Traditional hand fixed cladding still dominates the use of 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 positioning and type of loadbearing and restraint fixings will be determined by a number of factors including the stone thickness calculation, the location of the movement joints and the actual structure of the building.
The building designer will be aware that the structure and the stone cladding will be subject to movement.
Compression joints are horizontal movement joints and are designed to accept the vertical movement of the structure
Expansion joints are vertical movement joints.
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 manufacture tolerances as set out in BS 8298 allow for typically 5mm ( +- 1.5mm) joints for all but the largest stones.
Cement Lime Mortars
The mortars for jointing and pointing should match the colour of the Portland Stone and be slightly weaker than the stone itself.
The cavity between the stone cladding and the backing structure will incorporate an air gap, insulation and a breather membrane.
Cavity Fire Stops
The cavity design needs to incorporate cavity barriers to prevent the spread of fire both vertically and horizontally.
The insulation needs to be non-combustible, rot and vermin proof and non-absorbent. The insulation may have a foil layer to act as a vapour control layer or a breather membrane applied to the outer face to control condensation.
Weep holes will need to be designed where trapped moisture accumulates at cavity trays and at the d.p.c. level.
The cavity trays and the d.p.c. should be formed from materials that will not degrade and stain the stonework.
We recommend that the stone and the fixing contractor are selected as early as possible in the project programme.
The designer should be aware of the recommended lead-in period for the supply of stonework cladding.