View Full Version : Off Topic -Difference between galvanised iron & steel roofing?

15-08-2004, 03:56 PM
My family house was built as an 1890s cottage, then got add-ons later (e.g front verandah covered in, bathroom & back porch added)
There were also major interior changes in the 1930s/40s.
Although I know rough dates for some of these, I've no idea whether the roof is original (in part or whole), merely that it's now a rusty paint-flaking old corrugated iron job which needs attention - and perhaps replacement with a newer product?
I wasn't even sure it was galvanised, so tried to find out on the Web what year galvanising became common.
The trouble is, references are to galvanised *steel*. Too modern, I guess?
As mine is good old *iron*, I need to know the difference in components to make sense of it.
Google gives me too much to choose from, so I reckon a Press F1 person ( Any builders out there?) can probably enlighten me sooner...
And if anyone has any tips on how to decide whether it's better to patch up or replace an old roof, I'd be grateful.

Murray P
15-08-2004, 04:14 PM
Galvanised Iron and Galavanised Steel are one and the same. The iron bit is traditional usage but is a misnomer, roll formed roofing has always been steel.

Without seeing the roof I would be loathe to advise you re replace or patch, can you post a couple of photo's? The other questions I would ask is, what you intend to do with the property, will you get good usage from your investment in a new roof? Your in a rural area aren't you, if near the sea or an industrial zone you will need to get the appropriate corrosion grade to take this into account.

Cheers Murray P

Graham L
15-08-2004, 04:16 PM
"Corrugated Iron" is "Galvanised Iron" is "Galvanised Steel". :D

Wrought Iron gates are usually arc welded mild steel. ;-)

People use terms wrongly.

The difference between your 1890's roofing, and modern roofing is that yours is better. It's lasted that long. The modern rubbish is lucky to beat 10 years. Yours had a heavy coat of zinc, on quite thick steel sheet. Modern stuff is very thin zinc on very light steel.

Have a good look at the condition of the steel. It might be still sound ... the paint is the weakest part. If it's not, you might finish up replacing the lot ... you will find that the pitch of the corrugations(distance frompeak-peak) and even the coverage (width) of the sheets is different. I hope that the wood ("rafters, sarking, ...") to which it's nailed is in better condition than mine. :_|

15-08-2004, 04:45 PM
Murray, you're a gem
That's what I call a prompt & illuminating reply.
So useful to know that our good old corrugated iron is actually steel. It'll make my research so much easier.
No pix, sorry. I've not taken any & haven't a digital camera.

Actually, I'm not *rural* for this query. This is my Dunedin roof, and as I'm only a few blocks from the harbour (as is most of Dunedin) there are obviously some salt air corrosion problems...which will be ongoing.
As for *will I get good usage?*...Depends on how long I live. After 3 generations of my family in this house, I'm not actually planning on selling - despite the real estate agent cards in my letterbox.
There's no real panic about the roof. It merely looks dreadful, but amazingly it still doesn't leak. So I have time to work this out slowly...
Thanks again.

P.S. Concerning my rural *other* address as above - the Central Otago stone cottage of the 1880s does indeed still have its original roof. The paint's peeling, but there's no rust at all.
That's the difference between lowland coastal air & the clear dry Central Otago version you get at 1300feet. Horses for courses, indeed...

15-08-2004, 05:00 PM
You've encouraged me about my old stuff & demoralised me about the new stuff. (Just read your post after I finished replying to Murray)
Mind you, I don't know that my Dunedin roof is an 1890s job. It may have been part of a 1930s/40s tart-up before my knowledge time. Anyway, it'll obviously pay to get someone to do some scratching underneath that thar rust to see how thick the metal is underneath.
And yes, Mr Laura has already warned me that replacing *bits* would be hell - as it's old imperial corrugations/sizing & the new stuff is metric, so they wouldn't fit together.
Dunno about the rafters/sarking (perhaps just as well?) but I do know the spouting is definitely shot... though trying hard not to notice that yet...

Murray P
15-08-2004, 05:32 PM
Graham's right, the old, thick steel and galvanising they used to use is superior in most respects to the much thinner high tensile stuff they use now but the factory applied coatings are far superior to any on-site applied paint system, there's the tradeoff . Even so, a well installed and cared for pre-coated modern corrugated "iron" roof should perform well for the next generation. Most failures these days are installation error and/or ignorance rather than inferior materials, idiots using pencil to mark, friction blades to cut, wrong fixings, sealants, etc, and leaving swarf and debris on the roof all can greatly reduce the life of the roof.

