Cladding Systems - Structural Forms.

A variety of systems is available to suit environmental and financial constraints.The most common are listed below.

Single-skin trapezoidal roofing
This was widely used in the past with plasterboard or similar material as the lining material, and fibreglass insulation in the sandwich. The construction is susceptible to the plasterboard becoming damp due to condensation.An alternative is the use of rigid insulation boards, which are impervious to damp, supported on tee bars between the purlins. Unless the joints are sealed, which is difficult to achieve, con- densation is likely to form. Although inexpensive, this type is therefore limited in its applicability.

The minimum slope is governed by the need to provide watertight joints and  fasteners. If manufacturers’ instructions on the use of sealants and stitching to laps are rigorously followed, this type can be used down to slopes of approximately 4°.

Double-shell roof construction
In this form of construction the plasterboard has been replaced by a steel liner sheet of 0.4mm thickness with some stiffening corrugations. The lining is first installed and fastened to the purlins, followed by the spacing Zeds, insulation and outer sheet.

The liner tray is not designed to take full wind and erection loads, and therefore large areas should not be erected in advance of the outer skin.The liner tray is normally supplied in white polyester finish, providing a pleasing internal finish. The weatherproofing criteria are the same as for single-skin systems and generally the minimum slope is 4°. Differing thicknesses of insulation are accommodated by varying spacer depths. The norm is 80mm of fibreglass giving a nominal U value of 0.44W/m2 °C.

Standing seam systems
The traditional forms of construction described above suffer from the inherent disadvantage of having to be fixed by screw-type fasteners penetrating the sheet.

Traditional fixing methods also limit the length of sheet that can be handled even if, in theory, long lengths can be rolled; thus laps are required.

The need for weathertightness at the lap constrains the minimum slope. A  5000m2 traditional roof has 20 000 through fasteners and has to resist  around  1 million gallons of water a year. The difficulty in ensuring that this large number of fasteners is watertight demonstrates the desirability of minimizing the number of penetrations. This has led to the development of systems having concealed fastenings and the ability to roll and fix long lengths. In order to cater for the thermal expansion in sheets, which may be 30m long, the fastenings are in the form of clips which,while holding down the sheeting, allow it to move longitudinally.As discussed elsewhere, this may reduce the restraint available to the purlins and affect their design.When used in double-skin configuration the liner panel is normally conventionally fastened and provides sufficient restraint. The available permutations are too numerous to give general rules but purlin manufacturers will give advice.

It is necessary to fasten the sheets to the structure at one point to resist down-slope forces and progressive movement during expansion and contraction.With the through fasteners reduced to the minimum and laps eliminated or specially detailed, roof slopes as low as 1° (after deflection) can be utilized. The roofs must be properly maintained since accumulation of debris is more likely and ponding leads to a reduced coating life.

Standing seam systems are used to replace the traditional trapezoidal outer sheets in single- and double-skin arrangements as described earlier.

Composite panels
This most recent development in cladding systems provides solutions for many of the potential problems with metal roofing. The insulating foam is integral with the sheets and so totally fills the cavity, and with good detailing at the joints condensation can be eliminated in most environments.

The strength of the panel is dependent on the composite action of the two metal skins in conjunction with the foam. Theoretical calculations are possible although there are no codified design procedures. Since both steel and foam properties can vary, and these are predetermined by the manufacturer, it is a question of selecting the panels from load tables provided rather than individual design. In addition to having to resist external loads, the effects of temperature differential must be taken into account.The critical combinations are wind suction with summer temperatures and snow acting with winter temperatures. The range of temperature considered  is dependent on the colour and hence heat absorption of the outer skin; darker colours for roofs should only be considered in conjunction with the manufacturer, if at all.

Both standing seam and traditional trapezoidal forms are available with the same slope restrictions as non-composite forms.

A particular advantage is the erectability of the panels,which is a one-pass operation and, therefore, a rapid process. This is combined with inherent robustness and walkability.

Since the integrity of the panel is important, and it is difficult to inspect the foam and its adhesion once manufactured, quality control of the materials and manufacturing environment in terms of temperature and dust control is  vital. Reputable manufacturers should, therefore, be specified and their manufacturing methods ascertained.

External firewall
Where buildings are close to the site boundary the Building Regulations require that the construction is such that reasonable steps are taken to prevent fire spreading to adjacent property. It has been demonstrated by tests that walls of double-skin steel construction with fibreglass or mineral wool insulation can achieve a four hour fire rating. The siderails and fixings require special details which were included in the test arrangements of the particular manufacturer and it is important that these are followed closely. They include such things as providing slotted holes to allow expansion of the rails rather than induce buckling, which may allow gaps to open in the sheeting at joints.

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