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Sealing Joints in Natural Stone Cladding |
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New Build |
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Movement Joints
In stone cladding, movement joints are required and a full calculation method is given in BS 8298:1989 but generally as a minimum, 10 mm wide vertical joints should be provided at 6 m centers and horizontal joints at every floor level.
Sealants are used to seal these joints, while accommodating movement, for weatherproofing the outer skin of the structure. Various chemical types are available, but oil based mastics should never be used with stone.
Sealants For Stone
Sealants generally compatible with stones are silicones, polysulphides and polyurethanes. Care should be exercised in the particular sealant selection for individual stones; the sealant manufacturer should be consulted for advice to avoid problems such as staining. Extensive colour ranges are generally available.
In properly designed movement joints, installed with correct application procedures, the life of the sealants can be expected to be fifteen to twenty years. Clearly, the degree of exposure and local conditions will have effect on this.
Movement Joint Design
It is important to employ sealants in properly designed movement joints to place minimum stress on the sealant/substrate interface.
Normally a width to depth ratio of 2:1 is considered ideal for sealants, but this may need to be modified depending on particular designs encountered.
A non-bonding backing material should be employed, for a variety of reasons including:
i) to prevent three-sided adhesion which could lead to unacceptable stresses building up in the sealant,
ii) to control the sealant depth to the required value, and to provide a compressible space into which the sealant can deform,
iii) to provide support when gunning in and tooling the sealant.
Typical materials employed as bond breakers are closed cell polyethylene rod and polyethylene tape; the type selected normally depends on joint dimensions.
It is, of course, important to remember that sealants will be required to bond not only to stone but also metals, paint finishes, etc. In order to obtain the best 'system' for the job, primers may be needed and should be specified by the sealant producer. If specified they must be used.
NON MOVEMENT JOINTS
Sealants are extensively used to seal all the joints in facades of granite panels, in which case the performance requirement is lower. However, many of the above comments are still relevant, although the width to depth ratio is no longer so important and backing materials are not always employed.
Application
Application of sealants should always be carried out by trained personnel. These may be available within stone companies but generally independent firms are employed, and a list of suitable companies can be obtained from the Association of Sealant Applicators (ASA), see below.
Maintenance
Similar to other components used in buildings, correct maintenance will help to prolong the life of sealants. A simple schedule consists of visual inspections during application and upon completion, followed by maintenance inspections by the building owner after one year, then at five yearly intervals, with remedial action as recommended by the sealant manufacturer, for the particular product concerned.
Reference:
BS 8298 Code of Practice for design and installation of natural stone Cladding and lining.
BS 4254 Specification for two-part polysulphide based sealants.
BS 5215 Specification for one-part gun grade polysulphide based sealants.
BS 5889 Specification for one-part gun grade silicone based sealants (Type A)
BS 6093 Code of Practice for design of joints and jointing in building construction.
BS 6213 Guide to selection of constructional sealants.
BASA British Adhesives and Sealants Association 2A High Street Hythe Southampton S04 6YW Tel No: 0703 842765
ASA Association of Sealant Applicators 44 St Mary's Drive South Benfleet Essex SS7 2LB Tel No: 0268 751259 |
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