Most roofing sheet enquiries arrive as a single line: 0.5 mm corrugated galvanised sheet, 1000 mm wide. That description covers a dozen different products, and the gap shows up later as wind uplift damage, laps that track water, or rust staining the wall within two seasons. Three variables decide whether the sheet performs: profile geometry, base metal thickness, and coating system. Colour is the last thing worth arguing about.

Profile geometry sets coverage and stiffness

Two profile families dominate industrial work. The sinusoidal corrugated profile runs a wave pitch of roughly 76 mm with a wave depth near 16 to 18 mm. The wave shape itself consumes material, so nominal width and covered width differ considerably: a sheet supplied at 1000 mm typically covers about 762 mm once the side lap is counted. Trapezoidal, or ribbed, profiles use fewer but deeper ribs. A sheet fed at 1250 mm covers 1000 mm, and because the ribs are deeper, more of the steel sits in the flanges that carry bending load. For the same purlin spacing a trapezoidal sheet can therefore be supplied thinner, which usually makes it cheaper per square metre even at a higher price per tonne. A Classic Rib steel roof panel in Galvalume is a typical example of that geometry.

Profile choice also sets the minimum roof pitch. Shallow waves shed water more slowly than deep ribs, so corrugated sheets need a steeper slope. Keep end laps at 150 mm to 200 mm, always run the overlap down the slope, and plan the layout so side laps fall over a purlin rather than mid-span.

Thickness, purlin spacing and the tolerance nobody reads

Roofing sheet is supplied from about 0.25 mm to 0.80 mm base metal thickness. Sheets below 0.30 mm suit temporary covers and internal linings where nobody walks on the roof. The 0.40 mm to 0.50 mm band covers most industrial and agricultural buildings on purlins at 900 mm to 1200 mm centres. Above 0.60 mm the sheet becomes walkable and is specified for long spans, heavy snow loads, cyclone regions, and roofs that will carry maintenance traffic or solar mounting rails.

Span capacity depends on thickness, yield strength, purlin spacing, and the deflection limit the project applies. A working rule is to hold deflection under about L/150 for roofs that must not pond, and tighter where ceiling lines or gutters will show movement. Thickness tolerance matters here too. Standards such as EN 10143 permit a minus tolerance, so a sheet ordered as 0.50 mm may arrive at 0.47 mm. Bending stiffness falls with the cube of thickness, so a 6 percent loss in thickness costs noticeably more than 6 percent of load capacity. Ask for measured thickness on the mill certificate, not the nominal figure.

Coating system decides service life

Three coating families cover almost every corrugated roofing specification. Hot dip zinc, marked as Z, is the baseline. Zinc coating mass is quoted as the total on both surfaces, so galvanized corrugated steel plate at Z275 carries 275 g/m2 in total, roughly 20 microns per side. For rural and inland urban atmospheres that is generous. Coastal sites, fertilizer plants, and heavy industrial areas with sulphur and chloride in the air need more zinc, or a different coating altogether.

Aluminium-zinc coated steel, sold as Galvalume or Aluzinc and marked AZ, blends about 55 percent aluminium with zinc and a small silicon addition. The aluminium-rich surface gives better heat reflectivity and better long-term atmospheric performance than zinc in most environments, which is why roofing in coastal and hot climates increasingly specifies AZ150 rather than Z275. The known limitation is cut edge protection: zinc in a coating sacrifices itself at a cut edge, aluminium does not, so a roof assembled from many small cut pieces will show edge rust earlier. Sheets supplied as Galvalume corrugated roofing sheets or as larger corrugated Galvalume roof panels keep the cut edge count low and get the best of the coating.

Organic coatings are the third family. A prepainted sheet adds a primer and top coat over the metallic coating, commonly 15 to 25 microns of polyester, or PVDF where 20-year colour retention matters. A prepainted Galvalume corrugated roof combines aluminium-zinc corrosion resistance with a UV-stable finish, which is why it dominates visible commercial roofing. Thicker back coats are worth specifying where the underside will see condensation in an unventilated building.

For genuinely aggressive duty there are two further options. Corrosion resistant corrugated galvanized roofing board with a heavy coating is the economical answer for mildly aggressive interiors, while stainless steel corrugated roof panels or DX52D galvanized corrugated steel plate handle chemical plants, food processing halls, and coastal installations where even heavy zinc fails within a decade. Where ventilation or acoustic relief is needed, perforated corrugated Galvalume panels do the same job with an open area that passes air and sound while keeping rain out.

Fixings and laps cause more failures than coatings

Most leaking metal roofs fail at the fixing, not at the coating. Self-drilling screws with bonded EPDM washers are the standard, driven so the washer compresses slightly without bulging. On trapezoidal profiles the fixings sit in the crest of the rib; on shallow corrugated profiles many installers fix in the valley so the screw is not standing in the drainage path, and each of those holes then depends entirely on the washer to seal. Either way, do not overtighten, because a crushed washer splits and a backed-out screw leaves an open hole.

Thermal movement has to be allowed for as well. Steel expands about 12 microns per metre per degree Celsius, so a 12 m panel facing a 50 degree swing between a cold night and a hot afternoon moves around 7 mm along its length. Fixing on every purlin through tight holes works the fasteners loose. Oversize the holes at intermediate supports, fix rigidly near the panel centre and let the ends move, and use a profiled filler or lap sealant at the end laps rather than relying on sheet-to-sheet contact alone.

What to check on delivery

Three checks catch most problems before the sheets reach the roof. Confirm the metallic coating mass with a magnetic gauge or a stripping test on a sample, because coating mass is where cheap material hides. Measure actual thickness against the tolerance on the certificate. Inspect bundles for interleaving film or paper on prepainted sheets, since panels that have rubbed together in transit arrive with scuffed paint.

Matching profile, thickness and coating to the job

For an inland warehouse on a steep pitch with purlins at 1200 mm, a 0.45 mm corrugated sheet at Z275 will last decades for the lowest cost. For a coastal factory roof on a low slope, step up to 0.50 mm trapezoidal profile in AZ150 or prepainted Galvalume, and spend the difference on proper flashing and correctly torqued washers. For a chemical hall or a food plant, 0.60 mm stainless or a heavy-coat galvanized product removes the maintenance cycle entirely. Fix profile and thickness from the structural load case first, then buy the best coating the budget allows, because recoating an installed roof costs far more than the gap between Z100 and AZ150 at the mill.