Galvanisation and Why It Matters in Madinah's Climate

Galvanising coats steel sheet with a layer of zinc that stops rust in two ways: it is a physical barrier that keeps water and oxygen off the iron, and it is a sacrificial anode that corrodes in place of any steel exposed at a scratch or cut edge. In Madinah the threat is not marine salt as it is in Jeddah. It is condensation around air-conditioning ducts, fine dust that holds moisture on horizontal sections, and daily temperature swings that can exceed 20 °C. For non-structural gypsum-board framing the usual baseline is a G40 (Z120) coating, 120 g/m² over both faces, as referenced in ASTM C645, rising to G60 or G90 in wet rooms and in ceiling voids crossed by ductwork.

How a zinc coating actually protects

Steel rusts because iron wants to return to its stable oxide whenever it finds water and oxygen. Zinc interrupts that cycle with two mechanisms working together. The first is the barrier: a continuous metallic layer that keeps moisture away from the base steel for as long as it stays intact. The second is electrochemical. Zinc sits above iron in the galvanic series, so when steel is exposed at a scratch or a sheared edge, the surrounding zinc corrodes first and protects the bare iron. This cathodic protection effectively covers cut edges up to a sheet thickness of about 1 to 2 mm, which is why thin pre-galvanised framing does not need edge treatment after cutting on site.

Two products share the name and behave very differently:

• Continuous hot-dip galvanised sheet (pre-galvanised): the coil passes through molten zinc at roughly 450 °C before forming. The result is a metallurgically bonded layer, typically 7 to 20 µm per face depending on grade. This is the normal material for gypsum-board framing.
• Electro-galvanised sheet: a much thinner plated layer, usually 2.5 to 5 µm. It is bright and attractive but is consumed quickly in any damp environment. Acceptable for screws and dry interior accessories, not a substitute for hot-dip in load-bearing ceiling grids.

Madinah's climate: where the risk really sits

A common assumption is that an inland city more than 150 km from the Red Sea, with dry air and little rain, has no corrosion problem. Outdoors that is broadly true. Madinah's exterior environment generally falls in category C2 of ISO 9223, where zinc loses between 0.1 and 0.7 µm per year. But ceiling framing does not live outdoors. It lives in an enclosed plenum, and the plenum has its own climate.

The real drivers are:

• Condensation at air-conditioning ducts. Summer air outside exceeds 45 °C while rooms are held at 22 to 24 °C and the supply air inside the duct is colder still. Any gap in duct insulation or an uninsulated flexible connection becomes a dew point inside the ceiling void, dripping onto the Omega and C channels below every afternoon. Locally that is a C3 environment, whatever the city average says.
• Fine dust. Dust storms are frequent, and the layer that settles on horizontal sections absorbs night-time humidity and holds it for hours. Dust carrying soluble salts raises the conductivity of the wet film and accelerates attack.
• Daily thermal cycling. A 20 °C swing between afternoon and dawn is normal in winter. Every cycle expands and contracts the section. An organic paint film cracks over time, while a bonded zinc layer moves with the steel.
• Short, heavy winter rain. Annual rainfall is only around 50 to 60 mm, but it arrives in a handful of intense events. Sections stored on site, or installed before the envelope is closed, get fully wetted and then dry slowly between stacked layers.
• Wet rooms inside the building. Bathrooms, kitchens, laundries and public washrooms in hotels and hospitals run above 80 % relative humidity for hours a day regardless of the weather outside.

Reading coating grades on a specification

Zinc coatings are specified by mass, not thickness. ASTM A653 uses the letter G followed by ounces per square foot; the metric system and EN 10346 use Z followed by grams per square metre. In both cases the figure is the total for both faces combined. The grades you will meet most often:

• G40 / Z120: about 120 g/m², roughly 8.5 µm per face. This is the minimum ASTM C645 refers to for non-structural gypsum framing in ordinary dry interiors.
• G60 / Z180: about 180 g/m², roughly 12.5 µm per face. The sensible choice for plenums crossed by ductwork, and for buildings with a long design life such as hospitals and government facilities.
• G90 / Z275: about 275 g/m², roughly 19 µm per face. Used where humidity is permanent: pool halls, central kitchens, laundries.

