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Cladding

Generic types of cladding:

Boards (other than timber)
Decorative panels can be obtained that visually represent stone, exposed aggregate and textured or painted finishes. Some of these boards are manufactured from glass-reinforced composites, often with a claim from the manufacturers of high dimensional stability. It is important to give proper consideration to fire resistance when specifying such boards. Timber laminates may not provide the required 'spread of flame' condition. Compatibility with building movement is particularly important when installing high stability boards.

Concrete

Blocks
Weather resistant blocks conforming to BS 6528-1:1992 can be provided and may be laid unreinforced or strengthened/prestressed with steel. Detailing of reinforced blockwork is similar to that of brickwork and advice is given in BS 562-2:2000.

In-situ
These are rarely used in practice except, perhaps, on industrial buildings but should conform to BS 8110-1:1997 (to be replaced by EN 1992 Eurocode 2: Design of Concrete Structures).

Facing brickwork (including brick slips)
This should comply with the following:

  • BS 5628-1: 1992 Structural use of unreinforced masonry
  • BS 5628-2: 2000 Structural use of reinforced masonry and prestressed masonry
  • BS 5628-3: 1985 Materials and components design and workmanship

Proper provision should be made for differential movement. This is particularly important when attaching brick cladding to a reinforced concrete frame, as brickwork expands in the long term whereas concrete shrinks. Brick panels should be supported at regular vertical intervals not exceeding 10m. The use of brick slips to hide the faces of concrete supporting nibs should be avoided, if possible. In places where such supporting nibs are used, mechanical means of support would enhance adhesive fixings.

Faience and terra-cotta
Faience is a glazed terra-cotta often manufactured as large tiles and used as a form of tile-hanging. Alternatively, concrete-filled hollow blocks may be used and assembled in a manner similar to that used for concrete blocks.

There is no standard that covers the design and installation of faience but older books on building construction such as Mitchell's Advanced Building Construction give some guidance.

Failure can occur due to poor workmanship and/or poor quality materials.

Glass
Glass is, basically, a compound of silica (Si02), soda (Na2O) and lime (CaO) with trace impurities of iron from the sand (silica). To obtain special properties, these basic materials are augmented with controlled additions of other 'impurities' e.g. boron (borosilicate glass) for heat resistant applications such as laboratory glassware and Pyrex™ or silver and copper for photochromic applications.

In general, glass is brittle with virtually no ductility and gives little or no warning when about to fail, largely due to its amorphous molecular structure. Minute surface flaws, too small to be seen with the naked eye, can be caused during manufacture and handling and can seriously affect glass' strength. Safety in use must be paramount in the mind of the designer; this will be affected by many site factors including site location, liklihood of impact, size of individual panels, support conditions, fire safety requirements, design loading from weather, catchment arrangements in case of fracture and access arrangements.

Some guidance is given in BS 952-1 and BS 6262. Useful reference documents are Glass In Building by D. Button and IStructE reports Aspects Of Cladding (1995) and The Structural use Of Glass In Buildings (1999).

Failure may occur due to inclusions or discontinuities in the material (which may give rise to spontaneous rupture in service), poor fixing details, poor technique in on-site execution and impact or explosion.

Glass curtain walling
This is a generic term used to identify cladding which usually consists of a combination of glass used in conjunction with metal mullions. The mullions may be aluminium, coated steel or stainless steel. In low height applications the mullions may even be uPVC. In some architectural configurations opaque panels may be interspersed with the glass. Many proprietory systems exist and some faults may occur where some specialists attempt to 'mix and match'. Other common faults include defective sealing arrangements allow the penetration of damp or providing a cold bridge. When using an unfamiliar system, it is advisable to have a section assembled and tested, both for buildability and also environmental resistance.

A designer should take into account, inter alia, horizontal and vertical movement, glass deflection criteria, cold bridging and, for the mullions, structural and drainage continuity.

Expert advice on all aspects of curtain walling may be obtained from the Centre for Window and Cladding Technology (CWCT) in Bath, Somerset, England

Glass fibre reinforced cement (GRC)
This material usually consists of glass fibres in a matrix of Portland cement and fine sand but more recent formulations contain borosilicate E-glass in a polymer-modified Portland cement mix. Very careful attention must be paid to the lifetime environmental conditions in use as well as the production of such a composite material.

Alternative finishes include simulated concrete with decorative (off the mould) colouration.

Problems in service can arise due to top-hanging (bottom-support is preferable), potential long term material strength reduction, complex shapes, crazing and those due to the use of sandwich panels.

Further information can be obtained from BRE Digest 331 and the Glass Reinforced Cement Association (GRCA)

Glass reinforced plastic (GRP)
GRP panels may be fabricated by epoxy resins being applied in layers to a moulding surface which are interspersed with chopped-strand glass-fibre reinforcement. A hardening agent (a catalyst which causes cross-linking in the molecular structure of the copolymers forming the resin) ensures that the resultant composite material is thermo-plastic in nature and will not soften under the action of heat.

It is essential that the manufacture of GRP is strictly in accordance with the recommendations of the National Glass Reinforced Plastics Construction and Engineering Federation (NGRPC). It is preferable to bottom-support units made from this material.

Metal sheeting
This is usually manufactured in aluminium alloy, stainless or coated mild steel or copper. Sheets may be plain or profiled and are often backed by insulating material.

Finishes include:

  • Plain matt, bright, pearl or brushed
  • Textured leather, linen embossed or mosaic

Guidance for the design of steel sheeting may be obtained from BS 5950-6 and for aluminium from BS8118-1 and 2.

Problems may occur by edge damage (particularly to the coating of mild steel panels) which eventually leads to corrosion. In a limited number of cases, poor quality organic coatings have led to corrosion and there is some evidence that dark colour coatings (with their concommitant heat retention) may be more vulnerable to loss of adhesion than lighter coloured coatings.

Renders
External walls may be rendered for aesthetic reasons and/or weather protection. The normal form of render is that using two or three cement/lime/sand materials.

BRE Digest 196 identifies five types of render:

  • Pebbledash
  • Roughcast
  • Scraped-off texture
  • Plain coat
  • Machine applied finish

Problems may occur from crazing and patchy appearance. Poor preparation, bad workmanship and the incorrect choice of materials may lead to poor adhesion and the eventual breakdown of the render. The use of coated or stainless steel mesh may be prudent in some applications.

Slates
These can be of naturally occuring stone or synthetic e.g. GRC. Some synthetic slates carry a British Board of Agrément (BBA) certificate and potential users would be well advised to obtain a copy of this.

Good practice suggests that the foloowing be observed:

  • Fixing nails to be non-ferrous
  • Correct and generous lap lengths to slates
  • Tiling battens to have preserving treatment
  • A substrate which is adequate to support the battens and which is protected by a damp-proof membrane

Further information can be found in BS 5534

Technotrade Structural Systems Ltd. can advise on specification, procurement and erection of cladding systems. We can also test new cladding systems as well as carrying out condition surveys of existing cladding. Advice on methods of repair can be given.

Cladding to building structures fails for a variety of reasons. These include:

  • Lack of movement joints
  • Poor detailing
  • Badly executed fixings
  • Non-adherence of brick slips due to poor fixing techniques
  • Lack of attention to tolerances
  • Corrosion of metal sections
  • Bi-metallic action between electro-chemically active metals
  • Poor selection and/or application of sealant materials
  • Breakdown or displacement of sealant materials

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