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    How poor quality washers can cause corrosion on metal roofs

    DEKS Industries

    Andrew Meese from DEKS Industries explains how poor quality washers used on screws to fix metal roofs can play a significant part in corrosion and even accelerate deterioration of the roof.

    A washer may be the smallest and cheapest component of metal roofing structures, but substandard washers can make all the difference to the longevity of the roof.

    It is important, therefore to know how to identify the right washer. A washer used in a roofing system must be able to withstand all environmental conditions, including UV degradation, acid rain and extreme temperatures while remaining elastic, intact and watertight.

    Unfortunately, washers are often overlooked because of their minor cost per unit compared to fasteners and steel sheeting.

    Beware of blends

    Washers can be made of many different materials, but EPDM, a synthetic polymer derived from oil is considered the ideal material for washers. Although many companies state that they supply EPDM washers, the percentage of EPDM contained in each washer varies greatly due to the absence of regulatory standards. The number of ingredients in any given EPDM ‘blend’ can range from five to 20 with hugely varying properties such as UV stability, elasticity, compression performance and resistance to heat aging.

    One common washer additive one should be aware of is carbon black, which is used in a miniscule amount in EPDM blends for its ability to deliver important performance characteristics at a relatively low cost. These include:

    • High UV resistance
    • Improved elasticity
    • Weathering resistance
    • Assistance with manufacturing methods
    Carbon black’s low cost also makes it effective for binding blends with high-clay contents.

    As rising oil processes drive up the cost of EPDM, some manufacturers are using low EPDM, high-carbon blends that are almost impossible to separate visually from high-quality blends with longer lifespans extending beyond 20 years as against as little as 18 months expected from washers made with a poor blend.

    Conductive corrosion

    High levels of carbon black also result in bi-metallic corrosion facilitated by the conductive washer. The quality of carbon, grade of carbon and its dispersion in the blend can lead to an electrical pathway being developed through the washer itself. This problem is often misinterpreted as a failure of the fastener or roofing/cladding material.

    An indication that the washer is contributing to the corrosion can be small blisters of white rust on the surface of the steel sheet close to the base of the screw head. In severe cases, one can also see red rust as the coating is removed and the substrate begins to corrode, impacting the structural integrity of the construction envelope in the long-term.

    A simple test

    Testing the conductivity of a washer can be done simply by applying a resistance meter to the surface. Applying a charge of 1,000V across the washer and reading the resistance will provide a quick indication of whether the washer is conductive. The true measure of a perfect insulator, which is what a washer should be, is a reading of infinite resistance regardless of the voltage applied.

    The degree of conductivity is measured according to the amount of current that will pass though at any given voltage. The industry benchmark equates to a resistance of no less than 2000M Ohms. It is advisable to ask for a certificate of compliance from a screw supplier or a similar statement regarding conductivity. The test can also be performed with a simple multimeter.

    Recently, a major supplier of roofing and cladding material in Australia recommended that all roofing and cladding must be installed with fully non-conductive washers. The conclusion was reached after extensive real-world testing was carried out, which revealed that washers with even the slightest conductivity showed an increase in electrolysis compared with fasteners with non-conductive washers. This recommendation is to be adopted as part of the company’s overall roof material warranty.

    Quality washers are engineered to suit the fasteners, roofing and cladding profiles and materials, as well as the final application.

    When looking for a quality washer, always ask the following questions:

    • Does the washer have a warranty?
    • Is the washer quality traceable?
    • Is the technical data and support available to be viewed?
    • Has the washer been manufactured to suit the fastener and application?
    By asking these simple questions, one can be assured that their metal roofing project will not fail because of the washers.

    DEKS Industries is a leading manufacturer and supplier of roof flashing, roofing washers and plumbing products throughout Australia and the world.
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