air from inside the building. The vapour barrier will always be placed on the external face of the wall insulation. As most projects in the UAE use a foil faced insulation, the foil face of the insulation is placed on the outside and acts as the vapour barrier, meaning no separate vapour control layer is required.