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Translucent Sheeting

Translucent sheets should be fastened in the same way as metal cladding, except that extra or special fixings are required because plastic sheeting deflects more.

A different depth or screw gun torque setting may be required to avoid over tightening and subsequent distortion around the fixing. Because of their different rates of thermal expansion, plastic and metal sheet fixings should be fastened through oversized holes. Support for plastic sheeting is obtained from the metal sheeting for downward loads and span-breakers for uplift loads. See Midspan Supports.

All plastic roof light sheets must be laid on top of the metal sheeting at both edges.

Side laps must be fixed at a maximum of 600 mm centres (fixing details in Side-Lap Fixing) and are required to be sealed where the roof pitch is less than 10° and in high wind design load areas.

Oversized holes and load-spreading washers must be used when fixing roof lights to metal sheeting. All plastic roof lighting must have pre-drilled intermediate side lap fastenings with oversized holes on all spans larger than 1.2 m. See Side-Lap Fixing.

To avoid stress, roof lights should always be fixed starting from one end and fixing the intermediate purlins before doing the ends.

Deflection limits should be placed on the performance of plastic sheeting to ensure longevity and to avoid incremental failure due to flexing at the fixing points. This can be done by using mid-span supports. Plastic sheeting should not be used in applications where it approaches its deflection limits (50 mm).

Shallow symmetrical profiles of plastic roof lighting can be curved to suit draped curved metal cladding. However, the radius is determined by the profile, material type and thickness. See Curved Roofs.
When long lengths of plastic sheeting are not supported at mid-span, their longevity is reduced and incremental failure can be caused by the flexing at their fixing points.

The two main factors that limit spanning capability are deflection and pull-through over the fasteners.

As standard weight plastic roof lights have a low pull-over load and the critical condition is that imposed by wind suction; their performance is determined by the number of fasteners per square metre.

Load-spreading washers specifically made to fit the plastic sheeting must be used under all fasteners.

All fixings and washers must meet the same durability standard as the roof and wall cladding.

All plastic sheeting must have an enlarged fixing hole for the provision of thermal movement, predrilled to the following diameters for 12# or 14# self-drilling screws for both timber and steel purlins.

The fixing must be positioned in the centre of the oversize hole to ensure that the sheet has equal movement in all directions around the fixing.

GRP Fixing Positions

For G.R.P.
Sheets up to 6 m
Sheets 6 to 9 m
Sheets 9 to 18 m
8mm
10mm
12mm
(optional caption)

PVC Fixing Positions

For PVC and Polycarbonate
Sheets up to 4 m
Sheets up to 6 m
Sheets over 6 m
10mm
12mm
NR
(optional caption)

 

The minimum fixing pattern for roof lighting cladding is determined by the span, the design load and the profile thickness, but fixings with load-spreading washers are recommended for all profiled ribs, except for spans less than 1 m.
Tray profiled GRP roof sheeting must be mechanically fixed to provide for the uplift load specific to the site, and must be placed on top of the adjacent metal sheeting.

In extreme environments where there are chemical fumes and the fixings are prone to chemical attack from the inside of a building, stainless steel fixings should be used.

When pan fixing is used, the fastener should be positioned within 25 mm of the rib.

 

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