St Mawes
Products Used
Set within a large plot overlooking the Percuil River, Tregenna occupies a spectacular yet demanding coastal location in St Mawes, Truro. Positioned within an Area of Outstanding Natural Beauty (AONB) and exposed to severe south-facing coastal weather, the project required a roofing solution that could satisfy stringent planning requirements while delivering exceptional long-term performance.
Main contractor P Chapman Construction Ltd worked closely with specialist roofing contractor Wessex Slate and Tile to realise a design that balanced contemporary architecture with the conservation-led expectations of the local planning authority. Following the rejection of two earlier schemes, the final design was developed in consultation with the AONB, who acknowledged it as a significant improvement on previous proposals.
SSQ’s Riverstone Ultra Natural Argentinian Slate was specified for its proven durability, geological compatibility with indigenous Cornish slate and its ability to enhance the clean, low-profile lines of the single-storey design. As a phyllite stone, Riverstone offers exceptional density and strength, with extremely low water absorption of just .1%. This makes it ideally suited to harsh marine environments, where longevity and resistance to weathering are paramount. Its high mica content delivers subtle light refraction, closely mirroring the appearance of traditional Delabole and Trevillet slates, ensuring planning approval and visual harmony with the surrounding landscape.
The roof was constructed as an unventilated warm roof using Permavent Apex Air membrane, with Celotex GA4000 insulation installed between rafters in habitable areas and mineral wool laid over rafters elsewhere. Slates measuring 400 x 200mm were fixed with double 35 x 3.35mm copper nails at a 150mm gauge, providing a 100mm headlap appropriate for severe exposure zones along the south Cornish coast.
Careful setting out was critical. The roof was struck out using chalk lines at 815mm centres to achieve consistent perp joints of 3–5mm. Slates were meticulously sorted into thick, medium and thin grades, with thicker slates laid at eaves and progressively thinner slates towards the ridge, ensuring a refined, graded appearance. Close mitred hips, formed using 400 x 300mm Riverstone Ultra slates and Code 3 lead soakers, were a defining feature of the design, accentuating the building’s modern lines while respecting its low eaves height. Plasma Anthracite clay ridge tiles were mortar bedded and mechanically fixed to complete the roofscape.
Despite seasonal weather challenges typical of Cornwall between January and March, the project progressed smoothly. The roofing works, including slate installation, leadwork and the garage annex, were completed within eight weeks, on time and within budget.
The finished roof was well received by client, planners and conservation stakeholders. Tregenna stands as a compelling example of how SSQ’s Riverstone Ultra can deliver architectural clarity, planning compliance and a guaranteed 100-year performance in one of the UK’s most demanding coastal environments.
