Types of metal cladding system
The three principal forms of metal cladding are profiled sheeting, cassette panels and composite panels.
An appropriate way to classify metal-based cladding systems is by the method of manufacture. Most systems can fall into one of three categories. profiled sheeting, cassette panels and composite panels.
Profiled sheeting is one of the most common systems and has been in use for many years.
One of the earliest forms of industrialised metal cladding, often described as corrugated sheeting, stemming from the original manufacturing process. Initially a low cost cladding for industrial buildings with the development of new coating, profile cross sections and detailing it is used extensively in a wide variety of buildings.
Steel or aluminium is normally used for profiled sheeting. It can be applied to both roof and walls, in each case the performance requirements vary. On roofs the sheeting will be exposed to greater solar gain and hence thermal movement which has implications for end laps between sheets and fixings. The roof will also be subjected to foot traffic during both installation and maintenance. The finish has to be designed for greater toughness. On walls, as they are more visible, the colour fastness of the finish is more important.
The internal liner is not airtight and water vapour may pass into the construction.
In buildings where larger quantities of water vapour are produced or there is inadequate ventilation or air conditioning is used it is necessary to prevent as much vapour entering as possible. The metal lining being impervious will not allow vapour to pass but the joints and penetrations should be sealed. An alternative is to lay polythene sheets over the liner which can then be unsealed. The laps in the polythene should be taped to provide an effective seal.
A slightly more contentious form of condensation occurs through super-cooling.
On a clear night there can be considerable heat loss from a building through radiation. In the case of a metal roof this can cause the surface temperature to drop several degrees below the ambient temperature. If the moisture content of the air is close to dew point (100%RH) then water may condense on both the inside and outside of the outer sheet. In this case the cavity underneath the outer sheet should be ventilated at ridge and eaves which will then allow any condensed water to evaporate.
A key factor in the success of profiled metal cladding was the development of self drilling and tapping screws which enable the rapid and secure fastening of the sheets from one side.
Conventional bolts and hooks are much slower to install and require access from both sides of the sheet. Most screws are carbon steel with some form of metallic or metallic and organic coating to improve durability. Washers and heads are incorporated in screw. The screw heads can be covered with plastic caps to match the colour of the cladding. There are many varieties of screw, of different lengths, cutting heads and thread pitch and depth, depending on the type of cladding the substrate to which it is attached and the type of insulation used in the cladding system.
Self tapping screws and rivets are also used for secondary fixings, such as connecting side laps between sheets.
There is a number of proprietary composite cladding systems available in the UK, generally there is little difference in the actual core and facing material, the major differences lie in the joints between panels, the range of sizes and the range of architectural features such as corners and curved panels that are available.
Panels may also be laminated.
This process applies to the other core materials. Adhesive is applied to the steel faces and pre-formed slabs of insulation. The panels are then clamped together either in a press or by passing them through a series of rollers.
The table below compares the thermal performance of the different core materials
Thickness (mm) for a U value of 0.45W/m2K
Glass or mineral fibre
Thermal performance of core insulation (source: The Metal Cladding and Roofing Manufacturers Association)
The plastic cores are combustible however they are protected by the metal facings. Such panels can achieve class 0 performance with regard to the building regulations. If a fire resistant wall is required, at a boundary condition for example, then a fire rated construction is necessary. Composite panels with mineral fibre cores can be designed to achieve fire ratings up to 2 hours.