Three published rulebooks size the same hole three different ways, and the gap between them is roughly a factor of three in open area. What each rule actually requires, why open ends beat drilled holes, and the placement errors that put most hollow sections back on the truck.
A galvanizing plant can fix most things. A thin coating, a run, a rough patch, a bit of ash on a visible face: all of that gets sorted before the load leaves. What no plant can fix is a hollow section that arrives sealed. That piece is not a quality problem, it is a safety problem, and it goes back on the truck.
The conversation that follows is the same in every plant I have run. The fabricator says they put holes in it. Sometimes they did. The holes were 10 mm in a 200 mm box, or both at the same end, or drilled neatly in the centre of the cap plate where they do the least good. Venting and drainage get treated as detail work, and they are not. They decide whether the inside of the section gets coated at all, how much zinc leaves the plant as dead weight sealed inside a member, how long the piece spends on the jig, and whether anyone standing near the kettle gets hurt.
Steel reaches the kettle after degreasing, pickling, rinsing, fluxing and drying. Every one of those stages is wet. A hollow section with no opening, or with an opening too small to drain, carries some of that liquid inside it into a bath of molten zinc at 445 to 465 °C.
Water flashing to steam expands by a factor of roughly 1,700. Australian galvanizing guidance puts the pressure generated inside a sealed hollow section as high as 50 MPa. No structural hollow section is a pressure vessel at that level, and none is meant to be. The failure is not a contained bang. It is a rupture inside a kettle, which throws molten zinc across the working platform.
Take the moisture away and the problem does not disappear, it only becomes quieter. Sealed air heats, expands and holds the zinc out. The internal surface stays black. On a hollow section, that internal surface is close to half of the total area you are paying to protect, and it is the half that will never be inspected or maintained. The trapped air also makes the piece violently buoyant: molten zinc has a density of about 6,620 kg/m³, so every cubic metre of enclosed air pushes back with about 6.6 tonnes of uplift. Jigs and chains are sized for steel weight, not for that.
If a hollow item cannot be shown to be vented, it does not get dipped. Not at a reduced rate, not with a disclaimer, not as a favour to a good customer. Where venting is internal and cannot be seen, the piece needs a small external check hole so the galvanizer can verify it. A galvanizer who dips an unverified sealed section is gambling with somebody else's hands.
Most drawings treat the hole as an air escape. That is one of four functions, and sizing to the first alone is how holes end up too small.
| Job | Flow direction | What fails when the hole is too small or badly placed |
|---|---|---|
| Vent the air | Out, on entry | Air lock. The section floats, the jig fights it, and zinc never reaches the internal surface. |
| Admit the liquids | In, at every stage | Degreaser, acid, rinse water and flux never wet the inside. The internal surface is unprepared even if zinc later reaches it. |
| Drain the zinc | Out, on withdrawal | Zinc solidifies inside the member. Dead weight the customer pays for twice, in galvanizing weight and in freight, plus heavy runs at the hole. |
| Pass solids | Out, throughout | Dried flux, ash and drossy skin block the hole. The section behaves as sealed on the next dip or the next repair cycle. |
The fourth job is the one designers have almost never heard of, and it is the reason minimum hole diameters exist at all.
Ash is a solid powder, not a film. Australian guidance puts ash generation at roughly 200 g per square metre of galvanized surface. My own plants run 2 to 5 kg of ash per tonne of steel, which on a structural mix at around 30 m² per tonne works out to 65 to 165 g/m², the same order of magnitude. The number is not the point; the physical state is. Powder does not migrate out through an 8 mm hole drilled in a horizontal cap plate. It sits inside, and on the second dip that section is closer to sealed than it was on the first.
Holes have to exist before the piece enters the degreasing tank, not before it enters the kettle. A section drilled at the last minute has already been through pickling and rinsing with a sealed void, so the inside was never cleaned. It will take zinc unevenly if it takes it at all, and it may be carrying rinse water that no one can see.
There is no single global sizing rule. There are three widely used frameworks, and they are built on different bases, which is why two competent engineers can look at the same 100 mm box and specify holes that differ by a factor of three.
The North American practice standard is explicit that fully open ends are the most desirable situation. Where full-open venting is not possible, it calls for a minimum vent opening of 25 to 30 % of the cross-sectional area of the tubular structure, provided at each end of the capped section, and it recommends a larger percentage for small cross sections. For box sections where height plus width reaches 610 mm (24 in) or more, the area of holes plus clipped corners must equal at least 25 % of H × W. At internal intersections in tubular assemblies it requires internal venting plus one external hole of at least 9.5 mm (3/8 in) at each intersection, with the edge of the hole no more than 12 mm from the edge of the weld bead.
The Galvanizers Association of Australia design manual sizes from geometry rather than area: hollow sections require a vent and a drain hole each with a diameter of at least 25 % of the internal diagonal of the section, or several holes of equivalent total area. It sets floors of 10 mm or the wall thickness, whichever is greater, and long-standing practice guidance of 8 mm absolute minimum for a vent and 10 mm for a drain, with 12 mm and 25 mm as the preferred minimums. For large enclosed volumes it switches to a volume rule: one vent and one drain per 0.5 m³ of enclosed volume, each at least 50 mm diameter.
