The panel is bought for what passes through it

A perforated sheet is ordered on a pattern drawing, and the drawing usually shows a hole diameter and a pitch. What it does not show is how much steel is left, and that is the number the panel will fail on. A perforated corrugated panel is sold for facades, sunshades, fencing and screening, and each is a structural application with holes cut through it.

Open area is geometry, not a percentage

For a 60° staggered pattern the open area is 0.9069 times the square of the diameter-to-pitch ratio; for a straight pattern it is 0.7854. An 8 mm hole on a 12 mm pitch therefore gives 40.3 per cent staggered and 34.9 per cent straight, from the same steel and the same tooling budget.

Run it the other way and the pattern becomes a decision: 30 per cent open area needs a ratio of 0.575 — 6.3 mm on 11 mm pitch, or 8 mm on 14 mm; 20 per cent needs 0.47, a 5.6 mm hole on 12 mm. Above about 45 per cent the ligaments fall below the sheet thickness, and a punched ligament thinner than the material will stretch and tear at the edge.

Air, light and the shadow

Ventilation is the easiest to size. At 2 m/s a square metre of solid panel passes 2 m³/s; at 30 per cent open area it passes 0.6 m³/s and the air accelerates to 6.7 m/s through the holes. That acceleration is where the noise comes from, and why a screened plant room needs more panel than the fan duty suggests.

Light behaves differently. A 25 per cent panel does not give a quarter of the daylight: the holes act as short tubes and what comes through is scattered, so from three metres it reads as a translucent grey surface. Useful for a car park screen, useless for a shopfront, and set by hole diameter against sheet thickness, not by open area.

For a sunshade the pitch sets the shadow band: 40 per cent open at 15 mm depth throws stripes at low sun angles and a near-solid shadow at noon.

What the holes take out of the section

A 40 per cent panel keeps 60 per cent of its steel across any line through the pattern. Load it as though it were solid and the stress in the ligaments rises by two-thirds — in tension, in shear at the fixings and in bearing around every screw. Thirty per cent open area is a 43 per cent increase. The arithmetic is trivial, and is skipped when the panel is specified as a finish.

The edges are worse than the field. Tear-out at a fastener depends on the distance from the hole centre to the free edge, conventionally twice the hole diameter, and a perforated panel often has its fixing in a ligament the pattern has interrupted. Fix through the perforations rather than plain zones and the wind load is carried by a section the pattern has half removed.

The corrugation does more work than the steel

A flat 0.5 mm strip has a second moment of 10.4 mm⁴ per metre of width. Form it into a profile 15 mm deep with half the steel in each flange and the figure rises to about 28,000 mm⁴ — roughly 2,700 times the flat sheet, from a roll former and no extra metal. Perforate those flanges at 40 per cent and it falls to about 17,000 mm⁴, still 1,600 times the flat strip.

That is why the profile matters more than the gauge, and why moving from 0.5 mm to 0.8 mm to stop a panel drumming solves the wrong problem: depth beats thickness by the square. The profile dimensions and cover width carry the argument into the order.

Thin sheet, small holes, and what the punch leaves

At 0.5 mm the holes are punched, not drilled. A turret punch runs at a die clearance of 5 to 8 per cent of the thickness on each side — 0.025 to 0.04 mm — and that clearance, not the punch diameter, is what produces the burr. The burr sits on the underside, breaks off in handling and shows through paint as a ring, so the drawing must say which face is up.

Holes closer than about one and a half thicknesses distort the ligament between them, and a dense pattern at full stroke rate will bow a 1,250 mm coil by the end of the run. Punching remains the right process: laser or plasma at 8,000 holes per square metre costs more per hole and leaves a heat-affected edge nobody wants to paint.

The hole edge is bare steel

The substrate is usually DX51D with an AZ150 coating, 150 g per square metre total. At 3.7 g per cubic centimetre for the 55 per cent aluminium-zinc alloy, that is a little over 40 µm of coating, about 20 µm a side. Classic rib panels run from AZ30 to AZ150, and the top of that range is where a perforated external panel belongs.

Punching cuts through the coating and leaves a ring of bare steel at every hole. An 8 mm hole has 25 mm of circumference, and a 40 per cent pattern at 12 mm pitch puts 8,000 holes in a square metre — 200 metres of unprotected edge in one square metre of panel. The coating protects it by throwing power over a few millimetres, enough at a hole edge and not enough at a sheared end where the exposed face is the full sheet thickness. Shearing and bending both leave that wider bare face, and on a coastal facade it is where the rust starts.

Fixing a panel that is 40 per cent air

Screws go through the ribs, not the pans, and each needs a sealing washer with a head three times the hole. Fixing holes are slotted or drilled 2 mm oversize because a 1.8 m Galvalume panel moves about 0.9 mm over a 40 K swing, and a rigid fixing at both ends will elongate the hole or buckle the rib.

A perforated panel passes water: rain 20° off vertical reaches the inner face of a 40 per cent screen, so the cavity behind needs draining and venting, not sealing. The corrugated panel behind it is the weather line.

What goes on the purchase order

Substrate grade, coating mass and thickness; profile name and cover width; hole diameter, pitch and pattern type; open area as a percentage with the formula agreed; burr side; colour, and whether the sheet is pre-painted before or after perforation. Painting after perforation coats the hole edges and is worth the extra step near the sea.

Two substitutions are worth refusing. A Z275 galvanized substrate has 19 µm of zinc a side and no aluminium barrier, a different corrosion life at the same coating mass. And a flat galvanized plate in place of the profiled panel will meet the drawing and fail the wind load, because the holes are unchanged and the depth is gone. Surface treatment decides whether the panel can be handled at all before it is fixed.