DURABILITY PROBLEMS WITH ZINC CLADDED ROOFS DUE TO
INPROPER UNDERLAYS
Tommy Bunch-Nielsen (1) and Erik Brandt (2)
(1) Director, M.Sc., Bunch Building Physics, Vedbaek, Denmark
(2) Senior researcher, M.Sc., Danish Building Research Institute/Aalborg University,
Copenhagen, Denmark
Abstract
In Denmark the use of zinc as roof cladding has been popular among architects over the
last 10 years, and a number of corrosion problems has occurred after less than five years in
some of these roofs. It is corrosion from the back side of the zinc cladding and in most cases it
is so called hot water corrosion due condensation on the downside of zinc. The underlays for
the zinc cladding have changed over the years and the cladding is often layed directly on hard
mineral wool or plywood with a thin underlay of diffusion open textile roof underlay, which
is also used for clay tiles.
The corrosion is related to the moist environment between the zinc and the underlay and
the lack of ventilation of the underside of the cladding and lack of protection from oxidisation
of the down side of the zinc. The condensation is related to moisture in the underlays and the
sub cooling from radiation to the sky on clear nights.
Bitumen corrosion from organic acids has also been used as explanation for some corrosion
problems in zinc roofs when zinc is installed directly on an underlay of bitumen and the use of
structured underlays are discussed.
The paper presents results from a number of inspections of zinc roofs in Danish
environment and the laboratory test of the zinc plates and leaching of chemicals from the
underlays.
New design of zinc roofs to prevent corrosion is suggested in the conclusion of the paper.
1
INTRODUCTION
The use of metal roofs has increased over the last 10 to 15 years mainly due to
architectural trends. The design of roofs has also changed from the traditional design with
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wooden underlays to highly insulated roofs with underlays of insulation or prefabricated roof
components. Zinc has been widely used due to its nice ability to protect its surface on the
weathered side.
The change in design has been made without long term experience of the durability and
this has led to a number of problems with the service life of the zinc roofs.
2
DESIGN OF ZINC ROOFS
Zinc with standing seems is the most normal used application for zinc roofs and the
thickness of the zinc material is typically 0.8 mm or less.
The standing seems can be made on site and this makes it possible to design roof with a
length of up to 40 meters. In the long roofs the temperature movements must be taken into
account.
Figure 1: Zinc roof with standing seems
The traditional underlays has for the century been wooden boards of 25 mm thickness
spaced with 5 – 10 mm distance between the boards.
For the last 25 years plywood or OSB have been used instead of boards and some
experience has been gained with this underlay.
For the last 10 – 15 years zinc roofs designed as warm roofs with the zinc installed
directly on mineral wool or perhaps with an underlay of diffusion open roof underlays based
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on nonwoven textiles or nonwoven laminated with micro perforated foils as shown in figure
2.
Figure 2: Zinc with diffusion open underlay (of nonwoven laminated with micro perforated
foil??) on mineral wool
One of the problems with this design is built in moisture. The mineral wool will often be
left open for a period and in most European countries there is risk of rain all year around. If
the mineral wool becomes wet in the construction period the moisture will be trapped in the
roof structure.
The drying out is very difficult in a warm roof structure with a diffusion tight vapour
barrier on the warm side and a metal roofing on the cold side, an typically drying will take
several years.
There have been some indications from the suppliers and consultants that there is a
drying capacity in the standing seems. That means that the moist air in the roof should be able
to be pressed out of the roof when the roof is heated by the sun and the air in the roof
expands. This has not been proved by the experience from roofs in practice.
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Probably this would be compensated by in going air in the nights when the roof is cooled.
In low sloped roofs with slopes down to 2 or 3 degrees it is important to close the seams
with jointing material to make the roof water tight. To have watertight and diffusion open
joints at the same time is not possible in practical installation.
The use of moisture adaptive vapour barriers have been tried and can function for small
amounts of built in moisture. The sun will press the moisture down in the roof where it can
dry out to the interior of the building as the moisture adaptive vapour barrier opens when it
gets moist. Unfortunately the night and day cycles in temperature will to some extent send the
moisture back to top during the night time - in highly insulated roofs the moisture may stay in
the middle of the insulation.
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CORROSION
Zinc and titanium zinc in contact with the atmosphere will protect its exterior surface by
forming zinc carbonate.On the back of the zinc carbonate is not formed when the zink is in
close contact with a plywood or a roof underlay. Therefore the zinc surface is unprotected and
subject to corrosion if there is moisture present.If there is moisture present, e.g. due to built in
water or leaks, and the zinc surface is heated a phenomenon called hot water corrosion will
occur.
This hot water corrosion will cause pitting corrosion from the backside as shown in figure
3.
