Joints between sheets

Side laps of one corrugation will generally prevent driving rain passing through the outer cladding, but when exposure is 'severe' (e.g. in areas with a driving rain index of 7 or more) an increased lap may be needed (Fig. 5.28).

Side laps may sometimes need to be increased locally to avoid cutting sheets at corners and openings.

Table 5.3 Protective measures for aluminium sheetirr

Copper alloys

No protective measure possible; avoid

direct contact and water run-off from

copper onto aluminium, unless

organically coated.

Iron and steel

Direct contact. Unless organically coated,

use non-aggressive protective paint, e.g.

zinc chromate. Avoid copper-bearing

paints. In industrial and marine

atmospheres, use non-rotting chromate

insulating tape.

Water run-off. Paint iron, galvanize and

paint steel (non-copper-bearing, non-

aggressive paint).

Lead

Unless organically coated, avoid contact

and run-off.

Timber

Compatible non-copper-based

preservative (see CP 1430) bituminous

paint or felt or building paper

Boards

Moisture barrier such as building paper

or polythene.

Concrete, plaster

Bituminous paint and felt insert

asbestos cement

between contact surfaces.

5.26 Cartridge fasteners for metal cladding.

Such laps are easy to increase with regular profiles, but are difficult with irregular profiles and sheets with bonded insulation.

End-lap joints on walls will normally be I 00 mm, and on roofs 150 mm, but when exposure is severe the end-lap should be increased to 200 mm.

5.27 Aluminium roof sheeting at Stansted Airport, fixed on the ground and lifted into position as complete assembly

Sealing of lap joints with mastic is generally not used in vertical sheeting for weather protection. However, sealed side-laps are required for pitched roof-cladding below 15° pitch (Falconer, 1981). Swedish building practice HusAMA also recommends weather seals for profiled sheet roofing for pitches below 14°, and requires side-laps to be joined with screws with washers to protect from leakage, or rivets maximum centre-to-centre 500 mm. Corner details are an important design consideration. Figure 5.29

5.28 Side-laps between sheets, (a) Maximum flexibility in side-laps is provided by sheets with uniform troughs and half-corrugations, (b) Standard sheets with factory-bonded backings do not permit variations in side-lap. (c) Sheets in which corrugations are not uniform do not allow variations in side lap.

5.28 Side-laps between sheets, (a) Maximum flexibility in side-laps is provided by sheets with uniform troughs and half-corrugations, (b) Standard sheets with factory-bonded backings do not permit variations in side-lap. (c) Sheets in which corrugations are not uniform do not allow variations in side lap.

shows various ways of turning a corner using flashings. Over-sheet corner flashings usually need to be wide in relation to the profile of the main cladding and tend to conflict with it in scale. Appearance and weather-

5.29 Various methods of corner detailing using flashings: (a) over-sheet corner flashing; (b) over- and under-sheet corner flashing; (c) under-sheet corner flashing.

5.29 Various methods of corner detailing using flashings: (a) over-sheet corner flashing; (b) over- and under-sheet corner flashing; (c) under-sheet corner flashing.

ing are improved by inverting the edges of the angle, but this allows little flexibility in positioning the sheets with the profile shown. Curved-over-sheet corner flashings can also be used.

Using over- and under-sheet corner flashings, these can be shaped to match the profile of the main cladding.The upturned lip of the main cladding is also covered, offering a better water check.

With under-sheet flashings, in order to avoid the exposure of an upturned lip, one of the cladding sheets has to be reversed.The length of the under-leg of the flashing offers some tolerance in the assembly. In some cases the sheets are butted against the steel frame and the frame exposed at the corner (Fig. 5.30).

With horizontal cladding, various corner-joint details have been used including welded corners and special GRP closer ends with matching finish, as used at IBM Greenford (architects: Foster Associates). Tolerances in manufacture and assembly are even more critical with horizontal cladding, because any misalignment of the profiles can cause apparent waving. Slip trays can also be used at the joints between horizontal cladding; however; it is sometimes difficult to match the profiles exactly, and the thickness of the slip tray tends to make the edge of the sheet more obvious in appearance.

Profiled sheeting is best used in well-propor-tioned large areas, to enclose the main structure with the minimum of joints with other materials.Translucent sheets are available to match most profiles. If opening lights or transparent areas are required, sheets of patent glazing are better than small isolated windows, because they relate more naturally to the scale of the main cladding and reduce the problem of jointing around window openings.

Butt-end joints between sheeting and windows are normally made with Z-flashings or propriety neoprene sections (Fig. 5.3 I).

5.30 Corner detail with exposed steel frame.
5.3 I Neoprene flashing at window opening.

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