In the world of conservation architecture, the harmony between material, method, and craftsmanship is everything. The recent restoration of the Round House roof at Raby Castle, County Durham, exemplifies this ethos through a seamless marriage of heritage Cumbrian slate and traditional roofing expertise—earning it a finalist spot at the 2025 National Federation of Roofing Contractors (NFRC) Awards in the Heritage Roof Slating category.
At the core of this project was a close collaboration between KL Venning Slaters Ltd of Penrith and Burlington Stone, whose Burlington Blue slate, quarried from Kirkby Moor in Cumbria, was selected for its authenticity, durability, and natural aesthetic quality.
Design-led craftsmanship
Unlike rectilinear roofs, the Round House posed unique architectural and construction challenges. As a circular conical structure, the geometry required an advanced understanding of both materials and spatial dynamics. KL Venning Slaters were involved from the earliest design stages, advising on the precise shaping and timbering of the underlying roof structure. This early intervention was crucial in ensuring that the roof’s final form would support the slating system and achieve the smooth visual continuity demanded by the listed building’s heritage context.
The substructure itself was engineered using layered 6mm marine ply, moulded over curved timbers to produce the flawless cone onto which the slate would be applied. This layering provided the stable, continuous substrate necessary for a finish that would maintain structural integrity while honouring the original architectural form.
Slating methodology: diminishing courses with random widths
The roofing was executed using Burlington Blue slate in diminishing courses with random widths, a method traditionally associated with Cumbrian vernacular architecture. The visual rhythm created by this approach is unmatched, but the technical complexity is significant. Unlike uniform modular slates, each stone had to be hand-measured and individually prepared, with length-based batching carried out at ground level before application.
To maintain coverage consistent with BS 5534 standards, the team had to perform meticulous calculations to determine slate density per metre, adapting continuously as the roof’s radius reduced as it neared the apex. Every course was carefully scribed onto the roof—a challenging task given the non-linear nature of the structure.
Chamfering and tolerance control
One of the most technically demanding aspects of the job was chamfering each individual slate to allow it to conform to the curvature of the roof. This process involved dressing ½ inch off each side at the top of the slate down to a feathered edge at the bottom, increasing to ¾ inch for slates installed nearer the apex. This nuanced adjustment ensured each slate head sat flush on the substrate without disrupting head or side laps—an essential requirement to maintain waterproofing and aesthetic continuity.
Additionally, as a natural product, slate comes with its own variables: thickness, twist, and bow. The KL Venning team, comprising only experienced time-served slaters, continually graded and adjusted the slate during installation, ensuring precision and uniformity to make best use of the material’s organic irregularities.
Architectural legacy and regional pride
The successful delivery of the Round House roof not only demonstrates exemplary technical execution but also underscores the importance of regional materiality and artisan collaboration in heritage conservation. As Lisa and Mark Elliott of KL Venning Slaters reflected, being selected as one of just four national finalists was not only a validation of their craft but also a proud moment for Cumbrian slate and Cumbrian skills on the national stage.
This project is a reminder to architects and conservationists alike that the future of historic buildings rests not only in specification and design, but in the depth of experience and craftsmanship that connects us to architectural tradition—slate by slate, curve by curve.