The international design guidance works from a recommended minimum hole size tabulated against section size, with a practical floor of 10 mm that many European galvanizers now apply as a blanket policy on incoming work, on health and safety grounds rather than finish grounds. It also notes that long sections may need additional holes purely to drain properly, independent of the size rule.
Convert the Australian diagonal rule into open area and the difference stops being a matter of opinion. For a round section, a hole at 25 % of the internal diameter is exactly 6.25 % of the enclosed area. For a square section, 25 % of the internal diagonal gives a hole of 9.8 % of the enclosed area, and that figure is constant for every square size. For elongated rectangles it climbs, reaching about 12.8 % on a 200 × 100 RHS.
Set that against 25 to 30 % of the enclosed area per end under ASTM A385 and the gap is roughly three times the open area on a square section and nearly five times on a round one. Neither framework is wrong. They are calibrated against different plant practice, different hanging methods and different expectations of how long a piece may sit on the jig draining. But a fabricator who sizes to one and ships to a galvanizer working to the other will be told the section is under-vented, and both parties will be able to quote a standard at each other for the rest of the afternoon.
Size to the standard your purchase order or project specification names, apply the 10 to 12 mm floor whichever rule governs, and send the venting detail to the galvanizer for review before you fabricate. Reviewing a detail costs an email. Re-drilling a delivered truss costs a week.
| Section (outside × wall) | Enclosed area | A385, 30 % per end, one hole | A385, 30 % per end, two holes | 25 % of internal diagonal |
|---|---|---|---|---|
| CHS 48.3 × 3.2 | 1,379 mm² | Ø 23 mm | Ø 16 mm | Ø 10.5 mm |
| CHS 88.9 × 4.0 | 5,140 mm² | Ø 44 mm | Ø 31 mm | Ø 20 mm |
| CHS 168.3 × 6.0 | 19,187 mm² | Ø 86 mm | Ø 61 mm | Ø 39 mm |
| SHS 50 × 50 × 3 | 1,936 mm² | Ø 27 mm | Ø 19 mm | Ø 16 mm |
| SHS 100 × 100 × 4 | 8,464 mm² | Ø 57 mm | Ø 40 mm | Ø 33 mm |
| SHS 150 × 150 × 6 | 19,044 mm² | Ø 85 mm | Ø 60 mm | Ø 49 mm |
| RHS 200 × 100 × 6 | 16,544 mm² | Ø 80 mm | Ø 56 mm | Ø 52 mm |
| SHS 250 × 250 × 8 | 54,756 mm² | Ø 145 mm | Ø 102 mm | Ø 83 mm |
Enclosed area uses internal dimensions. Diameters are rounded up to the nearest millimetre and are illustrative of the rules, not a substitute for the governing standard. Apply the 10 to 12 mm floor to the small sections at the top of the table.
Read the bottom row before the top ones. Nobody drills a 102 mm hole twice into a cap plate, and nobody should try. That row is the standards telling you, in numbers, that above roughly 150 mm the correct answer stops being a drill and becomes an open end, a clipped cap plate, or both. The A385 clause allowing holes plus corner clips to make up the required area on large box sections exists for exactly this reason. On the smallest sections the table says the opposite: the calculated hole is smaller than the floor, so the floor governs, and on anything below about 50 mm the sensible answer is again an open end.
A correctly sized hole in the wrong place vents nothing. In my experience placement causes more uncoated internal surface than undersizing does, because an undersized hole at least works slowly.
Vent and drain holes work as a diagonal pair in the attitude the piece actually hangs. A hole that is at the high point on the drawing may be halfway down the section on the jig.
Run the same check on any CHS, SHS or RHS in a few seconds with the free Vent and Drain Hole Calculator. For fabricated assemblies where every node and every chord needs its own numbered hole, the members Hole Planner marks holes on photographs of the real steel and exports a driller-ready schedule for the workshop.
A truss is not one enclosed volume, it is as many enclosed volumes as it has sealed members. Every one of them needs its own path to atmosphere, and a node where four tubes meet needs four. This is where most rejections on complex work originate: the chords were vented and the diagonals were forgotten.
Two approaches exist. Internal venting keeps the outside clean, which architects like, and works by cropping or drilling the wall of the member at each intersection before the connecting member is welded over it. The internal opening should total at least 50 % of the internal diameter of the connecting member, and matching it fully is better. External venting puts holes in the outside face next to each connection. ASTM A385 sizes these at 9.5 mm minimum or 25 % of the pipe diameter, whichever is larger, one on each side of each intersection.
The problem with internal venting is that once the joint is welded, nobody can see whether it was done. That is what the check hole is for: a 10 mm external hole adjacent to each internally vented connection, so the galvanizer can confirm the path exists. ASTM A385 requires an external hole of at least 9.5 mm at each intersection for the same reason, positioned within 12 mm of the weld bead edge, and it says so explicitly in terms of preventing an explosion if the fabricator neglects the internal vent.
