Fibre sheets for daylight

Sadhana Fibre Glass Industries publishes its FRP sheet at 1 to 10 mm thickness, corrugated to match a GI or asbestos profile, in clear or in a translucent colour, and states a light transmission of 80 to 95 percent at the maximum end of that range. Matching the profile is the whole point: a corrugated FRP sheet cut to the same wave as the GC or cement sheet around it beds onto the same purlins and closes the same side lap, so it reads as one roof plane rather than a patch let into it.

This shop already treats the strip as a standard fitting rather than a special order. The accessory list behind the GC sheet, the colour-coated sheet and the fibre sheet itself all carry daylight sheets alongside purlins, ridge, flashing, screws, gutters and sealant, so a shed order built around any of the three can add a daylight run without a second visit.

Cladco publishes an exact rule for its own box-profile GRP rooflight: each sheet overlaps 250 mm over a purlin, and no more than two rooflights sit side by side before the roof goes back to metal sheet, a limit the maker gives mainly to keep the plane's strength, with the light as a second reason. It is a steel-roof rule from a steel-roof maker, published for that one box-profile product, and the reasoning behind it, that a wide unbroken run of translucent sheet weakens the plane it sits in, holds on a corrugated cement roof as much as on a box profile one.

On a cement roof the same job is done by a different fitting. Everest Industries' own accessories brochure names the corrugated north light curve, part BS9, sold in pairs, fixed at the apex of a north light roof, where the curved wing protects the glazing under it from sun and rain. A north light roof is the sawtooth-shaped factory shed roof, and the curve sits at the high point of each tooth instead of scattered down the pitch, the cement-sheet answer to the same question Cladco's spacing rule answers on steel.

Dyna Fiber sits on this counter's fibre sheet list alongside Tata and the Durashine group range, and the maker publishes no technical page to check a thickness or a transmission figure against. A Dyna Fiber sheet gets measured and held up to the light on delivery instead of read off a brand name, and it is ordered by size and colour rather than by a spec sheet that is not on record. It sits under the roofing and shed listing along with the GC and colour racks it is usually ordered beside.

Everest and Charminar sit on the same brand list for the cement side of the rack, and a strip has to be cut to whichever of their lines is going up beside it. Charminar sits under BirlaNu and carries more than one roofing line, at more than one width. So does the Everest side of that rack. Which line the quote names is what sets the profile and the corrugation depth the FRP or GRP strip is matched to, and it is worth settling before anything is cut. The Everest brand page and the Charminar page above carry what each maker names for itself.

Neither Everest's roofing page nor Charminar's BirlaNu page prints a light transmission figure for anything translucent; the 80 to 95 percent range above is FRP and GRP sheet makers' own figure for the strip, and it says nothing about the opaque cement or steel sheet beside it. Corrugation depth, cover width and screw spacing on the strip have to match whatever opaque sheet is already going up, cement, GI or colour, or the side lap will not close, so the profile in hand at the counter settles it faster than a catalogue number does.