A quick conversion: 100 g/m² on one face is close to 14 µm, because zinc has a density of 7.14 g/cm³. Combine that with ISO 9223 loss rates and the arithmetic is sobering. In a C3 plenum losing 0.7 to 2.1 µm per year, a 12.5 µm layer gives roughly 6 to 18 years before bare steel appears at the worst point. An 8.5 µm layer under a sweating duct can be gone in 4 years.

One product-selection note. The W angle is made in two versions, galvanised and painted white, both intended for hidden-lighting details that create a comfort space between wall and ceiling. Which one to specify depends on whether the edge is visible and what the finishing sequence is: the painted version gives a ready white surface, the galvanised version relies on the zinc layer described above. RUNNERS and STUDS are available in sheet thicknesses from 0.35 to 1.0 mm, thicker on request. Heavier gauge does not replace the coating, but it lengthens the time before corrosion removes enough section to matter structurally.

Site practices that waste the coating

Most rusted ceilings under 5 years old did not fail because the coating grade was too low. They failed because site practice stripped the protection:

• Stacked storage in the open. When galvanised sheet gets wet between tightly stacked pieces and cannot dry, it forms white rust, a zinc hydroxide bloom that consumes the layer rapidly. Store under cover, at least 10 cm off the ground, on a slope that drains.
• Cutting with an abrasive disc. The heat burns zinc off to about 5 mm from the edge and sprays hot iron particles that rust on the surface of the section. Use tin snips or a cold saw.
• Welding without repair. A weld strips zinc from a band 10 to 20 mm wide around the seam. If welding is unavoidable, repair with a zinc-rich paint as described in ASTM A780. Better still, connect with galvanised self-drilling screws.
• Mixing metals. Clamping a galvanised section against copper pipe, or fixing with brass or uncoated steel screws, creates a galvanic cell that eats the zinc at the contact point. Screws should be galvanised or carry an equivalent corrosion-resistant coating in line with ASTM C1002.
• Installing before the envelope is closed. Framing hung before glazing, doors and the AC system are in place sits through weeks of dust and night humidity. Sequence the programme so the envelope is weathertight first.
• Aggressive cleaning. Cleaners with a pH below about 5 or above about 12 dissolve zinc. Water and a neutral detergent are enough.

What to write in the spec and BOQ

A line reading "galvanised steel framing" protects nobody. A usable specification states:

• Coating process: continuous hot-dip galvanised sheet, coated before forming, to ASTM A653 or EN 10346.
• Coating grade by designation: Z120 minimum for dry voids, Z180 or Z275 for duct-crossed plenums and wet areas.
• Base-metal thickness in millimetres, stated separately. The zinc layer does not count toward structural thickness.
• A mill certificate for every delivery showing the measured coating mass, not just the nominal grade.
• Visual acceptance at delivery: no white rust, no scratches to bare metal, no dark staining.

The Saudi Building Code (SBC) draws its structural provisions largely from ASTM references, so naming the ASTM document in the contract gives both parties a clear basis if a dispute arises.

Checklist before you buy and install

• Classify each ceiling void separately: dry, duct-crossed, or permanently wet, and assign a zinc grade to each.
• Ask for the Z or G designation in writing and keep the batch certificate.
• Store sections covered, raised and sloped.
• Ban abrasive-disc cutting and unrepaired welding on site.
• Keep every metal in the system galvanised: sections, hangers, screws.
• Close the envelope and commission the AC before boarding, and fix any duct sweating before the ceiling is sealed.

In Madinah galvanising is not a cosmetic line item. It is the difference between a ceiling that serves for 25 years and one that shows rust spots through the board by its fourth winter.