Why any old galvanised iron won't do...
(This is the text of an article published in The Heritage Advisor, Issue 5 April 2000).
Prepared by Ian Wight with significant input by Lorraine Huddle and Robyn Mullens, which is
acknowledged with gratitude.
The Issues
Our very own Macquarie Dictionary defines galvanised iron as ‘iron coated with zinc to
prevent rust’ and ‘such iron formed into corrugated sheets and used for roofing etc, esp. in
rural buildings and outhouses.’
This definition only describes accurately the now rare zinc coated wrought iron of the
nineteenth century, which was replaced by mild steel from the 1890s. The ‘gal iron’ that
became such an important part of the Australian vernacular is in fact galvanised corrugated
steel.
Today, however, the most popular and best-promoted corrugated roofing is Zincalume® or
the factory coated version of Zincalume®, Colorbond®. These materials look and behave
quite differently from traditional galvanised sheet. Galvanised sheet is still available today but
is a rather different product from the hot dipped sheet produced up to the 1960s. These
changes have created a dilemma for heritage professionals advising on galvanised roofing
repair and replacement and on new corrugated roofing in heritage areas.
Zincalume® (55% aluminium, 43% zinc and 1.6% silicon) is superficially similar in
appearance to galvanised sheet but retains its bright shiny appearance for much longer. It
cannot be used in conjunction with galvanised gutters and downpipes, which must also be
replaced if a galvanised roof is replaced with Zincalume®. It is also not compatible with lead
flashing, which makes for difficulties in retaining or reconstructing original details.
However Zincalume® is more readily available and comes with a guarantee from the
manufacturers, BHP. Standard galvanised roofing is a little more difficult to obtain and has no
guarantee from BHP. Owners in heritage areas are often reluctant therefore to accept advice
to use galvanised roofing. They feel they are being asked to use an inferior product. They are
also often attracted by the additional protection provided by the prepainted oven baked finish
of Colorbond®.
Workshop discussion
In an attempt to understand these developments and assist advisors in making decisions
about the most appropriate materials to recommend, Alan Buchanan, a metallurgist with BHP,
was invited to make a presentation at one of last year’s advisor workshops. Lorraine Huddle
took on the role of respondent, and raised the difficulties that many advisors encounter on a
regular basis in dealing with the new materials.
To explain BHP’s position, Alan provided a detailed account of the history of corrugated
galvanised steel technology. The appearance and weathering patterns of corrugated
galvanised sheets are closely related to their method of manufacture. Since about 1850
corrugated galvanised sheets have been widely used in Australia for roofs and wall cladding.
Traditionally sheets were manufactured by a hot dip process where individual roof sheets,
which had been precut to standard length, were manually dipped in a bath of molten zinc to
galvanise the steel surfaces. The steel used was considerably thicker but softer than that
used today and the coating itself very substantially thicker. However there was a great
variation in the thickness of the coating with one side being typically much thinner than the
other as a result of the way the alloy developed as the sheets were racked for cooling. It was
common practice to alternate the sides of the sheet when laying, to save on overlap, and this
has resulted in the common checkerboard effect where one side has corroded much more
than the other.
However with changes in technology in the 1960s the process changed dramatically.
Continuous production of galvanised sheet was introduced, in a manner which offered a
closely controlled galvanising process and almost continuous lengths.
The high tensile steel allowed the steel base to become thinner with the coating being applied
continuously to specific tolerances. Where previously the market was supplied with a sheet
that may have varied greatly in coating mass, the market now has a choice between very thin
coating mass material for general manufacturing through a wide range to a maximum coating
of 600 grams/m2. Single run roof sheets greatly reduce corrosion by avoiding the horizontal
overlap.
The next development to the metallic coating of steel came in 1976 when John Lysaght
introduced Zincalume® metallic coated steel to the market. According to BHP, over the last
twenty years, Zincalume® metallic coated steel has become universally recognised as
offering a substantial increase in corrosion performance over the equivalent galvanised
product.
Colorbond® was originally produced on a galvanised base but is now produced with a
Zincalume® base. Zincalume® is BHP’s preferred product on which they offer a guarantee.
They produce galvanised corrugated roofing at a similar price to suppliers but do not provide
any guarantee for this product.
It was noted by participants in the discussion that galvanised products used in most other
parts of industry, even structural components, are hailed as the highest quality most costeffective product available. This reflects the performance of the heavier post dip galvanised
coatings available for these products.
