Traditional hand fixed cladding is still widely used for Portland Stone cladding, particularly for the medium and smaller projects. The traditional hand fixed cladding system typically carries the load of the cladding to a load bearing fixing situated at the floor plate. The stones above are simply restrained using Restraint fixings.
The design team should establish the maximum movement and shortening of the frame and how that will be accommodated in the stone cladding.
The fixings should be able to withstand the imposed loads on the cladding including dead loads, wind pressure and suction.
The cavity between the stone cladding and the backing structure will allow space for the fixings and bolt heads, stone and structural tolerances, insulation, an air gap to minimise transmission of rainwater and allow for free draining of any trapped moisture.
Cavity Fire Stops
The cavity design needs to incorporate cavity barriers to prevent the spread of fire both vertically and horizontally and reduce the effect of fire on the fixings.
The insulation needs to be non-combustible, rot and vermin proof and non-absorbent, and need to be at least 10mm from the back of the stone cladding.
Cavity Trays and DPC
The cavity trays and the d.p.c. should be formed from materials that will not degrade and stain the stonework and incorporated in a sufficiently wide bed joint in accordance with the manufacturer guidelines
Joints should be weathertight whether they are loadbearing or designed to accommodate movements of the cladding. Where the joints are narrower than 5 mm, the manufacturing tolerances and the effectiveness of the jointing material will need to be carefully reviewed.
Compression joints are horizontal movement joints and are designed to accept the vertical movement of the structure
They are normally situated at each floor level immediately under the course supported by the load bearing fixing and the width should be proved by calculation and is unlikely to be less than 15mm.
Movement (Expansion) joints are vertical. The spacing and the widths of these joints need to be designed to accommodate the anticipated movement, but the distance between the joints should not exceed 6m and should be between 1.5m and 3m from any corner. The joint width should be proven by calculation but is unlikely to be less than 10mm.
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.
The mortars for jointing and pointing should match the colour of the Portland Stone and be slightly weaker than the stone itself.
We recommend that the stone and the fixing contractor are selected as early as possible in the project programme.