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Single-Storey Frames, Shells and Lightweight Coverings 237
glass is that it provides a clear view and, with regular washing, maintains a bright surface
appearance.
Profiled, cellular and flat plastic sheets
Transparent or translucent plastic sheet material is used as a cheaper alternative to glass.
The materials used for profiled sheeting are:
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uPVC – PVC – rigid PVC. This is one of the cheapest materials and has a light transmittance
of 77%, reasonable impact resistance and good resistance to damage. The
material will discolour when exposed to solar radiation.
Glass reinforced plastic (GRP). GRP is usually inflammable, has good impact resistance,
rigidity and dimensional stability. The material is translucent and has a moderate
light transmittance of between 50% and 70%. Translucent GRP sheets comprise thermosetting
polyester resins, curing agents, light stabilisers, flame retardants and reinforcing
glass fibres. Three grades of GRP sheet are produced to satisfy the conditions
for external fire exposure and surface spread of flame.
The materials used for flat sheet rooflights, laylights and domelights are:
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Polycarbonate (PC). This material has good light transmittance, up to 88%, good
resistance to weathering, reasonable durability and very good impact resistance. PC is the
most expensive of the materials and is used principally for its high impact resistance.
Polymethyl methacrylate (PMMA). This plastic is used for shaped rooflights, having
good impact resistance and resistance to ultraviolet radiation, but softens and burns
readily when subject to the heat generated by fires.
Rooflights
The most straightforward way of constructing rooflights in pitched roofs covered with
profiled sheeting is by the use of GRP or uPVC, which is formed to match the profile of
the roof sheeting. The translucent sheets are laid so that they cover the lower sheet and
adjacent sheet to form an end and side lap, respectively. All side laps should be sealed with
self-adhesive closed cell PVC sealing tape to make a weather tight joint. End laps between
translucent sheets and between translucent sheet and roof sheets to roofs pitched below
20° should be sealed with extruded mastic sealant. Fixing of sheets is critical to resist wind
uplift, in common with all lightweight sheeting materials used for roofing, and the fixing
usually follows that used for the main roofing material.
Double skin rooflights are constructed with two sheets of GRP, as illustrated in Figure
4.74, which have the same profile as the sheet roof covering. Profiled, high-density foam
spacers, bedded top and bottom in silicone mastic, are fitted between the sheets to maintain
the airspace and also to seal the cavity. Double-sided adhesive tape is fixed to all side
laps of both top and bottom sheets as a seal. The double skin rooflight is secured with
fasteners driven through the sheets and foam spacers to the purlins. Stitching screws are
then driven through the crown of profiles at side and end laps. Factory-formed sealed
double skin GRP rooflight units are made from a profiled top sheet and a flat underside
with a spacer and sealer.