Traditional Handset Cladding

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.

Traditional Cladding

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.

We recommend the early appointment of the specialist stone cladding designer, who is normally part of the stone fixing contractor’s team. It is important to ensure they have the technical expertise and competence to advise on the fixing system and complete the stone cladding design. The designer should consider whether the fixings and the fixing system, including the stone, should be subject to testing to prove there is sufficient strength. Albion Stone’s Technical Data Sheets provide the dowel pull out test results (BSEN 13364:2002) which will assist with the selection of the fixing system.

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Fig. Restraint Fixings

 

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Fig. Load bearing Fixings

Movement Joints

The building designer will be aware that the structure and the stone cladding will be subject to movement. 

This movement will be initially due to shrinkage and/or elastic deformation under load, but ongoing movement will occur due to differential thermal movement of the structure, the fixings and the stone,  the moisture content of the stone and movement from wind loading. It is recommended that the structural engineer/designer sets out the calculations showing the effects of these various movements and ensures that the cladding  can accommodate them.


Compression Joints

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. The recommended minimum width is 15mm.


Expansion Joints

Expansion joints are vertical movement joints. 

The spacing and the widths of these joints need to be designed to accommodate the anticipated movement, but it is recommended that the joints are not less than 10mm per 6m length of cladding and should be between 1.5m and 3m from any corner.

Sealants

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.

Mortars

Joint Width

The manufacture tolerances as set out in BS 8298 allow for typically 5mm ( +-  1.5mm) joints for all but the largest stones. 

Some buildings are designed with 3mm ( +-€“  1mm) joints, but the designer needs to be aware that these joints are at the limit of the effectiveness of conventional mortars  and there will be a cost premium for the higher manufacturing tolerances.

The maximum width of a mortar joint is 13mm, but a sealant filled movement  joint can be up to 30mm wide.


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. 

To ensure the colour match, Portland Stone dust should always be used as the  aggregate. The recommended mixes are set out below:

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Cavities

The cavity between the stone cladding and the backing structure will incorporate an air gap, insulation and a breather membrane.

The cavity will need to be sufficiently wide to allow for free draining of any trapped moisture allow for insulation, accommodation of the fixings, bolt heads etc. and to take up all structural tolerances. The minimum width of the air gap should not be less than 10mm after all the construction tolerances have been included, but please note that a cavity width greater than 100mm may need supplementary framework to support the stone.


Cavity Fire Stops

The cavity design needs to incorporate cavity barriers to prevent the spread of fire both vertically and horizontally.


Insulation

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

Weep holes will need to be designed where trapped moisture accumulates at cavity trays and at the d.p.c. level.


Cavity Trays

The cavity trays and the d.p.c. should be formed from materials  that will not degrade and stain the stonework.

The cavity trays need to be placed at appropriate points to direct moisture out of the cavity where it could degrade the insulation or cause dampness to the backing structure

Fixing Contractor

We recommend that the stone and the fixing contractor are selected as early as possible in the project programme.

The stone fixing sub-contractors will normally take responsibility for the entire stone package including stone drawings, stone procurement and stone fixing on site. Therefore it is vital that the selected company has the necessary experience and expertise

Programme

The designer should be aware of the recommended lead-in period for the supply of stonework cladding. 

A typical programme for a medium sized office development in Central London has been set out below:Progress_Screen_Shot.jpg

Specification Clause