Prefinished sheet steel building products, such as wall and roof cladding, liner sheet, flashing, and associated items, have experienced an enviable growth record during the past twenty years or so. Coil coated prefinished sheet steel in a variety of colours and paint systems has greatly enhanced the appearance of thousands of industrial, commercial, institutional, recreational, residential and farm buildings, providing an economical, durable and attractive alternative to traditional materials. As with all materials, a little care and maintenance pays off handsomely in terms of longevity and repair costs. The recommendations which follow have been learned at first hand and represent the collective industry experience with thin-film paint systems applied to metallic coated sheet steel by the coil coating process. In this publication the term "thin-film paint system" refers to a modified silicone polyester or a fluorocarbon type having a coating thickness about 25μm.
When the guidelines listed below have been observed, thin-film paint systems have been used successfully for all types of environments other than severe industrial atmospheres which require special consideration. The guidelines are not intended for barrier coatings, laminates, and new formulations which have different characteristics than the thin-film paint systems on which these guidelines are predicated.
Design, Detail and Colour Considerations
- Architectural details should permit natural rain-flow cleaning of the cladding.
- On roofs or other horizontal surfaces, standing water can contribute to the premature failure of the paint system and substrate. Detailing should preclude damming or ponding of rain-flow at stacks, ventilators, air control equipment and other objects.
- Due to colour tolerances, there may be differences in colour shade between production runs. Where possible, ensure that each building elevation is clad with material from the same production lot. If different production lots must be used on one elevation, as may occur when making an addition to an existing building, try to begin the cladding on an elevation change or break in the building to minimize the effect of possible colour variations.
- A sufficient roof slope to permit drainage is recommended (e.g. 1 in 48 minimum, and greater where rainfall is heavy, drainage is restricted, etc.).
- Roof surfaces, defined as those up to 60 degrees from the horizontal, are subject to more severe exposure conditions than vertical surfaces. Conditions such as extended exposure to ultraviolet light may be resisted by using a light colour for the roof. Acid precipitation and drip edge puddling are other conditions that could affect the appearances and durability of the paint finish. Drip edge puddling may be minimized with a steeper roof slope or by modifying the edge details. If severe acid precipitation is experienced, a more resistant prefinish system may be required.
- The building design should seek to minimize the installation of mechanical equipment on a prefinished roof. Walkways should be provided where regular traffic is necessary for maintenance.
- In wall applications, horizontal portions of the cladding and base flashing should be sloped to prevent moisture from puddling.
- Walls shadowed by overhangs and all soffit areas have an increased time of wetness relative to other areas. The increased time of wetness creates a more aggressive environment for the cladding so affected; therefore, architectural details should try to minimize these areas.
- To decrease the visibility of "oil canning", select an adequate material thickness, a narrower flute and a lighter colour.
- To prevent unwanted galvanic corrosion, the architectural details should not allow the contact of dissimilar metals (e.g. steel and aluminum or copper) or should provide an adequate means of separation. The path of rainfall runoff should also be directed to prevent water runoff across one type of material to another which can also cause galvanic corrosion.
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