DuraGal is a brand of in-line galvanised steel hollow section made by Austube Mills, with a light zinc coating applied at the mill. Hot dip galvanised (HDG) steel is fabricated first and then dipped in molten zinc, which gives a much heavier coating inside and out. For steel posts and house stumps, that difference in coating can be the difference between decades of service and a rusted-out base.
The DuraGal steel post vs the hot dip galvanised steel post is a long-standing grudge match between two steely contenders. Which is better for your house? Here is the full blow-by-blow breakdown.
The Battleground
Whether it is a new house, an extension, a deck or a raised Queenslander, if there is a timber floor sitting up off the ground, you will probably meet the DuraGal vs hot dip galvanised battleground.
It’s not an easy one, either. At stake are cost, longevity, durability, corrosion resistance, availability and a whole host of other “-ities” that cloud the decision.
Steel stumps are often open to the weather, easily seen and usually ignored. Under a house they form a forest of columns stretching from the concrete footings to the underside of the floor frame.
They range from under 300 mm to more than 5 metres long, and they hold up floors, walls, roofs, belongings and families. They are critical structure. That’s why it matters that you and your building team get this decision right.
The Villain!
The villain of the battleground is corrosion. Rust. Steel cancer. Iron oxide. Fe₂O₃ (thanks John!).
Steel’s affinity for water and oxygen is like red wine to a musketeer. Corrosion is the downfall of shiny steel stumps and the kryptonite of steel’s super strength. So how do you protect yourself and your property?
The Hero!
Standing between steel and evil corrosion is its shiny coat of armour: zinc.
Applied as a molten coating, zinc bonds to the steel surface and protects it in two ways. It acts as a barrier, and it corrodes sacrificially in place of the steel. The thicker the zinc, the longer the armour lasts.
But steel must choose its armour before the battle: light enough to be affordable, yet strong enough to outlast the fight.
Behold the Contenders!
DuraGal: the lightweight
Weighing in at around 100 g of zinc per square metre on the outside is DuraGal, made by Austube Mills. It is the lightest and cheapest of the galvanised options. The zinc is applied in-line at the mill to square (SHS), rectangular (RHS) and circular (CHS) hollow sections, with only a thin paint-type coating on the inside.
DuraGal Plus: the middleweight
Next up is DuraGal Plus, which adds zinc protection to the inside surface as well. It costs a little more, but it is better suited where the inside of the section may be exposed, for example where posts are screwed into or end caps are left off. Whether that is acceptable depends on the environment, so check with your engineer.
Hot dip galvanised: the heavyweight
Finally, our superhero: hot dip galvanising. Typically weighing in at around 500 g of zinc per square metre or more, inside and out, HDG steel has the heaviest, toughest coat. It is also the most expensive. The steel is fabricated first and then sent to the galvaniser to be dipped in a bath of molten zinc, so it takes longer to arrive on site and is harder to alter once there. But it is super tough.
DuraGal vs Hot Dip Galvanised at a Glance
| DuraGal | DuraGal Plus | Hot dip galvanised | |
|---|---|---|---|
| Zinc coating (approx.) | ~100 g/m² outside | ~100 g/m² outside, plus internal protection | ~500 g/m² or more, inside and out |
| How it’s applied | In-line at the mill | In-line at the mill | Dipped after fabrication |
| Cost | Lowest | Moderate | Highest |
| Lead time | Off the shelf | Off the shelf | Longer (galvaniser) |
| Durability outdoors | Lowest | Better | Best |
Coating figures are indicative. Check the current manufacturer data and the galvaniser’s specification for your project.
Is DuraGal Hot Dipped?
No. DuraGal is galvanised in-line at the mill, which produces a much thinner zinc coating than hot dip galvanising after fabrication. They are not interchangeable, even though both look shiny and grey on the day they are delivered.
The Steel Stump Battle
It’s a fierce battle. Which coating is right for your project?
That depends on the environment (distance from the coast, industrial exposure, how wet the area under the house stays), how hard the posts will be to inspect and replace later, and the corrosion protection requirements in the National Construction Code. It is a decision for you, your structural engineer and your builder.
As a rule of thumb: the harsher the environment and the harder the post is to replace, the more armour it deserves.
The Provisos
Watch the base of the post
The base is where steel posts usually fail. Where a post is cast into concrete or sits at ground level, moisture, soil and debris collect at the junction and the thin coating on DuraGal can be consumed prematurely.
If you are casting DuraGal into a concrete footing, ask your structural engineer to document the precautions for your situation. Typical measures include keeping the post base above ground on a bracket or stirrup, finishing the concrete so water sheds away from the post, and adding protection at the ground line.
