Most Indian sheds are roofed in GI or other metal sheets, and most FRP roofing enquiries start with the same question: should I use FRP instead, or alongside it? The honest answer depends on three things: what is in the air around the building, whether you need daylight inside, and how much maintenance you can afford later. This guide compares the two so you can decide sheet by sheet.
The short answer
- Choose FRP where the roof faces acid fumes, chemical vapours, coastal salt air or constant humidity, and wherever you want natural daylight through the roof.
- Metal sheets still make sense on dry, inland sheds with no corrosive exposure, where no daylight is needed and the building is already designed around a metal roof.
- Many buildings use both: a metal roof with FRP sheets in the same profile at regular intervals for daylight, or FRP over the process areas where metal corrodes fastest.
1. Corrosion and chemical exposure
This is the biggest difference. Steel sheets rely on a zinc or paint coating, and once that coating is scratched or attacked by fumes, rust starts. In chemical plants, ETPs, fertilizer godowns, electroplating units, paper mills and coastal sites, that happens quickly. FRP does not rust at all. For the harshest conditions, FRP sheets are made with a chemical-resistant resin system instead of general-purpose resin, and cut edges stay waterproof because FRP does not rust or fray.
2. Daylight
Metal sheets block all light, so a metal-roofed warehouse needs electric lighting all day. Translucent FRP skylight sheets let through 60–85% of daylight depending on grade, and diffuse it so there are no harsh hot spots. A common layout is one strip of translucent sheets every 10–12 m of roof width, which in a typical warehouse removes the need for daytime electric lighting.
3. Fitting into an existing metal roof
You do not have to replace a whole roof to get the benefits of FRP. FRP sheets are moulded to match common Indian metal profiles, including GI corrugated and the trapezoidal box profiles used on PEB sheds. A matched FRP sheet bears on the full profile, laps with the metal sheets beside it and reuses the same fastener pattern. Our profile matching guide explains how to measure your existing sheets.
4. Fixing differences installers must know
FRP is fixed much like metal, with a few important differences:
- Pre-drill holes 2 mm oversize. FRP and steel expand differently with heat, and a tight hole can split the sheet within one season.
- Use roofing or stitching screws with EPDM washers, and do not overdrive them. The washer should be compressed, not bulging.
- Seal laps with neutral-cure silicone or polyurethane. Acid-cure silicone attacks the resin.
- Walk only along purlin lines. Never step mid-span in a valley.
Full details are in our FRP roofing installation guide.
5. Maintenance over the life of the roof
A metal roof in a corrosive environment needs regular inspection, rust treatment and repainting, and corroded sheets eventually need replacing. FRP needs no painting or rust treatment. What it does need is the right surface protection from day one: specify a UV-stabilised film or gelcoat on the weather side. That is the difference between a sheet that weathers in about 5 years and one that lasts around 15.
6. Cost
FRP sheets are priced per square foot by thickness, profile and translucency. Indicative ranges for each thickness band are on our FRP roofing sheet price list. When you compare against metal, include the cost of repainting and replacing corroded sheets, and the electricity saved by daylighting, not just the purchase price.
Quick decision guide
- Chemical plant, ETP, fertilizer or coastal site: FRP with chemical-resistant resin.
- Warehouse that is dark during the day: keep the metal roof and add FRP skylight strips in the same profile.
- Old shed with rusted GI sheets: replace the worst bays with GI-matching corrugated FRP sheets.
- New PEB building: metal or FRP cladding as designed, with trapezoidal FRP sheets for daylight and corrosive zones.
Not sure which suits your building? Send us the location, what the building is used for and a photo of the existing roof, and our team will recommend a combination. See the full FRP roofing sheets range.
