What the two coatings are
Both start as the same steel strip. What differs is the metal the strip is dipped in before it is painted.
Galvanised steel is coated in zinc. The coating is measured in grams per square metre, counting both faces, so Z275 (often written 275 GSM) carries 275 grams of zinc on every square metre of sheet. Lighter classes such as Z120 and Z180 are common on cheaper sheet. The Indian standard for it is IS 277.
Galvalume is coated in an alloy of about 55% aluminium and 43% zinc, with a little silicon. It is measured the same way, so AZ150 carries 150 grams of alloy per square metre, both faces. The standards are AS 1397 and IS 15961. Galvalume, Zincalume and Aluzinc are all trade names for this same coating.
Once painted, the trade calls them PPGI (pre-painted galvanised iron) and PPGL (pre-painted galvalume). Many sellers call every colour coated sheet "PPGI" out of habit, so the name on a quote proves nothing, and only the coating class tells you which one you are getting.
Side by side
| AZ150 galvalume | Z275 galvanised | |
|---|---|---|
| Coating | 55% aluminium, 43% zinc, a little silicon | Zinc |
| Coating mass | 150 g per sq m, both faces | 275 g per sq m, both faces |
| Coating thickness | About 20 microns a face | About 20 microns a face |
| Life on a roof in inland air | Longer, about two to four times at the same thickness | Shorter |
| Cut edges | Can show light rust early, then usually stops | Protected longer |
| Against wet concrete | Keep apart | Better, but still keep bare metal out of concrete |
| Welding | Not meant to be welded | Can be welded and touched up |
| Typical use | Roof and wall sheet | Heavy welded gutters that hold standing water |
| Painted version | PPGL | PPGI |
Why galvalume lasts longer on a roof
Zinc protects steel by corroding first, slowly and evenly, until it is used up. In galvalume the aluminium forms a thin, stable skin that wears away far more slowly, and the zinc in the alloy is left for the places that need it, such as scratches and cut edges. In ordinary inland air, the published exposure data of coil makers such as JSW and Tata BlueScope puts an aluminium-zinc coating at two to four times the life of a zinc coating of the same thickness.
The grams mislead people. AZ150 sounds lighter than Z275, but the alloy is roughly half as dense as zinc, so both work out at about 20 microns on each face, and at the same thickness the alloy lasts longer.
A colour coated galvalume roof that is looked after is designed to last 20 to 25 years. Coil makers back their colour coated sheet with warranties of 7 to 25 years, depending on the material specification: the coating, the paint system and the coil.
Cut edges and scratches
This is where zinc does better. Zinc gives itself up to protect bare steel next to it, so a cut edge on a galvanised sheet stays clean for longer. A galvalume sheet can show a thin line of rust along a cut edge in its first year or two. It usually goes no further, because the zinc in the alloy seals the edge, and on a roof it is cosmetic.
The bigger risk to either sheet is how it is cut on site. An abrasive disc throws out hot metal fragments, called swarf, that land on the paint, burn in and rust within weeks, leaving orange specks across the roof. Sheets should be cut with shears or a nibbler (a power tool that punches the cut out instead of grinding it), and any swarf swept off the roof the same day.
Where galvanised is the better choice
- Box and valley gutters that hold standing water, especially heavy welded ones. On the Delhi Metro stations we worked on, the specification called for welded 3 mm galvanised gutters at 275 GSM because they stand in water. On most factory sheds a colour coated gutter does the job, and the gutter material is the customer's choice.
- Parts that will be welded. Welding burns off either coating, but galvanised steel is easier to touch up afterwards with zinc-rich paint, and galvalume sheet is not meant to be welded.
Wet concrete, mortar and plaster attack both coatings, aluminium faster than zinc. Neither should be cast into concrete bare. The part that touches the concrete should be painted or kept apart with a separating strip, and we settle the base flashing and parapet details for each building to suit its site conditions.
Keep them apart where water runs between them
Where rainwater runs off one metal on to another, the run-off can attack the second one. Water running off copper is the worst case for either sheet, so keep both away from copper pipes and copper earthing strips upslope of them. Galvalume and galvanised parts should also be kept apart along a water path. Fasteners and flashings should come from a supplier who will state in writing that they suit the sheet. The fasteners we supply with our sheets carry a multi-layer rust shield coating.
What it costs
A sheet whose coating lasts longer costs less for every year it is on the roof. The bigger savings, and the bigger mistakes, are in the coating class and the thickness, so compare quotes on those first, as set out in our roofing sheet thickness and coating guide.
How to check which one you are getting
- Read the print on the underside of the sheet. It carries the coil maker, the grade, the coating class (AZ150, Z275 and so on), the thickness and the coil number.
- Read the mill test certificate that comes with the delivery. It states the measured coating mass, which should be at or above the class you ordered.
- Ignore the colour. Two sheets in the same blue can be AZ150 and AZ70, and the second will rust years earlier.
- Ask for the coating class in writing on the quote, as a line in the specification, and do not accept "galvalume" or "GI" on its own.
Our roofing sheets are rolled in AZ150 galvalume as standard.
Sources and standards referred to
- AS 1397: continuous hot-dip metallic coated steel sheet and strip
- IS 15961: hot-dip aluminium-zinc alloy metallic coated steel strip and sheet
- IS 277: galvanised steel strip and sheets
- IS 14246 and IS 15965: pre-painted galvanised, and pre-painted aluminium-zinc coated, steel strip and sheet
- MNF 2 Rib and 3 Rib specification letters (AZ150 to AS 1397, G550)
- URC Construction work order for the DMRC stations (3 mm galvanised gutters, 275 GSM)