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, 2950 Vedbaek, Denmark

(2) Senior researcher, M.Sc., Danish Building Research Institute/Aalborg University, 2450 Copenhagen SV, 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. and tThe design of roofs has also changed from the traditional design

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with 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.

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ForIn the last 25 years plywood or OSB have been used instead of boards and some experiencethere has been gained some experience with this underlay.

ForIn 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 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.

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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 in from roofs in practiceal roofs.

Probably this would be compensated by in going air in the nights when the roof is cooled.

In low sloped roofs with a slopes down to 2 or 3 degrees it is important to close the seams with oil jointing materialor similar 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 work 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 asif the moisture adaptive vapour barrier opens when it gets moist. Unfortunately the night and dsay cycles in temperature willth to some extent send the moisture back to top during the in night time - in highly insulated roofs the moisture may stays in the middle of the insulation.

3 CORROSION

Zinc and titanium zinc in contact with the atmosphere will protect its exterior surface by forming zinc carbonate.

On the backside of the zinc carbonate is not formed when the zink is in close contact with a plywood or a roof underlay.due to lack of carbondioxid from the air. Therefore the zinc surface is unprotected and subject to corrosion if there is moisture present.

If there is moisture present, e.g. due to builtd 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 on photos in figure 3.

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Figure 3:Hole in zinc roof due to corrosion originating from the back side seen from top side ( left) and from the backunderside (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.

Figure 4: Photo of zinc roof removed from a 6 year old zinc roof installed directly on a plastic foil.

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4 INSPECTIONS

The roofs can be visually inspected without the observation of very small holes or signs of corrosion are observed. To investigate corrosion destructive inspections is needed. and the rRepair 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 withll 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 and are used on plywood and other board, where there isare 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 diffusion open to diffusion as well as and open to air infiltration.

Practical experiences from a number of roofs from 2006 to 2009 have shown that installation on mineral wool will often lead to back side 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 there fore very small and availability of air the supply of carbon dioxide is not sufficientadequate to provide a surface protection of zinc carbonate to protect the zinc from corrosion. The corrosion starts as soon as if at the same time there is 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 must cannot comebe in direct contact with the zinc

surface – drainage effect- B: To ensure that he moisture from the underlying substrate, e.g. mineral wool must

not isbe transported to the zinc surface- C: To ensure that Aair and carbon dioxide is available and can protect the surfacemust

be transported to the zinc surface toby forming zinc carbonate.

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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 solutions to the structured separation layer. One typesolution is a product with stiff plastic fibres that gives a distance between zinc and substrate of 7-8 mm. This type and is diffusion open to diffusion and . This does not solve purpose B. and Tthe other typesolution is a plastic board with “knots” ‘ that provides a distance of 7- 9 mm between zinc and substrate. This type and is tight to diffusion and besides it is airtight. This type makes a barrier layer that preventsstops water from the insulation in to 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 ventilated so enough carbon dioxide has access to the surface to provide corrosion protection by forming zinc carbonate to avoid corrosion from the back. If air is not accessible the protective layer is not formed.

This can be provide in tThe traditional way of constructing zinc roofs by useing 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.

When zinc is used as roofing in warm roofs the access of air must be provided by a structured separation layer.

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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 securedprovided by a structured separation layer.

There is so far no préecise 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 haveavoid accumulation of moisture in the structured from builtd 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.

The combination of moisture and zinc without enough ventilation of the backside to form zinc carbonate is dangerous.

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

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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.

[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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