Check your roof space to confirm you have no leaks, delay to repairs can be very costly through hidden decay plus insect attack which can be encouraged by damp conditions (depending on species of beasty). If your not ready for a re-roof, small holes can be patched using a bitumen and aluminium foil composite tape and painted over to match existing. At least fix the guttering as this can lead to water ingress at the eaves or cladding that may not become visible for some time at which point remedial work could be costly.

Cheers Murray P

15-08-2004, 05:41 PM

We replaced our roof on an ~1890's house a few years back.
I know with some certainty that the roof was original, and it was replaced by us as an part of an overall refurbishment and to accommodate some additions.

We used Coloursteel, in the traditional corrogated (but metric?) format, however our architect opted for the heavier guage steel. He would not use the common (26 guage) thickness on any of his buildings and always specified the thicker (22 guage) product.

Its as thick as the old stuff, which was very solid (but was rusted quite badly in places). Having seen the product I agree with the architect.

But of course we were replacing the entire roof, so matching it was not an issue. Not sure there is much difference in the metric vs imperial though.

15-08-2004, 06:12 PM
Thanks, Godfather
Your information about Coloursteel was very interesting.
I wasn't aware it was available in 2 guages. I've not researched that far yet (still trying to canvas options of repair versus replacement) but have kept that choice at the back of my mind for checking later if necessary.
Your additions made it sensible to start from scratch, so you wouldn't have costed out my alternative choices - or perhaps you did?
Obviously, that decision comes down to how far gone my roof is -hence an expert opinion from someone who climbs over it, plus a quote - before looking at the Coloursteel prices (presumably found easily on a website?)
So I'll remember that the thick version is the way to go.

Murray P
15-08-2004, 06:58 PM
Color Steel (TM) or Colorcote (TM) do come in two gauges, as per GF's advice, .55 and .4 (24 and 26 gauge, or thereabouts, respectively). The .4 is paper thin, higher tensile steel, and not used by those with quality in mind IMO. However the grade of coating is more important for durability and has more impact on cost. As a general rule (there are other variables) if your within 500 metres of the sea you will need to use a product with a coating rated for severe marine environments.

For quotes and installation, use members of the NZ Roofing Association, make sure they pass on theirs and the manufacturers warranty.

GF, some of the old roofs were 22 guage but I don't think they were ever mad enough to use 26 gauge. There were also at least 2 different corrugated profiles. Roll forming was often done locally so profiles often varied from place to place or house to house (from batch to batch) due to wear or mis-adjustment of the machinery (or operator).

Cheers Murray P

15-08-2004, 07:18 PM
>f near the sea or an industrial zone you will need to get the appropriate corrosion grade to take this into account.
I'm pretty sure that Zincalume ( that spelling may be wrong) coated steel is a better product when near the sea.

Murray P
15-08-2004, 07:32 PM
Mike, your correct, both products I referred to use Zincalume rather than galvanising as the base corrosion resistant coating. Having said that, I'm not that sure whether it would be better than the old fashioned, thicker, method of galvanising that was the norm way back when.

Cheers Murray P

15-08-2004, 07:34 PM
Years ago corrugated iron was imported and often travelled as deck cargo. If you have ever seen a roof where all the edges were rusting but the rest was alright that was why.
If you live by the sea the warranty for coloursteel is not as long as for inland. Often you can leave uncoated corrugated iron for quite a time before painting. Almost as long as the warranty for colour steel. This can help reduce the initial cost of reroofing.
I was water blasting a roof back in Feb at 3000 psi. After doing so I suggested it was a waste of time painting it as I blew holes in it in parts.

Murray P
15-08-2004, 07:56 PM
Galvanised roofs were (are) left to weather to get rid of the lubricating oils from the roll forming process, more effective than etching or other cleaning methods IMO. A few months usually does the trick, then the paint will stick.

Most roofs rust at the laps because the laps have not been primed before installation or because of capillary action holding water in the laps for long periods of time along with buid up of salts where natural washing cannot remove them (same under nail heads) or, a combination of the preceding. Salt may have come from transport but I would have thought that roll forming would have removed it (very early corrugated was most likely to have been roll formed before arrival).