Then the piece is being dipped on trust. A welded joint is not a witness. In my plants a complex tubular assembly with no external check hole and no fabricator's venting drawing does not enter the pickle line, and that has never once turned out to be an overreaction.
Lapped plates, backing plates, doubler plates and stitch-welded connections fail differently. There is rarely enough volume for a violent event, but there is more than enough to hold pickling acid and rinse water between the faces. That liquid boils out during dipping, blows the coating locally, and what stays behind bleeds out over the following weeks and stains or undermines the coating around the lap. It is one of the most common entries in any defect gallery, and it is entirely a design decision.
| Overlapped area | ASTM A385, material under 12.7 mm thick | Alternative |
|---|---|---|
| Under 103 cm² | No vent hole required | None required |
| 103 to under 413 cm² | One hole, 9.5 mm minimum | Leave 25 mm unwelded |
| 413 to under 2,580 cm² | One hole, 12.5 mm minimum | Leave 51 mm unwelded |
| 2,580 cm² and above | One hole, 19 mm minimum | Leave 102 mm unwelded |
Thresholds change for material thicker than 12.7 mm. European practice reaches a similar place by a different route: vent holes of 12 mm or more at roughly 300 mm centres across the lap, or intermittent welding with 300 mm welds separated by 25 mm gaps.
The cleanest solution is not on the table at all. Either seal-weld the perimeter continuously so nothing can enter, or leave the lap genuinely open so anything that enters can leave. The half-welded lap, sealed on three sides and open on one, is the configuration that causes the trouble.
Above roughly half a cubic metre, sizing switches from cross-section to volume. The working rule is one vent and one drain per 0.5 m³ of enclosed volume, each at least 50 mm in diameter, placed at diagonally opposite corners. Internal diaphragms and baffles need cropped corners and, on large box sections, an access opening, or each compartment becomes its own sealed vessel.
Buoyancy is the governing constraint here rather than coating quality. A 2 m³ vessel that fills slowly is carrying something close to 13 tonnes of uplift while it fills, against jigging designed for its steel weight of perhaps 1.5 tonnes. The holes are not only there to let air out, they are there to let zinc in fast enough that the piece stops trying to leave the kettle. This is also why large vessels are dipped end-first at a steep angle and withdrawn slowly, and why a plant will quote a longer cycle time on them.
Across the three plants I run, under-vented hollow work is the most common reason a delivery is stopped at goods-in, ahead of oil, paint, weld slag and markings. The check takes an inspector under a minute per item and it is three questions. Are there holes at both ends, and are they diagonally opposite. Is the largest hole plausible against the section size, measured, not eyeballed. On anything with internal venting, is there an external check hole. If any of the three fails, the item does not enter the pickle line. The argument is far cheaper at goods-in than at the kettle, and immeasurably cheaper than at the hospital.
The commercial side deserves to be said plainly to fabricators, because it usually lands better than the safety argument. A section that drains badly leaves the plant with zinc solidified inside it that nobody can see and nobody will ever use. The customer pays for that zinc in the galvanized weight and then pays freight to move it. And when a hollow section has to be stripped and re-dipped because the inside did not coat, it takes a second full dose of zinc on steel that was paid for once, plus a second kettle cycle, which is exactly the arithmetic I set out in the zinc consumption KPI article. Venting is one of the very few design decisions that improves safety, coating quality and cost at the same time, and it costs the price of a drill bit.
Most venting disputes trace back to a drawing that said nothing. This clause is written to be pasted into a fabrication specification or a galvanizing purchase order. Adjust the named standard and the percentage to whichever framework governs your project.
Venting and drainage for hot-dip galvanizing
All hollow sections, box sections and enclosed voids shall be vented and drained for hot-dip galvanizing in accordance with [ASTM A385/A385M] [ISO 14713-2]. Vent and drain openings shall be provided at diagonally opposite ends of every enclosed volume. The total opening at each end shall be not less than 30 % of the enclosed cross-sectional area, with a minimum hole diameter of 12 mm. Holes shall be positioned off centre and as close to the section wall as the connection permits, and shall not be located at the centre of cap plates or end plates.
Enclosed volumes exceeding 0.5 m³ shall be provided with one vent and one drain of not less than 50 mm diameter per 0.5 m³ of enclosed volume. Internal diaphragms shall be cropped at all corners.
All holes shall be drilled and deburred before surface preparation. Internally vented connections shall include one 10 mm external check hole adjacent to each intersection to permit verification by the galvanizer.
Hole positions in load-bearing elements shall be approved by the design engineer prior to fabrication. The fabricator shall submit the venting and drainage detail to the galvanizer for review before fabrication commences. Items presented for galvanizing without verifiable venting will be rejected.
© Aladdin Mohammed · HDG Community. Hole diameters in this article are calculated to illustrate each published rule and are not a substitute for the governing standard, the project specification, or the galvanizer's own requirements. Any hole in a load-bearing element requires the design engineer's approval. Related tools: Vent and Drain Hole Calculator · Standards Library · all free tools.
