Guide · By Alex McCann, Managing Director
Galvanising Vent and Drain Holes: What Shop Drawings Need to Show
Steel that is hot-dip galvanised needs vent and drain holes so air, cleaning solutions and molten zinc can flow in and out safely. Industry guidance from the Galvanizers Association of Australia (GAA) says internal venting must be shown on shop detail drawings and approved by the galvaniser before fabrication. RDM shows vent and drain holes on shop drawings for the galvaniser and engineer to approve; we don’t design or certify them.
01 / Why
Why vent and drain holes matter
Hot-dip galvanising dips the whole fabricated piece into cleaning solutions and then molten zinc. Those liquids have to flow freely into, around and out of the steel, and every pocket of air has to escape.
- Safety: moisture trapped in a sealed section turns to superheated steam in the zinc bath, which can cause an explosion.
- Quality: trapped air stops the zinc reaching the surface, leaving bare spots, and can make a piece float in the bath.
- Finish: undersized or badly placed holes slow the zinc as it drains, leading to runs and a rougher finish.
Sources: Galvanizers Association of Australia, Venting and Draining Guide, and the GAA’s Design Guide for Hot Dip Galvanizing – best practice for venting and draining (Issue 2, October 2021).
02 / Guidance
What the published guidance says
A plain-English summary of the Galvanizers Association of Australia (GAA) guidance. Always check the current guide and your galvaniser’s requirements.
| Topic | What the GAA guidance says |
|---|---|
| Minimum hole size | No vent hole under 10 mm unless otherwise agreed with the galvaniser. The design guide adds: at least 10 mm or the steel thickness, whichever is larger. Bigger holes generally drain better. |
| Hollow sections (CHS, SHS, RHS) | At least one vent and one drain hole, each about 25% of the section’s diagonal, or several holes of equivalent area. Leaving the ends fully open is the preferred option. |
| Hole location | As close as possible to the high and low points of the section as it hangs in the bath, and not only in the centre of end plates and connections. |
| Connected hollow sections | External holes as close to the connection as possible. Internal holes, if used, at least 50% of the connecting section’s internal diameter, ideally the same size. |
| Check holes | A 10 mm external check hole at each location where internal venting is specified, so the galvaniser can verify it. |
| Overlapping surfaces | Large seal-welded overlaps need a vent hole, with the size increasing as the overlap area grows. |
This is a summary of industry guidance, not a design rule. Your galvaniser and engineer confirm hole sizes and positions for each job.
03 / Drawings
Showing holes on shop drawings
Vent and drain holes are part of the fabricated piece, so they belong on the shop drawings with everything else the workshop needs.
- Holes dimensioned on the assembly and single-part drawings
- Internal venting shown clearly, with its check holes
- Hole positions coordinated with end plates, cap plates and connections
- Drawings approved before fabrication starts
For how shop drawings and erection drawings fit together, read erection drawings vs shop drawings.
04 / Approval
Who approves vent and drain holes
According to the GAA’s design guide, internal venting must be shown on shop detail drawings and approved by the galvaniser before fabrication, and it is also recommended to be approved by the structural engineer.
Holes in structural members can affect the design, so talk to your galvaniser early and agree who approves what. Requirements for appearance, such as where holes can be seen, are also best raised with the galvaniser at design stage.
For more on roles, read who designs steel connections.
05 / RDM
How RDM handles galvanising holes
We show vent and drain holes on the shop drawings for the galvaniser and engineer to review and approve. We don’t design or certify them.
- Holes modelled in Tekla Structures and shown on the shop drawings
- Questions about hole sizes or positions raised as RFIs with the client or engineer
- Drawings issued for approval before IFC and fabrication drawings
- Holes shown in the model included in the NC and DXF files
See our structural steel shop drawings and how we handle RFI management and design coordination.
06 / FAQ
Frequently asked questions
Q01Why does galvanised steel need vent and drain holes?
So air, pre-treatment solutions and molten zinc can flow into, through and out of the steel. Without them, trapped moisture can turn to steam and cause an explosion, and trapped air can leave bare spots or make the piece float in the bath.
Q02Do vent and drain holes have to be shown on shop drawings?
The GAA’s design guide says internal venting must be shown on shop detail drawings and approved by the galvaniser before fabrication. Showing all vent and drain holes on the drawings keeps the workshop, galvaniser and engineer working from the same information.
Q03Who approves vent and drain holes?
The galvaniser, and the GAA recommends the structural engineer also approves internal venting. Confirm the arrangement for your project in writing.
Q04What size should vent holes be?
The GAA publishes minimum recommendations, such as a minimum 10 mm vent hole and larger holes for hollow sections based on their size. Your galvaniser and engineer confirm the sizes for each job.
Q05What is a check hole?
A small external hole the GAA recommends at each location where internal venting is specified, so the galvaniser can confirm the internal venting has been made before dipping.
Q06Does RDM design or certify vent and drain holes?
No. We show vent and drain holes on the shop drawings for the galvaniser and engineer to review and approve.
Q07Are the holes included in NC and DXF files?
Yes. Holes shown in the model are included in the NC and DXF files we supply for fabrication.
07 / Quote
Request a quote
Send your drawings, scope and your galvaniser’s requirements to info@rdmdrafting.com.au. Turnaround times are provided with every quote.