Cheers Murray P

15-08-2004, 08:13 PM
The deck cargo I was talking about was the final product.
A little bit of plumbers spirits of salts in water ( about a desert spoon or less in 10 litres) is great for washing a new roof off to get rid of the oils.
I have seen a lot of builders prime the edges but they have not removed the oils so the paint does not adhere properly which in my opinion is worse than leaving the edges unpainted initially.

15-08-2004, 11:17 PM
Fascinating. I've certainly learned something today.

There are quite a few examples of 100 year old unpainted corrugated iron in Central Otago, still doing the job. Remarkable.

I'd guess your roof is newer in Dunedin, Laura. Wetter and more salt in the air. Good luck.

15-08-2004, 11:44 PM
>idiots using pencil to mark,
That has me curious have probably been guilty of that myself. :8} What does the lead in a pencil do to the galvanizing. I have never heard of that being a no no.
Those niblers are a great little invention but I have been on a number of roofs where they have not cleaned the swarf off.
Was on a new Wharehouse roof recently and there certainly did not feel like there was much between me and the floor about ten meters below.

16-08-2004, 12:05 AM
Laura, our roof had a few sections of old galv-corrugated steel that had rust holes and spots, and I am sure a roofer would have said to replace with new. But I simply removed the loose bits and applied a rust kill (turns iron oxide into a blue-black different material), then patched the holes with a trimmed venetian blind (aluminium) and stuck them on with silicon cement. Then I painted the patched areas with a rust kill paint. Finally I applied 4 layers of roof paint. That was 4 years ago and no rust has come through. By the way, the venetian blind slats are curved and are a good fit on the corregated iron.

On the other hand, older homes like your's and mine have small sections of corregated iron (not the modern long lengths). It is really easy to pull up the rusted small sections and hammer in a new one..... and much cheaper to do it yourself.

Murray P
16-08-2004, 11:27 AM
> What does the lead in a pencil do to the galvanizing

I'll retract the idiot's comment then ;)

Carbon black in black rubber (for washers and flashings) and pencils etches the surface of galvanised, zincalume and colour steel materials which will promote corrosion, besides being hard to get off. Any colour pencil other than black is ok, neoprene or silicon rubeer is ok.

Cheers Murray P

16-08-2004, 12:08 PM
Thanks once again to all you helpful gents.
Lots of options there, plus enough background info to keep us from falling into some of the roofing traps.
And as we've all got some kind of roof, maybe Winston wasn't the only other PF1er to find it fascinating...
(Today, of course my roof looks simply dazzling - glorious in its pristine white.
What a pity I can't keep the snow glued on there, once it has melted everywhere else...)

Graham L
16-08-2004, 03:25 PM
Final note: "gauges" as in 24 gauge, 26 gauge, etc, are non-intuitive. Bigger numbers mean thinner. With thickness as in 0.4mm, 0.6mm, etc, bigger numbers mean thicker.

Old Tom
16-08-2004, 08:10 PM
Not been mentioned, but with advent of long run things have improved,mostly went on the joins.

16-08-2004, 08:48 PM
Laps Tom, laps. :D

16-08-2004, 09:09 PM
Laps, like this? (http://www.reediejournals.com/blogs/149/archives/Salt%20City%20Cat%20Club%20-%20Cat%20Photo%20Fun%20Gallery%20page%20H%20pg%202 .jpg)

16-08-2004, 10:39 PM
I have one that laps like that. Ours happens to be black and white.

Cute Tabby though you have.

The difficulty of the English language.

To Laura:-
Hope I didn't hijack the thread. Galvanised Iron is usually steel these days.

When it comes to roofing very often a new roof corrogated will be put on and it is suggested you wait before painting. This so the new roof will have time to weather so the first time it is painted the paint will not fall off.

16-08-2004, 11:21 PM
If your roof is sound, paint it with a good system.

If it has severe corosion, found by looking at the underside of the sheets through the manhole.. looking at the laps, (if you see building paper there then your roof may not be as old as you think) then replace it.

Depending on your budget, the best coloursteel roof will last 25 years or so that close to the sea, not the 100 years or so from the original.

I will reccomend that if you have to re-roof you do NOT use a metal type roofing. There are some excelent products from bitumastic tiles, sheet products to liquid applied membrains. You will never get the durability of the "old Iron" ever again

Houses are constructed to today to be re-cycled every 25 years or so, if no maintanence is done. .. Less in Auckland ;-) .. thanks to the BIA.

