Toby Marlow of Haddonstone explains how architects can achieve the best outcomes when using cast stone and what advice they should expect from a manufacturer
There is a virtually endless variety to the forms which cast stone can take, from fully structural elements weighing three or four tonnes, to decorative reproductions of different architectural periods. Cast stone is regularly used to create lightweight retrofit designs which can quite literally create a Trompe L’oeil just a few millimetres thick, while appearing to have been carved from a slab of quarried rock.
Sourcing the right products for individual projects, however, means engaging with an experienced manufacturer able to offer the best possible technical and aesthetic advice based on long practical experience of producing bespoke, as well as standard cast stone designs.
Established in the early nineties, the UK Cast Stone Association (UKCSA) has grown over the decades to the point where its 11 full members – as well as an impressive list of associate members – offer a truly comprehensive selection of products ranging from garden planters to large scale structural components. UKCSA members’ capabilities range from producing stonework for modern, new developments to sensitive restorations of historic and listed buildings. In fact, it is Britain’s rich architectural heritage, from Roman times through to influential periods including the Georgian era, that drive customers and their consultants from across the continent, North America and Asia to seek the services of UK cast stone manufacturers.
It is a truly eclectic industry, and one which can appear complex to the uninitiated. This means that cast stone manufacturers need to ensure they fully understand their customers’ unique
project specifications, as well as being able to educate their clients on the different categories of product available, and their characteristics.
Significantly, while cast stone products manufactured in the UK are normally compliant with overseas building standards, the converse is not the case: and UK specifiers should not assume that foreign made goods will match up in terms of production standards, especially regarding frost resistance – or with our increasingly onerous safety requirements.
UKCSA full members manufacture three main types of products, with wet cast being the closest relative to conventional precast concrete units. In practice, these typically use a 5% water content to ensure the cement is properly hydrated, though particle size within the mix affects compaction; and the end results.
The biggest contrast to semi-dry alternatives, however, is the use of super-plasticisers, which afford the mix the consistency or slump of structural concrete normally delivered to construction sites, with a strength of around 45 N, which with reinforcing offers superior load carrying capabilities, as well as allowing complex profiles. Then, fibre reinforced, hollow castings are ideal where weight is to be kept to a minimum.
While some architects will be sufficiently experienced to know the type of product they require for specific projects, in most cases they will rely on the cast stone specialist to advise on the most suitable solution; normally involving a collaborative process to determine the correct mix and stone colour, whether reinforcement is required, and the various decisions to be made in relation to installation.
Although UKCSA full member manufacturers offer standard product ranges, some 70-80% of orders involve totally bespoke castings, for which individual moulds must be fabricated – arguably the most skilful part of the process; and one requiring exact measurements – while offering the most aesthetic and technically appropriate answer.
Normally beginning with the architect’s drawings or at least a sketch, the manufacturer will help decide where to position the joints in castings like cills and provides expertise on the correct positioning of fixings to ensure secure installation. The positioning and type of reinforcement is also particularly of importance on thinner section products. With a 40 mm cover being required to protect against corrosion, it may be necessary to switch to thinner bars or to use stainless steel: whereby the cover can be cut to 25 mm. However, the design of the reinforcement will remain the responsibility of the project structural engineer.
It has further become commonplace for manufacturers to receive design information in the form of 3D images and STL files that are combined to help create a full-size, printed PU model of items like ornate plaques or corbels. The manufacturer then creates a fibreglass and rubber mould from which the castings are produced. As with all bespoke moulds, the normal practice is to keep them for at least three months after the product is made, but this can be extended indefinitely where it is known further castings will be required.
There is also the choice of whether the manufacturer’s role is extended by involving one of its experienced independent installers. Conscientious UKCSA full member manufacturers and their experienced independent installers, in fact, consider the entire process through scheduling production and deliveries, to protect castings appropriately during transport, and ensure a safe and timely installation is achieved.
Haddonstone is one of the UKCSA’s founding members, and one project involved a two-year plan to extend and refurbish the 12 Apostles Greek Orthodox Church at Brockmans Park in Hertfordshire. For this bespoke project, Haddonstone worked closely with the lead consultant, GLA Architecture & Design, as well as main contractor, MP Build, to supply a package of dry cast enrichments to closely match the building’s style and history. Now, the cast stone elements frame the main door and three different style windows, while the roof gables feature reduced sized quoins and sculpted apex stones.
In conclusion, the cast stone industry in the UK, exemplified by UKCSA members, offer a diverse range of high-quality products and bespoke services, essential for both modern construction and the restoration of historic buildings. By collaborating closely with architects and contractors, these manufacturers ensure that each project benefits from expertly crafted, durable, and aesthetically appropriate solutions, tailored to meet the specific needs and standards required.
Toby Marlow is building & construction director at Haddonstone