Yes, that includes keeping galvanised posts out of contact with the ground, uric acid (we’re looking at you, Fido) and other corrosive chemicals.
Check the NCC
For engineers and builders: the National Construction Code sets minimum corrosion protection for structural steel in houses, depending on the exposure environment. In many locations, a light in-line galvanised coating alone is not enough for external steel, including steel under a house, and paint or a heavier coating is required. Check the current edition of the NCC rather than relying on an older clause reference.
Painting DuraGal
Yes, you can paint DuraGal, and in many external situations you should. Clean and degrease the surface, then use a primer and paint system made for galvanised steel. Ordinary paints can peel off zinc.
Wherever the zinc is cut, drilled or welded, the bare steel needs touching up. A zinc-rich cold galvanising spray is the usual repair. Welding galvanised steel also produces zinc fumes, so make sure there is good ventilation and appropriate protection.
Some product links on this page are Amazon affiliate links. If you buy through them, Cornell Engineers may earn a small commission at no extra cost to you.
How Long Do Steel House Stumps Last?
There is no single number. A heavily galvanised stump under a dry, well-ventilated house well away from the coast can last for decades. A lightly coated stump in a damp, poorly drained subfloor near the sea can be in trouble much sooner.
The things that shorten stump life are the same ones we see again and again: water ponding around the base, soil or garden beds built up against the post, coastal salt, damaged coatings, and posts cast into concrete without protection at the ground line.
Rusting Stumps Under Your House?
Corroded steel stumps are common in older raised houses, and the damage is often worst where you can’t easily see it: at the ground line or inside the section. Some corrosion is cosmetic. Some means the stump needs replacing before it gives way.
Cornell Engineers assesses existing stumps, posts and subfloor structures, works out what needs replacing and what can be left, and gives practical repair advice. Where drainage under the house is part of the problem, see our drainage advice. For investigations more generally, see our forensic structural engineering page.
Send us the property address, photographs of the stumps (including close-ups at the base), a short description of what you have noticed and any reports you have. We will review the information and let you know whether the matter is a good fit for Cornell Engineers before preparing a fee proposal.
Further Information
- Austube Mills: current DuraGal and DuraGal Plus product information
- Galvanizers Association of Australia: hot dip galvanising coatings and durability
- National Construction Code
25 replies on “Steel Posts – Duragal vs H.D. Galv”
Problem with tradie advise us most times you don’t see or hear about the job down the track so I would say the restumper who replaced so many is the only tradie that really knows the answer and you heard it
I am a painter with 45 yrs exp and painting any steel with appropriate coating is a must imo
But also I’m going for hdg…plus painting the whole stump with a bitumen type compound and cap both ends no welding
I just found the above article on posts and was after some info. Given that the hot dipped gal posts are suspended in the concrete footing is there any chance over the lifetime of the posts that the footing cracks and possibly allows termites to enter through the holes in the bottom flange then up into the house through the top flange.
Also if using hot dipped posts for a restump with steel beams to open up an area, How are the posts attached to the beams. Bolts or welded or both. If Welded would that not burn off the gal coating and reduce its effectiveness even after the application of a cold gal over the welds
Thanks
Derek
Some good questions Derek!
It is possible, but not probable that termites will go down into a cracked concrete stump, find their way through the missing baseplate (we prefer there to be a baseplate) up the steel stump, through the poorly welded cap plate into the timber. Phew. It sounds like the roadtrip from hell.
So many things have to go wrong for that to happen.
So in no particular order:
– Have a base plate on your columns burined in concrete
– Have a fully welded cap plate on top of your post.
– Repair damaged galvanising by touching up with cold gal.
– Have regular pest inspections.
– Ensure the steel is painted before it is cast into the concrete.
Pray that termiotes don’t like hellish roadtrips.
Best of luck.
Matt Cornell
Thanks all for the good info and links.
IF the base of a steel stump does have signs of rust because it has been n contact with earth, is it effective to treat the rust; wire brush, rust converter, cold gal, sealer x n, waterproof bituminous membrane, and then build the concrete pad up higher?
Does the concrete keep moisture away or would it merely keep the earth and its attendant chemical away?
Is there any concrete additive that makes it more waterproof?
Being a House restumper myself, most of my work is replacing duragal and supergal steel columns that have only been in the ground between 5 to 9 years with genuine hot dipped galvanized steel house columns that will not rust.
The start of 2022 to the end of 2022 I have taken 14 ton of duragal and supergal steel columns to the scrap recycling people.
Great article written in a interesting format,
however,
I find it difficult to believe that an shs Dura Gal posts in the majority of cases rust out in 5 to 9 yrs. This would be exceptional circumstances likely where there is exceptional moisture present.