Most importantly .. put a saucer of milk in the attack to ensure the borer are well fed and don't le go hands. :D

17-08-2004, 12:03 AM
I'm noting your advice about the borer - being aware that probably only their hand-holding has kept the house up for so long.
I just never thought of MILK as the answer.
Truly, we learn something every day...

17-08-2004, 12:20 AM
I have been informed it is too cold down in Dunedin for the borer to live... well that is what I 've just been told.?

>>Less in Auckland .;-). thanks to the BIA.

Far too much rain up here, stop sending it on ... PLEASE

17-08-2004, 12:45 AM
Sadly for me, your informant was wrong, fairway
Borer just love our Dunedin climate.
(Maybe they thrive so well because they've no competition from those other wood-chewing greeblies that need Auckland's humidity?)
Sorry about your rain. Didn't come from here. We're only doing snow today...

Murray P
17-08-2004, 01:33 AM
No roof will last 100 years in even a moderate marine environment without regular maintenance. Higher grades of pre-coated steel roofing (long run) usually carry a 25yr warranty in very severe coastal environments (0-100 metres of breaking surf, heavy salt deposits, etc,) but, this only applies with regular maintenance, washing, inspection... As fairway says BIA via the NZ Building Code (B2/AS1) only require non-structural roof claddings to last 15yrs (50yrs for structural and those not easily replaced, which is almost non [simplified for brevity]) which is at least ten years shy IMO for a major component. You can stretch the life of the roof cladding well beyond 25yrs by repainting it, before it deteriorates too much.

Most membranes carry much the same warranty or less than pre-coated steel. A major brand of torch on modified bitumen membrane (two layers), for eg, carries a 20yr warranty with a stipulation that regular maintenance and inspections be carried out, that's for approximately 2-3 times the cost before you put the plywood substrate down.

I would not use a liquid applied acrylic membrane on a timber based substrate of reasonable surface area (and many junctions) and expect it to last for much more than 10 years, I come across ones that have barely lasted five and more particularly on decks that are letting in water almost immediately (including other types of liquid applied membranes). Applicator error and too high an expectation as to what the material can tolerate is usually the problem. Chip coated fibreglass/bitumen tiles have a habit of coming off where temperatures are a little lower and winds a tad higher, besides anything other than corrugated iron looks naf, IMO, on a old villa or bungalow unless it has been designed with solid clay tiles in mind ;)

The miss-use of roofing products and their application in conjunction with other components is one of the main causes of leaky buildings in NZ.

Unfortunately the more common little beasties that chew through your investment are well distributed throughout NZ. Some like it a little dryer, some like the damp, some like a bit of sun and some like to hide in the shade. There's usually a niche (or 3) somewhere in your house to suit. At least you wont get the ants they have in Auckland and north that like to chew on water damaged cellulose, ie, the sweet part of timber and they love untreated pinus radiata. Charming little critters :D

Cheers Murray P

17-08-2004, 02:35 AM
Well then, Murray, this started with my old roof & now you're getting me curious about the microscopic wee beasties that obviously live in my pantry shelving - untreated pine, I presume - & leave no signs apart from a fine powdery dust on the shelf below.

(Quite separate from my borer, which has lived here off & on since before I was born. They'll have many more generations in the house than me - and are probably more entitled to live here by now, if they had their rights.)

As these other wee fellers leave no holes and merely require some extra dusting from me, I've been operating a live-and-let-live principle. Obviously they weren't going to bring the house down etc..
Now you've reminded me those shelves are only a 1950s job - done by my parents - so I should be resolute in identifying & eradicating the bugs.
(Then again, I could always take out the shelves and build some more convenient ones. Melamine doesn't quite fit the period, but wotthell...)
And speaking of period, you're quite right about the tiles/membranes etc. in terms of fitting in.
I value fairway's advice, but this house needs a corrugated metal roof to look right.

Old Tom
17-08-2004, 06:33 AM
I had no idea that cats lapping the joins caused rust;)Thanks Win and G_F.

Billy T
17-08-2004, 07:56 AM
My home was built in 1958 so it is just a new kid on the block compared to yours Laura. We extended in the mid 80's and found some rust in the original roofing.

Minor damage was rest treated per an earlier post, minor holes were treated and patched with aluminium tape (where do you get the bitumen & aluminium variety?) but we found it easy to buy demolition iron of equivalent age and in good condition to replace sheets on the southern side that were rusted beyond repair. Some people were giving it away in the free columns of Trade & Exchange. Patching unwanted nail holes was no problem (tape again) and it looked as good as new.


