Coated steel sheet: how to assess the steel, profile and finish

Beneath the visible finish, a metallic coating helps protect the steel. This metallic layer and the outer organic coating do different jobs. The reverse face also has a specified finish. Check the product data for the sequence and thickness of the layers; these vary between sheets.

Distinguish the steel core thickness from the coating thickness. They describe different things and are generally quoted in different units. Extra paint cannot make up for missing steel, and a good finish will not compensate for misaligned supports. Check how the selected sheet will work in the structure.

Check which face the coating description refers to. On an open-sided shelter or soffit you may see a backing coat in a different shade from the weather face. Agree the appearance of both faces before ordering; a photograph of the top surface shows only one of them.

Forming ribs makes a thin sheet stiffer than flat steel of comparable thickness. T18 and T35 trapezoidal steel sheets differ in rib height and are available in different cross-sections. Tile-effect sheets have a different pattern of ribs and steps, while deep structural decking serves a different purpose again. Price per kilogram tells you little about suitability.

Load capacity also depends on steel thickness, grade and support arrangement. Even profiles of equal depth can have different capacities. Use the data for the exact cross-section. Before substituting a sheet, check it against the design loads and the spacing of the supports.

A sheltered residential site differs from a coastal, industrial or agricultural location. Choose a finish intended for the actual exposure. Consider the inside face separately, particularly where humidity is high or aggressive substances are present. Shelter from direct rain does not rule out corrosion.

Compare declared corrosion resistance, UV resistance and warranty terms. Micrometres alone are a poor basis for ranking different coating systems. Matt and gloss describe appearance, not a common durability class. Inspect a physical sample alongside its technical data sheet.

A warranty covers specified defects and may have separate terms for colour and appearance. Read the installation requirements, inspection duties and environmental exclusions. The warranty period is neither a promise that the finish will remain unchanged nor a prediction of service life on a particular building.

Drainage, detailing and use affect the life of the finish. Standing water, metal swarf and contact with unsuitable substances can damage even a well-chosen coating. Keep purchase records and batch labels. If sheets are added later, new material may look different from a surface that has weathered for years.

Protect the finish during unloading and handling. Lift sheets clear rather than sliding them across one another. Support long lengths to prevent kinks. Follow the storage instructions and keep packs from getting wet: water trapped between stacked sheets is particularly troublesome.

Cut with tools approved for the coated sheet. Sparks and excessive heat can damage the protective layers. Remove swarf after cutting or drilling. Peel off the protective film within the stated time; prolonged heating can make it harder to remove and spoil the finish.

List the profile, steel core thickness, coating, colour and sales unit in each quotation. For roof sheets, compare effective cover width rather than overall width. For flashings, check the developed width of the blank and the length of each piece. Similar prices can buy different quantities or levels of protection.

Include accessories, fixings and delivery. Specify the face that will be on view, particularly for soffits and fencing. The most expensive coated sheet is not automatically the right one. Choose a specification suited to the site and an installation method that preserves the protection you paid for.