Vent and Drain Holes for Hot-Dip Galvanizing: How to Size and Place Them

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.

What Happens to a Sealed Section at 450 °C

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.

A Vent Hole Has Four Jobs, Not One

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.

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.

Three Rulebooks, Three Different Answers

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.

ASTM A385/A385M: an area rule

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.

Australian and New Zealand practice: a length rule

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.

ISO 14713-2: a table with a floor

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.

How Far Apart the Three Rules Actually Sit

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.

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.

Placement Beats Size

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.

• Diagonally opposite, one at each end. Two holes at the same end vent that end twice and the far end not at all.
• Never in the centre of a cap plate or end plate. Air collects in a ring against the section wall, which is where the weld is and where the coating matters most. Put the hole hard against the wall, as close as the weld permits.
• High point and low point in the hanging attitude, not the drawing attitude. Tell the galvanizer how the piece is intended to hang, or accept his hanging and place holes so any reasonable attitude still drains.
• Same plane as the fabrication. A hole that rotates out of the low point when the piece is tilted for drainage stops being a drain hole.
• Keep clear of lugs, gussets and stiffeners. A lifting lug welded over a vent hole reseals the section, and it is a genuinely common find at goods-in.
• Drill and deburr, do not flame cut where you can avoid it. A ragged flame-cut edge is a thick coating and a run point, and it collects ash.
• A drain hole should still drain in service. A hole that sits at the low point in the kettle but fills with rainwater in the final orientation becomes the corrosion cell you were trying to prevent. Check both attitudes.
• Any hole in a load path belongs to the design engineer. Not to the fabricator, and not to the galvanizer. Agree hole positions at detailing, not at the kettle, and note them on the drawing.

Worked Example: 100 × 100 × 4 SHS, Capped Both Ends

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.

Assemblies, Intersections and the Check Hole

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.

Details by Product Type

Everything above gives you an area. It does not tell you where the opening goes on the product actually in front of you, and that is where drawings still go wrong long after the arithmetic is right. What follows is the detailing practice for the product families that come through a plant most often, from Chapter 9 §9.4.3 and the ASTM A385 figures. The illustrations are the ones used inside the Hole Planner, so a detail agreed here is the detail the workshop is shown.

Gussets, stiffeners and end plates on open sections

A beam or a channel drains along its length without any help. It traps liquid wherever a plate closes a pocket: gusset plates, stiffeners, bracing, end plates and base plates. Cropping beats drilling nearly every time, because a cropped corner cannot be plugged by ash and cannot be welded over by accident.

• Crop at least 19 mm at every corner of stiffeners, gussets and bracing. On gusset plates crop generously; there is no structural penalty at the corner and every millimetre helps the flow.
• Where cropping is impossible, drill 13 mm minimum as close to the corner as the weld permits. A hole in the centre of a gusset drains the centre of the gusset and nothing else.
• End plates on rolled shapes take 13 mm holes hard against the interior corners. The alternative is a 13 mm hole through the web within 6 mm of the plate, which is usually easier to drill and works just as well.

Box sections and rectangular end plates

Box sections are sized on height plus width rather than on area alone, and the required share of the opening climbs as the section gets smaller. That is deliberate. A small box has proportionally more internal surface for each unit of open area, and it drains through a shorter, tighter path.

Internal gussets sit no closer than 914 mm (36 in) apart, with all four corners clipped and, in large sections, an additional opening at the centre. On a rectangular rather than square box, work the area out and agree the opening positions with the galvanizer before you fabricate.

Pipe columns, poles and tapered arms

Street lighting columns, transmission poles, pipe columns and pipe girders share one detailing problem: a base plate, and often a cap plate, closing a long slender volume. The most desirable answer is the one fabricators resist, which is to leave the end completely open at the full bore of the section, top and bottom.

Where a plate has to close the end, the openings in it are equal substitutes provided they meet the area rule. The opening at each end must be at least 30 % of the cross-sectional area for pipe of 76 mm (3 in) bore and above, rising to 45 % below 76 mm, because the smaller bore drains more slowly against surface tension. On a 150 mm section that works out to a 19 mm slot with a 76 mm centre hole, half circles of 45 mm radius, an oval of the same radius, or 16 mm radius notches in the pipe wall for the no-holes-in-the-plate case.

Option 5 is worth dwelling on, because it is the diagonal pair rule wearing different clothes. Two notches 180° apart at opposite ends of the pole means one of them is high and one is low whatever attitude the pole is hung in, which is exactly what the placement section above asks for.

Internal gusset plates and end flanges inside a tapered arm need their own vent and drain holes, placed diametrically opposite each other at opposite ends of the member. Where the member has flanges or end plates at all, it is cheaper to put the holes in the plate than in the section, and it leaves the structural wall untouched.

Pipe and rectangular tube trusses

The truss is the assembly that generates the most rejections, for the reason given earlier: the chords get vented and the verticals get forgotten. The rule is specific, and it is easy to check on a drawing before anyone cuts steel.

