The tile is the visible part, not the working part

A 600 x 600 ceiling tile looks like the product on a specification sheet. It is a 0.5 mm sheet with four folds on the edge, and on its own it carries almost nothing. The grid beneath carries the tile, the luminaires, the diffusers and the sprinkler drops. A ceiling that sags, rattles or loses tiles in a pressure test is usually a grid problem that the tile was blamed for.

That has an ordering consequence. A ceiling package is two purchases: a coil order for the tile and a system order for the grid, hangers and trim. Quote one without the other and the installer guesses the module.

What a square metre actually weighs

Steel at 7.85 g/cm3 gives 3.93 kg per square metre at 0.5 mm, so a 600 x 600 tile weighs 1.41 kg. At 0.6 mm the same tile is 1.70 kg. A square metre holds 2.78 tiles, and the grid adds roughly 3.3 m of T-bar, so a finished system lands between 6 and 7 kg per square metre before anything is hung on it.

A common design case for an office ceiling is 0.25 kN per square metre, or 25 kg, including a share for small services. With hangers on a 1.2 m grid, one hanger supports 1.44 m2 and therefore 0.36 kN. A 4 kg luminaire on two tiles adds 5.6 kg per square metre locally, so those tiles should be wired back to the structure rather than resting on the grid.

Zinc indoors is not free, and not always needed

A heated office ceiling is dry, and a Z100 coating is enough there: 100 g per square metre across both faces is 50 g a side, or about 7 microns. A pool hall, a commercial kitchen or a covered car park is a different environment, and the same tile in Z275 gives 137.5 g a side, or 19.3 microns.

The difference matters at the cut edge and at every punched hole, where there is no coating beyond the zinc's own throwing power. An electro-galvanized sheet such as SECCN2 is offered at 40, 60 and 80 g per square metre, or 2.8 to 5.6 microns a side. That is a dry-interior product; a pool hall is more often clad in 304 coil at 0.5 mm, because chloramine condensate sits on the panel for years.

The Z275 galvanized coil runs from 0.12 to 2.0 mm and 600 to 2000 mm wide, with coating mass from 30 to 275 g per square metre. Thickness on a ceiling tile is set by handling and screw pull rather than by strength; the coating is set by the room.

The tile is painted before it is punched

Most visible tiles are made from prepainted galvanized coil at 0.12 to 2 mm, coated in a RAL colour on a polyester system of roughly 20 to 25 microns. The coil is coated, slit, roll-formed and then perforated, so the paint film stops at the die entry and the hole edge is bare zinc over bare steel.

The hole pattern sets how much of that edge exists. A 2.5 mm hole on a 5 mm pitch in a 60 degree staggered array is 22.7 per cent open area, or about 46,000 holes and 362 m of cut edge in every square metre. It is the same geometry used at a larger scale on a perforated panel for facades, and it is why both products are sold on pattern rather than steel grade.

Acoustic fleece behind the pattern is what turns a perforated steel tile into an absorber. Without it the holes are a visual texture and little else, and the room still rings.

Punching is the cheap route for this hole count, and the punching process decides burr height, which decides whether the paint at the hole edge cracks when the tile is handled on site. A burr above about 0.05 mm will show through a 20 micron film on the first wipe-down.

Flatness at 600 mm is a different tolerance

A ceiling is viewed at a grazing angle under a window, so a wave of 0.3 mm across a tile is visible even though it is inside any coil flatness standard. The down flange is usually 20 mm and formed in four bends, and the corners are where the sheet pulls. A 0.5 mm tile has a stiff edge and a soft centre, so the middle is where oil canning appears if the coil was levelled badly.

Two numbers govern the visible joint. The tile sits on a 24 mm grid flange with 1 to 2 mm of clearance, and the grid should be level within about 2 mm over 3 m. A flat tile on an uneven grid produces a shadow line that wanders down a corridor, and no amount of tile sorting fixes it.

Thin sheet is also easy to over-form. A cold rolled sheet at 0.5 mm work-hardens at the bend and springs back about 2 degrees, so the flange angle is set on the bending tool rather than on the drawing.

The plenum is the real client

Above the tile there is usually 400 mm of space holding ductwork, cable tray, sprinkler pipe and the top of a partition. A sprinkler head needs a tile removed within reach, so allow one access tile per 2 m2 in a serviced office. Nothing heavy should hang from the grid: a diffuser supported by the T-bar transfers its weight to two hangers and its vibration to the whole bay. The processed galvanized products used for edge trim are light for the same reason.

Fire, seismic and the clip nobody quotes

A steel tile is non-combustible, but a ceiling's fire performance belongs to the tested assembly of tile, grid and hanger, not to the sheet. Seismic restraint is the item most often dropped from a quotation: each tile needs two wires or a clip to the grid. Steel grows by roughly 0.012 mm for every metre and every kelvin, so a 3.6 m runner moving through 15 K changes length by 0.65 mm, and a fixed perimeter turns that into a buckle.

Where the ceiling also acts as a diaphragm for wind uplift, as in an open plant room, the grid resists it and the tile does not. That needs a fixing schedule, not a row of shot-fired pins into a metal deck.

What belongs on the enquiry

Five lines cover most of the risk: the module, the tile thickness and coating mass, the paint system and colour, the grid type with hanger spacing, and the environment. Add the perforation pattern if acoustics are specified, and say whether a fire rating or seismic restraint is needed, because both change the grid rather than the tile.

The sheet is the easy part to quote. The galvanized plate and DX51D+Z coil behind it are commodity items at 0.12 to 1.2 mm with 20 to 120 g of zinc, and a SECCN5 sheet will do the same job in a dry room. What separates two ceiling packages is whether the grid and the edge trim were priced with the tile.