Figure 3:Hole in zinc roof due to corrosion originating from the back seen from top side ( left)
and from the back (right).
The pitting corrosion cannot be observed until a hole occurs and the roof leaks. The
corrosion cannot be seen from the topside and can develop in few weeks.
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Figure 4: Photo of zinc roof removed from a 6 year old zinc roof installed directly on a
plastic foil.
4
INSPECTIONS
The roofs can be visually inspected without the very small holes or signs of corrosion are
observed. To investigate corrosion destructive inspections is needed. Repair is difficult and
will be visible afterwards and furthermore the problem with corrosion is not solved and will
consequently come back. To avoid further problems the only solution is a total refurbishment
with a suitable substrate.
It is possible to inspect with a cover meter to determine the thickness of the zinc plate but
this is not an accurate measurement and it is difficult to perform over a whole roof surface.
5
SEPATION LAYERS
The use of structured separation layer were introduced approximately 10 years ago.
Structured separation layers can be used on board materials like plywood and OSB where
there is no air penetration from the underside.
When zinc is installed on an underlay of mineral wool in a warm roof there has been no
clear rules for the use of structured underlays, as the mineral wool by some suppliers have
been considered open to diffusion as well as to air infiltration.
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Practical experiences from a number of roofs from 2006 to 2009 have shown that
installation on mineral wool will often lead to corrosion of the back side of zinc roofs. The
mineral wool used has a relatively high density of 120 to 180 kg/m3 and the air penetration is
therefore very small and availability of air is not sufficient to provide a surface protection of
zinc carbonate to protect the zinc from corrosion. The corrosion starts as soon as moisture is
present and the formation of zinc hydroxide starts.
The use of structured separation layers has three purposes:
- A:To ensure that the moisture cannot come in direct contact with the zinc surface –
drainage effect
- B: To ensure that moisture from the underlying substrate, e.g. mineral wool not is
transported to the zinc surface
- C: To ensure that air is available and can protect the surfaceby forming zinc carbonate.
Figure 4: The backside of zinc plates taken from a roof with (right) and without (left)
structured separation layer. It is clearly seen that no zinc carbonate is formed and
consequently no protection against corrosion achieved without separation layer.
There are two different types of structured separation layer. One type is a product with stiff
plastic fibres that gives a distance between zinc and substrate of 7-8 mm. This type is open to
diffusion and does not solve purpose B. The other type is a plastic board with “knots” that
provides a distance of 7- 9 mm between zinc and substrate. This type is tight to diffusion and
besides it is airtight. This type makes a barrier that prevents water from the insulation in
reaching the zinc surface.
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Figure 5: Typical structured separation layer of the type open to diffusion.
6
CONCLUSION
The backside of a zinc roof cladding must be accessible for air in order to form a protective
layer of zinc carbonate to avoid corrosion from the back. If air is not accessible the protective
layer is not formed.
The traditional way of constructing zinc roofs by use of board with a spacing of 5 – 10 mm
over a ventilated gap, se figure 5, is one way of achieving accessibility of air. This type of
construction has by long times experience proved to have a long service life.
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Figure 5: The traditional solution with an underlay of wooden boards with a spacing of 5 10 mm over a ventilated gap.
When zinc is used as roofing in warm roofs or directly on a substrate of plywood and
similar the access of air must be secured by a structured separation layer.
There is so far no precise knowledge of the maximum length for a roof design with
structured separation layer. For a typical structured separation layer of 8 mm the length of the
roof must probably be limited to 20 m to provide enough air infiltration.
It is important not avoid accumulation of moisture in the structure from built in moisture
(from the construction period) or water from leaks in flashings around windows or ventilation
equipment on the roof. The design and workmanship of details, e.g. ventilation ducts and
chimneys are therefore important.
REFERENCES
List and number all bibliographical references at the end of the paper under the major heading
REFERENCES. Number the references in order of appearance, using Times New Roman 11
pt. Authors are cautioned to give complete information about books and authors and to
carefully check spelling and dates. The presentation of references is given below. Commercial
papers or documents not available to the public cannot be referenced.
[1] Sterman, S. and Marsden, J.G., 'Silane coupling agents', Ind. Eng.. Chem. 58 (3) (1966) 33-77.
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[2] Alexander, K.M., Wardlow, J. and Gilbert, D., 'Aggregate-cement bond, cement paste strength
and the strength of concrete', in 'The Structure of Concrete', Proceedings of an International
Conference, London, September, 1965 (Cement and Concrete Association, London, 1968) 59-81.
[3] Crank, J., 'The Mathematics of Diffusion', 2nd Ed. (Clarendon, Oxford, 1975).
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