Chord end plates are fully open by preference. Where that is not possible, a pipe chord needs openings of at least 30 % of the bore area, and a rectangular chord follows the height-plus-width table in the previous section. Each hole of a vertical pair is sized from half the required combined area, which is why the pair is so much easier to drill than the single equivalent hole would be.

Handrail

Handrail is the product where venting is most visible to the end client and most often argued about, because the fabricator wants an unbroken run of pipe and the galvanizer wants to see that the inside of it will coat.

• External vents go as close to the weld as possible and are never smaller than 9.5 mm (3/8 in). They have to be visible from outside the assembly. This is the check hole principle from the previous section, applied at every joint rather than every intersection.
• Internal holes at each rail-to-post junction should be the full internal diameter of the pipe. It gives the best coating quality and, because the piece spends less time draining, the lowest galvanizing cost.
• If full internal holes are not used, the external vent at each weld must be at least 25 % of the pipe bore, not merely the 9.5 mm floor.
• Vents in end sections and similar closed sections are 13 mm.
• Ends of horizontal rails and vertical legs are left completely open. Any field erection fitting that would close an end is galvanized separately and attached afterwards.

Overlapping and Faying Surfaces

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.

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.

Tanks, Vessels and Large Enclosed Volumes

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.

Openings also have to be placed so that flux floating on the bath can escape from the vessel rather than being trapped against the roof of it, and so that no air pocket forms where the cleaning solutions cannot reach. Where a vessel or a heat exchanger is galvanized on the outside only, it needs snorkel tubes or extended vent pipes carrying the air exit above the zinc level in the kettle. Those are temporary fittings that require special equipment, so agree them with the galvanizer well before fabrication.

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.

From the Plant

The Clause to Put on the Drawing

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.

Key Takeaways

• A sealed hollow section cannot be galvanized. Trapped water expanding roughly 1,700 times as steam generates pressures reported up to 50 MPa, and the failure happens over an open kettle.
• The hole does four jobs: vents air, admits pretreatment and zinc, drains zinc, and lets dried flux and ash out. Size for all four, not just the first.
• The three published frameworks are not equivalent. ASTM A385 asks for 25 to 30 % of the enclosed area at each end. The 25 % of internal diagonal rule works out to 6.25 % of area on round sections and 9.8 % on square ones. Size to the standard your specification names, and review the detail with the galvanizer.
• Above roughly 150 mm the area rule stops being drillable. That is the standard telling you to use an open end or a clipped cap plate, which is also the faster-draining answer.
• Placement decides more outcomes than diameter. Diagonally opposite, off centre, hard against the wall, at the high and low points of the hanging attitude, clear of lugs and gussets.
• A truss has as many enclosed volumes as it has sealed members. Internal venting needs a 10 mm external check hole so the galvanizer can verify it, and ASTM A385 requires an external hole at every intersection for the same reason.
• Overlapping surfaces should be either fully seal-welded or genuinely open. The half-welded lap holds acid and rinse water and bleeds it out for weeks.
• Above 0.5 m³, size on volume, not cross-section, and remember that every cubic metre of trapped air is about 6.6 tonnes of buoyancy against the jig.
• Holes must exist before pretreatment, not before dipping. A section drilled late has an internal surface that was never cleaned.
• Put the venting clause on the drawing. Almost every dispute at goods-in starts with a drawing that was silent on the subject.

References

• Aladdin Mohammed, The Complete Hot-Dip Galvanizing Guide, Chapter 9, Design and Fabrication Guidelines. §9.4 vent and drainage holes, §9.4.3 sizing, §9.4.4 hole position and the geometry of immersion, §9.5 overlapping surfaces, §9.8 blind areas and hollow sections.
• ASTM A385/A385M, Standard Practice for Providing High-Quality Zinc (Hot-Dip Galvanized) Coatings. ASTM International. Venting and drainage of tubular products, box sections, overlapping surfaces and internal intersections.
• ASTM A123/A123M-24, Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products. See also what changed in the 2024 and 2022 editions.
• ASTM A384/A384M, Standard Practice for Safeguarding Against Warpage and Distortion During Hot-Dip Galvanizing of Steel Assemblies.
• ISO 14713-2, Zinc coatings. Guidelines and recommendations for the protection against corrosion of iron and steel in structures. Part 2: Hot dip galvanizing.
• ISO 1461:2022, Hot dip galvanized coatings on fabricated iron and steel articles. Specifications and test methods. Fourth edition.
• Galvanizers Association of Australia, Design Manual: structural sections, venting and draining of hollow sections, and the associated venting and draining guide.
• Galvanizers Association (UK), Design for galvanizing: filling, venting and drainage.
• American Galvanizers Association, design and fabrication guidance on venting of hollow structural sections.
• Mohammed, A., Hot-Dip Galvanizing: Science, Operations and Profitable Management, Chapter 9: Design and Fabrication Guidelines. Independently published, 2026. ISBN 979-8-17010-961-6.

© 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.