Kumar IronUpdated
In this guide
Why gauge causes confusion
Gauge is a number from old wire and sheet systems, and a higher gauge means a thinner sheet. The trouble is that there is more than one system. SWG (Standard Wire Gauge, British) and BWG (Birmingham Wire Gauge) agree for some numbers and differ for others. Roofing sheet makers today mostly work in millimetres, but the market still says gauge.
Also, in the trade the word 'gauge' is often used loosely for any thickness, and 'GSM' is often used for the zinc coating weight, not the sheet thickness. They are different things. Thickness is in mm. GSM (grams per square metre) is the coating mass.
Gauge to mm: two systems side by side
Values below are the standard figures for the SWG and BWG scales, rounded to the nearest mm fraction. Real sheets carry a tolerance and many makers round to a nominal size such as 0.45 or 0.50 mm, so use this as a guide, not a spec.
| Gauge | SWG (mm) | BWG (mm) | Common roofing use |
|---|---|---|---|
| 18 | 1.22 | 1.25 | Heavy fabrication, not usual roofing |
| 20 | 0.91 | 0.89 | Heavy duty roofing, industrial |
| 22 | 0.71 | 0.71 | Heavy roofing, large spans |
| 24 | 0.56 | 0.56 | Sheds and factories |
| 26 | 0.46 | 0.46 | Sheds, houses, general roofing |
| 28 | 0.38 | 0.36 | Light roofing, short spans |
| 30 | 0.32 | 0.31 | Very light cladding |
Notice that 26 gauge is about 0.46 mm in both systems, but 28 gauge differs slightly. Colour coated sheets from the large mills are usually sold by exact mm (for example 0.35, 0.40, 0.45, 0.50), not by gauge. Our weight chart lists these mm sizes with kg per square metre.
BMT vs TCT: which thickness are you being quoted
BMT is base metal thickness: the steel only. TCT is total coated thickness: steel plus zinc plus paint on both sides. A colour coated sheet can have a TCT noticeably higher than its BMT, because the paint and zinc layers add up.
So a '0.50 mm' sheet may be 0.50 BMT (steel is really 0.50 mm) or 0.50 TCT (steel is thinner, maybe 0.45 to 0.47 mm). The steel is what carries the load and resists dents. Two sheets quoted at 0.50 mm, one BMT and one TCT, are not the same strength, and the BMT one will cost more for good reason.
- Ask: is the 0.50 mm you quote BMT or TCT?
- Ask for the mill test certificate or the label on the coil or bundle.
- Weigh a few sheets if the lot is large. Our weight chart gives the expected kg per sheet for steel thickness.
Zinc coating: Z120, Z275 and what they mean
The zinc layer is what stops the steel rusting. It is stated as mass of zinc per square metre, counted on both faces together. Under IS 277, the coating mass is the total of both sides. So Z275 means about 275 g per square metre of zinc across both faces combined, not 275 on each.
Z120 means 120 g/m² and Z275 means 275 g/m² as minimum coating mass. More than double the zinc means a much longer life against corrosion, especially near cattle sheds, poultry farms, factories with fumes, or damp areas. Note that the Z-classes come from the international system. Older Indian grade names used plain numbers, and some mills use AZ (zinc aluminium) classes. Check which designation is printed on the product.
| Class | Minimum coating | Good for |
|---|---|---|
| Z120 | 120 g/m² | Dry, low corrosion areas, short term sheds |
| Z180 | 180 g/m² | General houses and light sheds |
| Z275 | 275 g/m² | Farm sheds, coastal or humid areas, longer life |
Z180 is a common intermediate class, but check the product data sheet for the exact classes a given brand offers.
What to ask the dealer
- Thickness in mm, and whether it is BMT or TCT.
- Zinc or zinc aluminium coating, and the class or g/m² (Z120, Z275, AZ150 and so on).
- Brand and mill, and whether the sheet is primary quality (not seconds or off-grade).
- Paint system for colour coated sheets: the paint thickness and warranty terms in writing.
- Length, profile and cover width, so sheet count is correct.
Frequently asked questions
- Is 26 gauge the same as 0.5 mm?
- Not exactly. 26 gauge is about 0.46 mm in SWG and BWG. A sheet sold as 0.50 mm is slightly thicker. Always ask for the mm size.
- What does GSM mean on a GI sheet?
- In the roofing trade GSM usually refers to the zinc coating mass in grams per square metre, not the thickness. Ask for it clearly, for example Z275 or 275 g/m².
- BMT or TCT, which is better to quote?
- BMT is the more honest measure of the steel. If a quote does not say, ask. A TCT figure can hide a thinner steel core.
- Which zinc coating should I choose for a cattle shed?
- Choose the higher coating class, such as Z275, because ammonia and humidity speed up corrosion. For a dry store room, a lower class can be enough.