Alan acknowledged that glare is a problem, but not that it was confined to Zincalume®. It is
causing BHP enough concern to undertake research on methods to reduce the glare. Several
methods are being tested, including an extra coating. Another issue, the fading or ‘chalking’ of
Colorbond®, was attributed to problems with the earlier products, as well as some known
pigmentation problems which have since been improved. Alan maintains that the BHP CSA
product is branded and any such premature failures are acknowledged. He also drew our
attention to the availability of cheaper imported products some of which he suggested would
not have the performance of Colorbond®.
It was pointed out that there are numerous anecdotal examples provided by heritage advisors
of galvanised corrugated iron roofs over one hundred years old and still functioning. (although
this could not of course be assumed to be a general rule)
There was also the question of the size of the 'heritage market'. There are hundreds of
thousands of houses in Victoria alone that are most appropriately repaired or reclad, from a
heritage point of view, with galvanised corrugated sheet. This is not an insignificant market to
supply.
Given these circumstances, the suggestion was put that the manufacturers might consider
producing a quality galvanised roofing that could attract similar guarantees to those applied to
Zincalume®.
Heritage Viewpoints:
The following examples of the application of Burra Charter philosophy, practice and
procedures to corrugated steel roofing indicates where and why galvanised sheet will usually
be preferred by heritage practitioners.
Preservation - to prolong the life of the original galvanised sheets that may be corroding
(where they are lapped) by placing 'sleeves' of galvanised iron in between the lapped
originals.
Reconstruction - replace with galvanised iron materials similar to those known to have been
used on that structure.
Adaptation - modifying a place to suit proposed new uses for economical reasons, e.g.
replacing a damaged slate roof with galvanised sheet, being a cheaper material used on
buildings of the same style and/or period, etc.
Compatible use - Cover an existing roof, such as a timber shingle or slate roof that can no
longer be maintained in a waterproof condition, with a material such as galvanised iron. This
material will have minimal impact on the cultural significance of the place because corrugated
galvanised iron was used on many roofs in the same period that timber shingles and slate
was used. Many old shingle roofs were covered with galvanised corrugated iron as soon as
that product became available in the 1850s as it provided good water collection.
Repair and maintenance - replacing worn materials with the same type of materials.
Maintaining the Patina of Age - The heritage character of many places, including heritage
precincts, is made up of several small features which, in themselves may not appear to
matter, but it is their cumulative effect that is important. In the case of a heritage precinct with
free-standing buildings, roofs are usually substantial and visible parts of the building. The
precinct will lose its appearance of being an old heritage area if all the unevenly patterned
rusty roofs are systematically replaced with shiny Zincalume® or Colorbond®. The town of
Maldon is a good example of the importance of the appearance of slowly corroding but
waterproof roofs. Pitched roofs on which galvanised corrugated iron are fixed are typically
very visible as they contribute to the picturesque skyline and consist of a very large proportion
of the total external cladding of a structure. In hilly terrain the roofs are even more visible from
within the garden or from the windows of individual properties as well as from the street.
Reflection - The reflective properties of new galvanised iron and Zincalume® cladding,
immediately after fixing to the frame, are an uncomfortable inconvenience for a few months
until the galvanised iron dulls to its typical light grey colour. However, Zincalume® takes
several years before the reflective properties have dulled noticeably.
New Buildings to be Recessive - New buildings and additions in heritage areas should be
recessive in the streetscape relative to the visually connected authentic heritage places.
However, buildings with highly reflective roofs will reduce the degree to which they recede
into the background.
Dealing with the Issues
While Zincalume® is the most readily available steel roofing, our enquiries reveal that
corrugated galvanised steel roofing is available at reasonable cost. We know of two suppliers
who access high tensile galvanised sheet from BHP and roll corrugated roofing. These are:
JH Stephenson & Son Pty Ltd, 152-154 Victoria Street, North Geelong Ph. 5278 7044
Metroll Pty Ltd., 1 Bellevue Crescent’ Preston. Ph. 9480 3744
Despite BHP supplying the material at a similar cost to Zincalume®, both firms charge extra
for galvanised (Stephenson + 6%, Metroll + 8%), perhaps because of the lower turnover.
