Why We Use Jesmonite AC730 for External Architectural Mouldings
At Chelmer Mouldings, Jesmonite AC730 has become our material of choice for external architectural work. Here we explain why and what it means for the projects we work on.
A Smarter Alternative to Cast Stone
When clients and architects first ask us about external architectural mouldings, the conversation often starts with cast stone. It is familiar, it looks the part, and it has a long track record in traditional and heritage-influenced architecture. But cast stone comes with real practical limitations. It is heavy, expensive to fix structurally, and its sheer weight restricts the complexity and scale of moulding profiles you can realistically achieve.
Jesmonite AC730 gives us a way to deliver the same visual result, and often a significantly better one, without those constraints.
We use it for cornices, string courses, coping details, column capitals, pilasters, window surrounds, fascia panels and decorative features on facades ranging from high-end residential to commercial and mixed-use developments. The material is a water-based, polymer-modified composite that cures to a dense, hard finish. It looks and feels like stone. It takes paint, limewash and decorative finishes exceptionally well. And because we cast it in our own moulds, the level of detail and crispness we can achieve is something that cast stone simply cannot match.
The Detail We Can Achieve
This is probably the aspect that surprises people most when they first see finished Jesmonite work. Because we are casting into high-quality rubber or fibreglass moulds, we can reproduce intricate classical profiles such as egg-and-dart, dentil courses, acanthus leaves and fluted columns with a sharpness and consistency that would be impossible to achieve in poured concrete or individually hand-cut stone.
Every piece comes out of the mould with the same crisp geometry. For large-scale projects where you might need hundreds of repeating brackets or cornice sections, that consistency matters enormously, both aesthetically and when it comes to installation.
We can also produce very large format pieces and unusual curved profiles that would be prohibitively expensive to source in natural stone. If an architect wants a curved cornice to follow the radius of a bay window, or a bespoke capital that does not exist in any standard catalogue, we can design it, produce a mould, and cast it.
Weight and Fixing
Cast stone and natural stone are heavy. A deep classical cornice in cast stone can weigh upwards of 150 to 200 kg per linear metre. That weight has to be carried by the structure behind it, which means structural fixings, lintels, potentially engineered supports, and the associated cost and programme implications.
Jesmonite AC730 is significantly lighter, typically around 30 to 45 kg per square metre for a GRC-backed cladding panel, which means it can often be fixed directly to a masonry or framed facade using conventional stainless steel fixings, without the need for structural engineering input. On high-rise residential projects in particular, this can translate into meaningful savings on both programme time and cost.
It also simplifies the work at height. Lighter panels are safer and faster to handle on scaffolding, and the installation team we work with can fix more lineal metres per day than would be possible with stone.
Fire Performance
Since the introduction of the Building Safety Act 2022 and the continued evolution of Approved Document B, fire performance on the external envelope has become a primary specification consideration on most projects above 11 metres.
Jesmonite AC730 carries a Class A2-s1, d0 classification to EN 13501-1, the second highest non-combustibility classification in the European system. In practical terms, this means limited combustibility, very low smoke production (s1), and no burning droplets (d0).
It also achieves Class 0 to BS 476 Parts 6 and 7, confirming very low surface spread of flame.
This is the critical distinction between Jesmonite and GRP (glass reinforced polyester). GRP is widely used for architectural mouldings and performs well in many respects, but its fire classification is significantly lower. GRP combusts, produces toxic opaque smoke, and generates burning droplets, all of which are unacceptable on high-rise residential, schools, hospitals, or any project where the external cladding system must meet non-combustibility requirements. Jesmonite AC730 has none of those concerns.
For planning departments and building control, we can provide the full fire test certification for AC730.
Durability and Weather Resistance
We would not use a material externally that we did not trust to perform over the long term, and Jesmonite AC730 has an excellent durability track record. The acrylic polymer matrix gives the cured material a degree of flexibility that purely cementitious products lack. It accommodates thermal movement without micro-cracking and has been independently tested to BS EN 539-2 with no degradation after 50 freeze-thaw cycles.
Water absorption is very low, under 3% after 28 days of full immersion, which means the material resists saturation, frost damage and the biological staining that can affect more porous materials over time. It is also inherently UV stable, so painted and pigmented finishes hold their colour without the yellowing or fading that can occur with some polymer-based alternatives.
For a well-specified and properly installed Jesmonite facade, a 30-year design life is readily achievable.
Sustainability
Compared to quarried stone or precast concrete, Jesmonite has a meaningfully lower carbon footprint. It is water-based and contains no solvents or VOCs, which matters both from an environmental perspective and in terms of handling on site. Because pieces are lighter, transport emissions are also reduced, something that is increasingly relevant as clients and contractors report on embodied carbon.
In Summary
We use Jesmonite AC730 on external projects because it lets us produce architectural mouldings of exceptional quality, with a level of detail, a breadth of profile and a crispness of finish that heavier traditional materials cannot match, while meeting the fire, durability and weight requirements of contemporary construction.
If you are working on a project where external mouldings are being specified, we are happy to discuss how Jesmonite might be the right solution. Get in touch with the team at Chelmer Mouldings to talk through your requirements.
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