Rainscreen Cladding

Ventilated rainscreen cladding is a system of cladding in which the stone panels are used as part of a system that shields the majority of the supporting structure from direct rainfall. It combines a cavity and drainage system behind the panels to remove any moisture that gets past the rainscreen panels. There are two types of rainscreen panels:

Pressure-Equalised

The cavity behind the panels is divided into compartments to generate pressure to impede the ingress of water through the open joints.

Drained-and-Ventilated

The open joints between the stones permits air movement, thereby encouraging the drying out of moisture that gets through the joints.

Fixings & Fixing Systems

The individual stones can either be fixed direct to a backing wall or more typically to a framing system comprising of a grid of vertical and horizontal aluminium profiles. 

The fixings to the stone will normally have combined load bearing and restraint capabilities. They can be brackets recessed into the joints of the stones or undercut anchors in the back of the stones. There are a number of different systems that offer differing advantages and disadvantages to the cladding designer.

The Fischer fixing systems have been used extensively on natural stone buildings across the world.  The system uses undercut anchors pre-drilled in the backs of the stones that are simply connected to the frame system that is attached to the building structure.
The Fischer undercut anchors can typically resist up to twice the pull out loads of traditional dowels and therefore mean that the stones can be significantly thinner and lighter.  Fischer will be happy to provide full and detailed technical support. 

Click here to see the Fischer fixing's Framework Support Systems document

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Kerf Slots are an alternative to the Fischer undercut anchors.  It is important that the designer has calculated the strength of the slots and compared this to the loading that the panel will need to resist.  Typically a stone continuously supported by a Kerf Slot will provide more support than a simple dowel solution, so the stones are probably going to be thinner and lighter, but still thicker than the Fischer system.

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An alternative to the continuous kerf slot is the Aztec self-supporting attachment system which enables 20mm, 30mm or 40mm stone panels to be fitted by means of a stainless steel clip. It is very versatile and quick to install.

The system allows for the attachment of four stone panels by means of a clip that is used to hold the two lower pieces and to support and hold the two upper pieces. The clips are attached to a vertical aluminium T-rail by means of self-tapping screws. 

It has been developed with the view to making the structure as light as possible, whereby the system only adds 1.5 Kg/m2 of weight to the stone, which means a ventilated facade weighing only 55 Kg/m2 for 20mm stone.  Stone Cladding Systems will be happy to provide full and detailed technical support.

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Ultra Thin Cladding

There are a number of different methods for attaching very thin stone onto a strengthening substrate, these include the honeycomb aluminium, ceramic and lightweight concrete.

These systems can reduce the weight of the stone cladding significantly thereby saving structural costs, but the cladding itself is typically more expensive than just natural Portland Stone.

The designer should ensure that the most appropriate fixings system is selected and is structurally compatible with the building's structure. We recommend the early appointment of the specialist stone cladding designer. It is important to ensure that they have the technical expertise and competence to advise on the fixing system and complete the stone cladding design. The designer should consider whether the fixings and the fixing system including the stone should be subject to testing to prove there is sufficient reserve of strength. Albion Stone's Technical Data Sheets will provide the relevant data relating to the stone.

Movement 

The designer needs to be aware of the various movements; thermal, dead & live loads, settlement, moisture and wind loadings, between the structure, the framing system and the individual stones.


Joints

Joints in a rainscreen panel system are designed to permit air to circulate behind the panel and to allow moisture to drain from the cavity. 

In most projects, all the joints will be open, but some joints can be filled with an appropriate sealant if required. If a sealant is used it is important to ensure that it has a good service life, will match the colour of the stone and will not cause staining of the stone. A primer may be necessary.  The joint width needs to be carefully controlled to ensure that it is wide enough to be classed as open at all times. An open joint is normally classed as a minimum of 10mm and will allow free draining of water.  Baffled (a component inserted into the joint) or Labyrinth (stepped) joints to impede the direct passage of water may be considered.

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Cavities

The cavity between the stone and the backing structure will incorporate an air gap, insulation and a breather membrane. 

The cavity may also be divided into compartments if a pressure-equalized system is being designed, ensuring that the compartment's air volume is correctly proportionate to the open joints.

The cavity wall needs to be sufficiently wide to allow for free draining of any trapped moisture, insulation, to accommodate the framing system and to take up all structural tolerances.


Cavity Fire Stops

The cavity design needs to incorporate cavity barriers to prevent the spread of fire both vertically and horizontally.


Insulation

The insulation needs to be non-combustible, rot and vermin proof and non-absorbent. The insulation may have a foil layer to act as a vapour control layer or a breather membrane applied to the outer face to control condensation.

Fixing Contractor 

We recommend that the stone and the cladding contractor is selected as early as possible in the project programme.

The stone cladding contractor will normally take responsibility for the entire stone package including stone drawings, stone procurement and stone fixing on site. Therefore it is vital that the selected company has the necessary experience and expertise

Programme

The designer should be aware of the recommended lead-in period for the supply of stonework cladding.

A typical programme for a medium sized office development in Central London has been set out below:

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Specification Clause