Fig. 3.48 Damp-proofing basements by tanking with asphalt

16Lightweight walling systems

16.1Sheeting

Vertical cladding usually consists of aluminium or mild steel galvanised sheets with a substantial protective coating system. The sheets are normally corrugated or profiled to give additional uni-directional strength. The wall cladding including aluminium is usually coated with a film of alkyd amino-based paint er other approved material. All sheets can be obtained in various colours.

.Steel sheets are normally 0.8 mm thick and alumi­ nium sheets 1.2 mm thick. The zinc coating of the steel sheets should be in accordance with BS2989 [30]. Plastic films can be applied to sheets at the works.

Drying may be carried out using stove techniques or air drying processes depending on the type ol coating used. The film thickness may be from approximately 25 microns for the coated finishes to films in excess (if 250 microns for the laminated finishes. When comparing different materials, however, film thickness is not a measure of the relative durability characteristics of different materials.

Sheets can be cut by the manufacturers to the client's requirements and in many cases it is transport and erection difficulties which limit the length of sheet. Nevertheless, sheets up to 12 m long are commonly used for large walls to reduce the effects of horizontal joints. Cladding is also more rigid where sheets are continuous over a number of sheeting rails.

16.2 Insulation

Where exclusion of weather is the only design consider­ ation, single sheets of cladding without a lining arc­ sufficient. In locations where human occupancy and the necessity to preserve temperatures above those at which condensation forms (i.e., dew point tempera­ tures) require some form of heating, then to prevent excessive heat losses a lining of thermal insulation such as plaster board or similar material is essential.

16.3 Fixings

External sheeting is normally fixed by self-tapping screws, hook bolts, clips or other suitable means directly to the angle sections forming the sheeting rails. The insulation layer may be fixed to the back of the outer sheeting or it may be supported on the inner face of the sheeting rails, thus forming a cavity which increases the thermal insulation value. Although clad­ ding of this type can be used on a structure with a reinforced concrete frame, the spacing of the sheeting rails and other characteristics make it most suitable for use with a structural steel framework. Figure 3.49 shows^a typical arrangement of insulated cladding used in conjunction with a steel frame.

To obtain natural light inside a building with this type of fabric, windows of the normal pattern or, alterna­ tively, patent glazing consisting of wired cast-glass in aluminium or lead-covered T-shaped steel glazing bars, may be used. Another option is to use sheeting that incorporates panels of transparent material made from polyester resin reinforced with glass fibre. These sheets can be obtained with similar profiles to the main cladding material and may be incorporated directly into the cladded area. Alternatively, sheets of a different profile may be used with flashing at the top and bottom of the line of sheets.

In their application to power stations, these methods of construction have been developed to meet the need for lightweight cladding to structures. Their main