Stone on Precast Cladding

Stone on Precast cladding is stone panels on precast concrete cladding units. The technique was imported from the USA in the 1950s and has a proven track record. The cladding façade will look very similar to the traditional handset cladding and often Stone on Precast projects will include elements of handset stone.

Fixings

The individual stones are supported by the series of dowels and they do not rely on adhesion with the concrete. 

Stone on Precast panels are normally a single storey in height and up to a structural bay in width. Movement between these panels and the structural frame will need to be assessed and movement joints around the panels designed by the specialist stone cladding designer. It is important that they have the technical expertise and competence to advise on the fixing system and complete Stone on Precast cladding design.

Movement

The debonding membrane between the concrete backing panel and the individual stones will allow for differential movement between these different products. 


Sealants

Movement joints need to be filled with a sealant that has a good service life (above 20 years), has good adhesion properties, will match the colour of the Portland Stone and will not cause staining of the stone. A primer may be necessary.


Joint Width

The manufacturing tolerances as set out in BS 8298 allow for  typically 5mm (+-€“ 1.5mm) joints for all but the largest stones. Please note that the maximum width of a mortar joint is 13mm, but a sealant filled movement  joint can be up to 30mm wide.

The debonded surface should mean that there is little or no contact with the concrete and so minmise the risk that contaminants and staining from the concrete can leach into the stone. The stones are attached to the large concrete backing panels in a factory environment, then hauled to the site and craned into position. The panels can include the window units and are attached to the structure using a variety of methods as outlined in BS 8297. Stone on Precast panels can have the advantage of allowing for faster on site construction and earlier enclosure of the dry envelope to enable internal works to proceed faster.

The stones on a precast concrete panel are subject to very little stress. They are individually supported and supported at the back by the concrete. The only stress exerted on the stone is through the dowels themselves.

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Fig. Stone / Dowels interface detail

Cement Lime Mortars

The mortars for jointing and pointing should match the colour of the Portland Stone and be slightly weaker than the stone itself. 

To ensure the colour match, Portland Stone dust should always be used as the aggregate. The recommended mixes are set out below:

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Stone on Precast Contractor 

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

The Stone on Precast contractors 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 and the availability of production slots in the pre-cast company's factory.

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

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Specification Clause