The home owner with the correct not expensive maintenance schedule could prevent this issue.
hello i am just wondering if u can paint the duragal post and bull nose to make it more attractive as a house fence if so what kind of paint would be best to use thank you
Hi Trisha
I’m not a paint expert. Perhaps try explaining your painting project to your paint supplier – especially if they are a professional paint supplier like Paint Place https://www.paintplace.com.au/.
Regards
Matt Cornell
Cornell Engineers
Hi
I am wanting it use Duragal as my fence post when building wing fences to the house. What precautions do I need to take when cementing in the posts.
How long could I expect these posts to
Last in comparison to using Hardwood posts.
Many Thanks
Wayne
Wayne, Duragal posts for fencing need the same care as house posts. Have a look at this website for some more information: https://profence.com.au/installing-fence-posts-concrete/
thanks Matthew
We have to replace our 30 year old wooden private power pole with a H.D. Galvanised 4.5mm thick steel pole due to site constraints.
I found your article as I was researching whether I could proactively increase its service life by applying a zinc coating before our electrician installs it.
From what I’ve read there would be no real advantage to doing this. Please correct me if I am wrong.
Also, is the use of quick setting concrete likely to exacerbate corrosion more so than standard concrete or using a tampered blue metal base?
Grateful for any advice
Might want to fix this up – ‘rust’ is Fe2O3, the Iron Oxide III.
Thank you John. You are of course quite right. Fe2O3.
Everyone knows and agrees that DuraGal posts need additional protection if cast into concrete pads, if we want it to last many years , but what protection ? .. Everyone will tell you … “Ask your structural engineer … ” Shame that we do not have structural engineers in Australia , otherwise someone would “dare” to take a “risk” and recommend something. Are we all afraid to be sued if our opinion work out no to be the best possible ?
Actually Nick, there’s some very handy advice contained in the National Construction Code NCC2016 available here for free. https://www.abcb.gov.au/ncc-online/NCC
Kane,
I fully agree with this document. Durgal in soil, is like having a Tissue Paper for a Rain Coat.
As a Forensic Structural Engineer, I have seen too many failures to allow Durgal to be in contact with soil. A SHS may last 5 years, and not 50 Years.
What would be the effect of including a simple layer of damp proof course stocking/sock…between the durogal post and it’s stump or other such support surface? whaddya mean they don’t make’em?
A very useful blog
I had a Welding contractor weld 10mm Baseplates to my Steel House stumps and Verandah Posts.
The Duragal is 3.5mm thick and the stumps are around 1300mm long.
He sprayed cold Galv over the plates and the welding.
He assures me that this will be there for 70 years.
Does anyone have an opinion on this.?
Here’s a second opinion about the effectiveness of cold galvanising.
https://galvanizeit.org/knowledgebase/article/what-does-the-cold-galvanizing-process-consist-of
Here’s a third opinion:
https://www.itwpf.com.au/galmet/faqs/
Check out what they say about surface preparation before using cold gal spray and then have another chat with your welding contractor.
Matthew what is your opinion about the concrete protecting the steel ?
I am a builder and an Engineer and soon I am doing a house project with stump and pad and planning to use 75 by 75 by 3 mm square hollow section Dura Galv
I know dura galv is much less resistant to rust than hot dipped but the cost is like half the price so I thought I could raise my concrete pads above the ground level and put a cone to them so nothing is in contact with surface water , what is your opinion is this wise ?
Hi Ali Mohamad
The NCC says you can use DuraGal stumps (yes they are much cheaper than full hot-dip galvanised stumps) so long as you paint the entire SHS stump. The doming of the concrete is another important aspect. Doing both will give your DuraGal stumps a good chance at remaining rust free. Don’t forget to fully seal your stumps also so that the inside of the steel column doesn’t rust.
Matt Cornell
hello I think if the steel is embedded inside concrete with good cover then the concrete protects it not your cold galv , in fact, the concrete has chemicals which will probably strip the cold galv coating anyway but the concrete protects the steel because its alkaline, I have seen rio bars come out of decades old pavings you see the steel is like new with no rust ,,,
Ali you are the only one to mention RIO STEEL. I am a Carpenter learned my trade in Commercial construction IE steel and concrete.
What I learned was this Attention to the amount of concrete COVER is to protect steel from moisture penetration. EG above ground may be 40mm or below ground 60/100 mm. Exposed steel at the concrete surface is vulnerable even if hot dip gal. There was always a specification to protect this area from dampness and AIR. Hence the very common use of TAR. Good use of pencil vibrators were commonly specified to increase the concrete density by releasing as much air as possible from the concrete. Steel and concrete are highly compatible. All we have to do is give attention to the vulnerabilities. Damp and Air, exclude them by any means and you will succeed in prolonging the structure’s life.