Billy 8-{)

Murray P
17-08-2004, 10:40 AM
The critters are most likely house borer (Anobium punctatum) and have entered the end grain or edges of the shelving, (which would be OB rimu [sap wood] or pine?) and left by the same route. Once you see the frass (borer dust) they have departed, mated and started a new family who will be munching on your shelves. The frass will be slightly gritty in consistency rather than fine talcum. Depending on how they like your shelves and how warm you keep them they should be out in about four years, looking for some action.

If you find flight holes (1.5-2.5mm as opposed to 1-2mm) or grubs in heartwood or across the face of weather boards (for eg) and on lighter sunnier aspects then you probably have the native house borer (Leanobium flavomaculatum). Goddie is probably more acquainted to them as they tend to prefer dryer east coast locations.

If the timber is different to that above then you may have another beastie entirely.

Best treatment for house timbers is replacement with treated timber or heartwood but gas and then application of a contact insecticide will knock their socks off. Furniture may warrant injection or flooding with an insecticide. Smoke bombs only get the ones in flight around November to January, not much use for anything except killing spiders and making you feel better. Cats are absolutely useless as a control technique, effective eradication is strictly a pro job unfortunately.

Cheers Murray P

Nigel Thomson
17-08-2004, 02:41 PM
> minor holes were treated and patched with aluminium
> tape (where do you get the bitumen & aluminium
> variety?) ..........
>Patching unwanted nail
> holes was no problem (tape again) and it looked as
> good as new.

Where do you get this Aluminium tape?

does it have a trade name perchance, as I have been looking for something to patch some old nail holes and minor rust patches, on our roof, down here in Invers


Murray P
17-08-2004, 04:35 PM
> Where do you get this Aluminium tape?

There are probably a few that make it. Get it at any good(ish) hardware store. You'll get stung if you buy is in a little package rather than a roll of several metres. IIRC, it comes in 50, 75, 100, 150 mm wide rolls about 3.5M long, and it ain't cheap.

You could try getting some off-cuts from a torch on sheet (1x10M approx rolls) from your friendly local membrane applicator. Stick them down using a bit of heat from a hot air gun as per those used for paint stripping or a gas torch (carefull now, don't want to lose the lot). One disadvantage is that it will require coating will a bituminous aluminium paint or similar, before applying anything else, otherwise it will start to break down due to UV.

These little patches can easily outlast the life of the roof.

Cheers Murray P

17-08-2004, 05:09 PM
Nigel - I saw a roll in Wrightson's in Winton a few months ago. Never seen it before and was tempted to purchase. Dear enough though - about $9 for 5m maybe? Mind you, that would do plenty of nail holes.

17-08-2004, 05:14 PM
This is a splendid thread for all the DIY readers. Keep it going !
I will need some of that tape for our roof, which has all the symptoms mentioned earlier. The first owners of our place seemingly did not clean off the iron before painting and we have had a continuing peeling problem not fixed by blasting. We have a bit of lap rusting, so roll on good weather so fixing can start.
Cheers T

Graham L
17-08-2004, 05:18 PM
But it doesn't leak in fine weather. :D

17-08-2004, 10:06 PM
Get Murray to run you a GZ2 (correct me if I'm wrong) colour of your choice .7 guage steel roof .. it is about the best on offer .. known as "gold plated" to those with a small budget.
This is one interesting subject for sure .. learning that the "Best Roof" on the market today has the durability 1/4 of your original roof of a hundred years ago... ...I'm stunned....

And now untreated framing rears it's ugly head again ( first used in the 1890's in Nelson as the sapwood of the native white pine, a cast off from the heartwood used in boat building) I hear guys calling it "Liquid Pine" it washes out with the rain in a couple of years.? (no, Auckland is not washed out yet)

Thinking we have put you off buying a new home, perhaps you should swap the milk for a borer bomb.... anually, and go buy a cheap steel roof, paint it and let your kids worry about the next one ? ... or the one after that ??

17-08-2004, 10:43 PM
Now, all you farmer people ..
There were two products on the market, both bitumastic aluminum paints, with one considerably thicker than the other, Ideal for a quick patch to a roof or gutter... one called "Coolite" and the other "Alumicool" so if anyone find's some at their local supply co.. let me know, Laura has had me up on my roof, 18 years old and ready for the knackers yard, I hate this disposable society we have.