---
title: What steel grade and coating should my brackets be, and can my fabricator substitute?
description: The steel grade and galvanised coating for ShedMate connection brackets by plate thickness, what applies to column base plates, and when to ask before your fabricator substitutes.
---
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# What steel grade and coating should my brackets be, and can my fabricator substitute?
**Plate thickness sets the grade and coating.** Brackets up to 3 mm are cut from pre-galvanised coil: G450 Z350 for structural brackets such as apex and haunch brackets, and G250 Z275 for lighter-duty brackets. Plates 6 mm and thicker, including every column base plate, are G250 mild steel, hot dip galvanised after fabrication. If your fabricator wants to use a different grade or coating, ask us first with the job number and the bracket code.
**Before your fabricator orders steel**
1. Read the plate thickness on the drawing, for example PLT 3 or PLT 6
2. Look up the grade and coating for that thickness in the table below
3. A 6 mm column base plate is G250, hot dip galvanised, even if the drawing's general note says G450 Z350
4. Want a different grade or coating? Ask us before you order
### Grade and coating by plate thickness
| Plate | Steel grade | Coating | How it's galvanised | Standard |
| --- | --- | --- | --- | --- |
| 1 to 2 mm | G250 | Z275 | Pre-galvanised coil | AS 1397 |
| 3 mm, structural brackets | G450 | Z350 | Pre-galvanised coil | AS 1397 |
| 3 mm, lighter duty | G250 | Z275 | Pre-galvanised coil | AS 1397 |
| 6 mm | G250 mild steel | 500 g/m² | Hot dip after fabrication | AS/NZS 3678 and AS/NZS 4680 |
| 12 to 16 mm | G250 mild steel | 600 g/m² | Hot dip after fabrication | AS/NZS 3678 and AS/NZS 4680 |
### Bracket types at a glance
| Bracket | Plate | Steel grade | Coating |
| --- | --- | --- | --- |
| Apex bracket | 3 mm | G450 | Z350 |
| Haunch bracket | 3 mm | G450 | Z350 |
| Stiffener | 3 mm | G450 | Z350 |
| Mezzanine bracket | 3 mm | G450 | Z350 |
| Mullion base plate | 3 mm | G450 | Z350 |
| Girt and purlin bracket | 2 to 3 mm | G250 | Z275 |
| Fascia bracket | 1 to 3 mm | G250 | Z275 |
| Mullion and header plate | 1 mm or 3 mm | G250 at 1 mm, G450 at 3 mm | Z275 or Z350, as the drawing shows |
| Column base plate | 6, 12 or 16 mm | G250 mild steel | Hot dip galvanised, 500 or 600 g/m² |
| Bridging beam bracket | 6 mm bracket with a 3 mm stiffener | G250 mild steel and G450 | Hot dip 500 g/m² and Z350 |
| Filler plate | 6 mm | G250 mild steel | Hot dip galvanised, 500 g/m² |
Where an assembly combines a thick plate with a thin one, such as a bridging beam bracket, each plate follows its own row. The drawing says which.
**Column base plate drawing says G450 Z350?** Some bracket shop drawings still carry an older general note that reads "ALL BRACKETS: G450 ... Z350". For 6 mm column base plates that note no longer applies. Use G250 mild steel, hot dip galvanised to 500 g/m², as in the table above.
### Can my fabricator substitute a different grade?
- **Structural brackets** (apex, haunch, stiffener, mezzanine and the 3 mm mullion base plate) are specified in G450. A lower grade, such as G350 or G250, isn't the specified grade, so check with us before ordering one.
- **Lighter-duty brackets** (girt, purlin and fascia brackets) are G250 at least. A higher grade also meets the specification.
- **Plates 6 mm and thicker** are G250 mild steel plate, galvanised after fabrication. If a supplier says G450 isn't available in that thickness, that's expected: high tensile galvanised coil isn't made in heavy gauges, which is why the thicker parts use mild steel plate.
- **A different coating** from the one in the table? Ask us before you order.
To ask, email support@shedmate.com.au with the job number, the bracket code and what your fabricator wants to use. We'll check it with our engineer and confirm before you order.
### Why the changeover is at 3 mm
Brackets come from two different products, split by thickness because of how each is made.
| | Up to 3 mm | 6 mm and thicker |
| --- | --- | --- |
| Material | High tensile structural steel, galvanised at the mill to AS 1397 | Uncoated structural plate to AS/NZS 3678 |
| How it's coated | The coil arrives coated, and the bracket is cut from it | The bracket is cut, folded and drilled first, then dipped in molten zinc to AS/NZS 4680 |
| Strength | G450 has a minimum yield strength of 450 MPa, so a thinner bracket carries the load. G250 is used where that strength isn't needed | Grade 250 plate. The strength comes from the thickness, which is why these parts are 6 mm and up |
| Coating figure | A Z class, such as Z275 or Z350. Cut edges and holes are protected by the surrounding zinc | A mass in g/m². Every surface is coated, including cut edges and the inside of holes |
The two coating figures are measured on different bases, so they can't be compared directly. Read a Z class against AS 1397 and a g/m² figure against AS/NZS 4680.
### Turning a drawing callout into an order
| Drawing callout | Order this | Coating to specify |
| --- | --- | --- |
| PLT 3 with G450 | 3 mm structural galvanised coil, grade G450 | Z350 minimum, AS 1397 |
| PLT 3 with G250 | 3 mm galvanised coil, grade G250 | Z275 minimum, AS 1397 |
| PLT 1 or PLT 2 | 1 or 2 mm galvanised coil, grade G250 | Z275 minimum, AS 1397 |
| PLT 6 | 6 mm mild steel plate, Grade 250, AS/NZS 3678 | Hot dip after fabrication, 500 g/m², AS/NZS 4680 |
| PLT 12 or PLT 16 | 12 or 16 mm mild steel plate, Grade 250, AS/NZS 3678 | Hot dip after fabrication, 600 g/m², AS/NZS 4680 |
Quote the thickness and the standard together, and your supplier can find the right product.
### Bracing members aren't brackets
Apex bracing and knee bracing are cold-formed C sections, not plate, so they aren't in these tables. They follow the member specification on your engineering drawings, the same as columns, rafters, purlins and girts. The brackets that connect them are in the tables.
### If a drawing doesn't match this article
Bracket shop drawings are workshop documents. Where a shop drawing and the certified engineering plans differ, the certified engineering plans govern. If you find a grade or coating that doesn't match this article, email support@shedmate.com.au with the job number and the bracket code, and we'll confirm it before you order.
### Related articles
- [Where do I get bracket drawings for my builder and fabricator?](https://help.shedmate.com.au/knowledge/where-do-i-get-bracket-drawings-for-my-builder-and-fabricator?hsLang=en)
- [Which plans and engineering documents can I send, and when?](https://help.shedmate.com.au/knowledge/which-plans-and-engineering-documents-can-i-send?hsLang=en)
- [Why can't I use a bolt-down base bracket on larger columns?](https://help.shedmate.com.au/knowledge/why-cant-i-use-a-bolt-down-base-bracket-on-larger-columns?hsLang=en)
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