Both sheets start life as the same steel coil. What separates them is what happens to the surface afterwards — and in a coastal climate that difference decides how long the roof lasts.
Galvanized (GI) and pre-painted galvanized (PPGI) sheets are the two workhorses of roofing and cladding across Dubai, Sharjah and Ajman. They look similar on a delivery note and behave very differently on site, so it is worth being clear about what you are ordering.
The base material is the same
Both begin as a cold rolled steel coil that is hot-dip galvanized — passed through molten zinc so the surface carries a metallic coating. That zinc layer is what protects the steel underneath. PPGI takes that galvanized coil one step further: it is cleaned, chemically treated, primed and finished with a baked-on paint system in a continuous coil-coating line.
So every PPGI sheet is a galvanized sheet. The reverse is not true.
Coating weight matters more than thickness
Buyers tend to focus on sheet thickness and ignore the zinc coating weight, which is written as Z120, Z180, Z275 and so on — the number is grams of zinc per square metre across both faces. For a covered warehouse roof inland, a lighter coating is often fine. Within a few kilometres of the coast, or anywhere with industrial fallout, the heavier coatings pay for themselves.
- Z120–Z150 — internal partitions, ducting, light structures under cover
- Z180–Z220 — general roofing and cladding away from the shoreline
- Z275 and above — coastal sites, humid or chemically aggressive environments
Where PPGI earns its cost
The paint system on PPGI does two jobs. It seals the zinc from moisture, and it reflects a share of the solar load so the space underneath runs cooler. On a large shed roof through a Gulf summer that reflectivity is not a cosmetic detail — it changes the cooling bill.
Polyester finishes are the common default and hold up well for standard projects. Where the specification calls for a longer service life or heavy UV exposure, silicone-modified polyester and PVDF systems are the step up, at a corresponding price.
Corrugated, trapezoidal or plain sheet
Profile is a structural decision, not a styling one. Corrugated and trapezoidal profiles add stiffness along the span, which lets you increase purlin spacing for the same sheet thickness. Plain sheet is for flashings, fabrication and anything that will be rolled or pressed later.
What to send when you ask for a price
Quoting galvanized sheet without these five details always ends in a second round of emails. Send them together and the quote comes back the same day:
- Thickness in mm (base metal thickness, not including coating)
- Width and length, or coil width if you are buying coil
- Zinc coating class — Z120, Z180, Z275
- Profile: plain, corrugated, trapezoidal, chequered
- For PPGI: colour reference (RAL), paint system and top/back coat thickness
A short checklist before you order
Ask for the mill test certificate and check that the coating class on it matches your purchase order. Confirm whether the quoted thickness is BMT or TCT, because the two differ by the coating and that difference shows up in the weight you pay for. And if the roof is within sight of the water, do not economise on the zinc — the cost gap between Z180 and Z275 is small next to the cost of replacing a roof early.