Typically the sheet is supplied in specified lengths up to 30m. Availability seems to be good
and freight costs quite minimal
The standard roofing sheet is ‘Z450’ which means 450 grams per square metre of coating
(225 grams on each side!) on a base metal 0.42-mm thick. This is nothing like the thickness
of coating on the hand dipped sheets of the past, which BHP estimate to have been the
equivalent to 800 gsm, but does not suffer the variation discussed above. It is generally
accepted that its corrosion resistance is inferior to Zincalume® and BHP have drawn our
attention to research by CSIRO indicating that it is much inferior in a marine environment (i.e.
within 1 km of Port Phillip Bay). It has however been in widespread use since the 1960s and
there are no doubt many roofs surviving from that period.
‘Z600’ coating (300 grams per square metre on each side = 600 gsm) is readily available in
soft steel which is used for water tanks and curved verandas. This would seem to be the
closest approximation to the old hot dipped sheets.
The owners of a long established South Australian firm, Fielders, have introduced ‘Z600’ high
tensile galvanised corrugated steel to appeal to the heritage market. This steel with a coating
of 600 grams per square metre (300 grams per side) comes with a ten year guarantee from
Fielders and is, in their opinion superior to Zincalume® for a number of applications. There is
a snag - this heavier duty coating costs about 25% more than standard Zincalume®. They
consider their product to be competitive with Colorbond®. Fielders are setting up a national
distribution network and their Victorian address is; Unit 1, 87 Tennyson Street, Essendon,
3040 Ph. 9375 3047. Head Office in Adelaide is (08) 8292 3611.
It was ironic to discover, while researching this article, that the ‘XSE’, or extreme and severe
environment Colorbond® which had been recalled by BHP, had a Zincalume base and has
had to be replaced with a Colorbond® with a galvanised base (BHP is quick to point out that
this had nothing to do with the corrosion resistance of the Zincalume®, which they would
prefer to continue to use if certain compatibility problems can be solved).
What to Recommend?
This discussion does not offer simple answers to the heritage advisor. It is possible however
to suggest some solutions for particular circumstances:
Repairing rather than Replacing - We must accept the simple fact that short hot dipped sheets
are no longer available. We will therefore never see the picturesque checkerboard pattern of
corrosion develop on a new galvanised roof. Where this patina is considered important this
lays greater emphasis on repair rather than replacement of roofs and/or the use of sheets of
old iron that have lasted well.
Replacing like with like - Where new sheet is required to replace all or part of the roof of a
significant building, the soft steel Z600 would seem to be the best option. This can be cut in
short lengths if it is considered important to retain the horizontal shadow line created by the
overlap. Fielders’ new high tensile Z600 offers another option where lightness and strength is
important.
Roofing in Heritage Areas - For buildings of less significance and new buildings in heritage
areas there will often be cases where only galvanised roofing will be acceptable due to the
visual impact of the alternatives. In these circumstances the owners would have a choice
between the cheaper but inferior Z450 or the 10-year guaranteed Z600 produced by Fielders.
Owners who would have preferred to use Colorbond® might be persuaded to use the Z600.
In less visually dominant situations advisors have often been prepared to approve the use of
Birch Grey Colorbond® as this colour is similar to that of recently weathered galvanised
finish. While this blends in quite well, it will never lose its neat and crisp look. Heritage Red
Colorbond® can approximate the paint finish on roofs from the federation period. Advisors
might be more inclined to approve alternatives in coastal environments where galvanised
roofing seems to perform less well.
The suggestions above probably reflect most advisor’s current general practice except that
the introduction of high tensile Z600 provides a further useful alternative. It is encouraging to
find that the perceived demand for a higher quality galvanised sheet, discussed at the
workshop, had been recognised in South Australia by a firm that is now seeking to develop
that market across Australia.
Key Reference:
The Maintenance of Heritage Assets: A Practical Guide, Information Sheet 4.1 Corrugated
Roofing NSW Heritage Office 1998.
Other reading:
'Corrugated Galvanised Iron: The Profile of a National Culture?’ Peter Meyers, Transition
June 1981 p24
'The Corrugated Iron Aesthetic’ Miles Lewis, Historic Environment Vol. 2, No. 1, 1982 p.50
'A Corrosivity Survey on a Grid of Sites Ranging from Rural to Moderately Severe Marine.
Part 1. Steel, Galvanized Steel and Zincalume Coated Steel’ G. A. King, CSIRO Division of
Construction and Engineering, published in Corrosion Australasia, February 1988, pp. 5-14.