NAAMM Bar Grating Designations Explained
What "19-W-4" actually means, how to read any NAAMM MBG 531 designation code, and how to convert it to the metric dimensions used by Anping manufacturers.
The NAAMM MBG 531 Designation System
NAAMM MBG 531 is the North American standard for metal bar grating, published by the National Association of Architectural Metal Manufacturers. Every standard bar grating product has a three-part designation code:
19 – W – 4
19
Bearing bar depth
in 16ths of an inch
(19/16" = 1-3/16")
W
Bar surface type
(W = plain/smooth,
I = serrated)
4
Bearing bar thickness
in 16ths of an inch
(4/16" = 1/4" = ~6mm)
The bearing bar pitch (c/c spacing) is not encoded in the designation itself — it is specified separately. The most common pitch used with W and I series is 1-3/16" (30mm), but 1-1/4" (32mm) and 1-11/16" (43mm) pitches are also standard.
Bar Surface Type Codes
| Code | Description | Typical Use |
|---|---|---|
| W | Plain smooth welded bearing bars | Dry indoor, general industrial |
| I | Serrated (notched) welded bearing bars | Wet, outdoor, oily, inclined surfaces |
| SW | Plain smooth swaged cross bars | Architectural, lighter duty |
| SI | Serrated swaged cross bars | Architectural with anti-slip requirement |
| DW | Deep plain welded | Long spans, heavy loads |
| DI | Deep serrated welded | Long spans, anti-slip requirement |
Common Designations and Metric Equivalents
The table below converts the most widely ordered NAAMM designations to metric bearing bar dimensions. Use these when sourcing from Anping, China.
| NAAMM Code | Depth (in / mm) | Thickness (in / mm) | Metric Equiv. |
|---|---|---|---|
| 11-W-4 / 11-I-4 | 11/16" / 17mm | 1/4" / 6mm | 20×5mm |
| 13-W-4 / 13-I-4 | 13/16" / 21mm | 1/4" / 6mm | 20×5mm (approx) |
| 15-W-4 / 15-I-4 | 15/16" / 24mm | 1/4" / 6mm | 25×5mm (closest) |
| 19-W-4 / 19-I-4 | 1-3/16" / 30mm | 1/4" / 6mm | 30×5mm |
| 19-W-2 / 19-I-2 | 1-3/16" / 30mm | 1/8" / 3mm | 30×3mm |
| 25-W-4 / 25-I-4 | 1-9/16" / 40mm | 1/4" / 6mm | 40×5mm |
| 32-W-4 / 32-I-4 | 2" / 50mm | 1/4" / 6mm | 50×5mm |
Note: metric equivalents are the nearest standard Chinese mill size. Confirm with your factory if exact imperial dimensions are required.
Cross Bar Designation
NAAMM also specifies cross bar type separately. The most common cross bar specifications sourced from China are:
- Round cross bar — 6mm (¼") diameter at 50mm (2") pitch. Most common for China export grating.
- Twisted square cross bar — 5×5mm or 6×6mm twisted flat bar. Common in Chinese GB/T 12755 grating. Slightly heavier than round bar.
- Flat cross bar — square-edged flat bar, often in architectural or close-pitch applications.
When ordering metric equivalents from Anping, specify: bearing bar H×T mm @ P mm c/c pitch + cross bar d mm round @ P_xb mm pitch. This gives the factory everything needed to quote accurately.
Example order specification
25×5mm plain bearing bar @ 30mm c/c pitch
6mm round cross bar @ 50mm pitch
Panel size: 6000×1000mm
Finish: hot-dip galvanized ISO 1461
Standard: GB/T 12755 (equivalent to NAAMM W-series)
FAQs
What does 19-W-4 mean in NAAMM bar grating?
19-W-4 means: bearing bar depth of 19/16" (≈ 30mm), W = plain (smooth) welded construction, and 4 = bearing bars at 4/16" (1/4") thickness. The c/c pitch is typically 1-3/16" (30mm) unless separately specified. The metric equivalent is approximately 25×5mm @ 30mm pitch.
What is the difference between W and I series NAAMM grating?
W-series has plain smooth bearing bar tops. I-series has serrated (notched) bearing bar tops for anti-slip performance. I-series is required by OSHA and most offshore standards for wet or sloped surfaces. Both are welded construction.
How do I order NAAMM-equivalent grating from China?
Specify the metric equivalent: bearing bar H×T mm at P mm c/c pitch. For 19-W-4, specify 25×5mm at 30mm c/c pitch, plain top. Chinese factories manufacture to GB/T 12755 which is dimensionally compatible with NAAMM MBG 531 metric equivalents.
Is GB/T 12755 the same as NAAMM MBG 531?
Not identical, but dimensionally compatible for standard specifications. GB/T 12755 uses metric dimensions; NAAMM MBG 531 uses imperial. Both cover welded bar grating with equivalent load capacity when the bearing bar cross-section and pitch are matched. Key difference: NAAMM specifies exact material grades (A36); GB/T uses Q235/Q345.