Perforated Metal Dry Façade

A screen, not a rainscreen

Before anything else, one distinction that changes how this system is used.

Ceramic, stone, composite and HPL are rainscreens. They shed most of the water and the drained cavity behind them handles the rest.

Perforated metal is a screen. Rain goes through it. It is not a weather barrier and it is not asked to be one. It sits in front of something that is already weather-tight — a curtain wall, a rainscreen, an insulated wall — or in front of nothing at all, as a car park skin or a plant enclosure.

• Rainscreen Ceramic, stone, composite and HPL shed most of the water; the drained cavity behind handles the rest.

• Screen Rain goes straight through. The wall behind must be complete on its own — and its rails and brackets are seen through the holes.

Sections, outside on the left. Simplified.

Which means two things:

• Whatever is behind it must be complete on its own. A perforated screen never makes a wall watertight and it never substitutes for a weather line.
• Everything behind it gets wet and gets seen. Fixings, rails, brackets, the wall behind, the drainage, the daylight coming through — all of it is visible through the holes, especially at oblique angles. The substructure of a perforated façade is part of the design in a way that a solid rainscreen’s never is.

The cavity fundamentals this family shares — depth, drainage, cavity barriers, brackets — are covered in full on Ceramic Dry Façade. What follows is what is specific to a perforated screen.

Open area: how it is actually calculated

Open area is the proportion of the panel that is hole, as a percentage. The specification gives a range of 10 – 50%, and everything else about the screen is downstream of where in that range you land.

Open area

10 – 50%

Aluminium sheet

1.5 – 3.0 mm

Folded return

20 – 30 mm

System weight

4 – 9 kg/m²

For a given hole size d (or side a) and centre-to-centre pitch p, it is geometry:

• Round, 60° staggered The most common, and the densest packing of circles — which is why it reaches 50% open comfortably.

• Round, straight pitch Circles in a square array. At the same open area the bar of metal between holes is thinner.

• Square holes, straight pitch The simplest arithmetic: the hole’s share of one square cell.

One repeating cell of each pattern, each drawn at 30% open.

The two constants are not arbitrary. 90.69 is π⁄(2√3) as a percentage — the densest packing of circles in a triangular array. 78.54 is π⁄4 — circles in a square array. Staggering the rows is what makes 50% open practical with round holes; a straight pitch has to leave a much thinner bar of metal between the holes to get there.

Worked: what pitch gives you what, for 5 mm round holes, 60° staggered

Read the right-hand column, because it is scale-independent: the ratio of pitch to hole is what sets the open area, not the absolute sizes. A 5 mm hole at 8.7 mm pitch and a 20 mm hole at 34.8 mm pitch are both 30% open. They will look completely different, and they will shade identically.

That is the useful separation. Open area is the performance. Hole size is the appearance. Decide the performance first, then choose the hole size that gives the elevation the grain you want, then set the pitch from the ratio.

Estimating the weight

The perforated sheet weighs about thickness × density × (1 − open area) — 2,700 kg/m³ for aluminium, about 7,850 kg/m³ for steel. A 2 mm aluminium sheet at 30% open is about 3.8 kg/m² before its folded returns and any stiffeners.

The published 4 – 9 kg/m², depending on open area, is the figure for the system. At high open area the folded returns and stiffeners become a meaningful share of the total rather than a rounding error.

What open area decides

Solar shading — and why the figure moves with the sun

At normal incidence a 30% open screen passes roughly 30% of the direct beam. But the sun is rarely at normal incidence, and as the angle becomes oblique you stop seeing through the holes and start seeing the thickness of the sheet through them. The effective open area falls, and at grazing angles a perforated screen approaches opaque.

• Square-on Light at normal incidence passes roughly the open area — 30% open passes about 30% of the direct beam.

• Steep sun, thin sheet At an angle the walls of each hole start to block it, and the effective open area falls.

• Steep sun, thick sheet A deeper barrel cuts off sooner. At the same open area, a 3.0 mm sheet does not shade like a 1.5 mm one.

Sections through a perforated sheet, sheet thickness exaggerated.

That has two useful consequences.

• It shades best when the sun is steep or raking. High summer sun on a south elevation, and sun travelling along the face of the building, arrive at a steep angle to the sheet and the walls of the holes cut them off. Low sun square-on to the screen — morning on an east elevation, evening on a west — passes close to the full open area, and that is the case to check.
• Sheet thickness is a shading parameter, not only a structural one. A thicker sheet gives each hole a deeper barrel and a sharper angular cut-off. Two screens at the same open area, one 1.5 mm and one 3.0 mm, do not shade the same.

Where the shading has to be predicted rather than estimated, it is a daylight and solar model, run with the geometry. Where precise control at a specified sun angle is the brief, a fixed screen is the wrong instrument and the answer is Aluminium Louvre Façade, where blade angle does the work.

Wind load — reduced, but not by the open area

A perforated screen carries less net wind pressure than a solid panel of the same size. It does not carry (1 − open area) of it. Air passing through the holes changes the pressure distribution in a way that is not a simple scaling, and it depends on the open area, the pattern, the sheet thickness and — decisively — on whether the screen stands free or sits in front of a solid wall with a cavity behind it.

A free-standing screen is loaded on both faces. A screen in front of a wall traps a cavity that pressurises, and the net load on the sheet can be higher than intuition suggests.

• Free-standing Air passes through and pressure acts on both faces. The net load is reduced — but not to (1 − open area) of a solid panel’s.

• In front of a wall The trapped cavity takes up pressure, and the net load on the sheet can be higher than intuition suggests. Use the code’s provisions for porous elements.

Sections, wind from the left. Schematic — the loads come from the code, not from the drawing.

The specification is explicit that design wind load is project-specific, to EN 1991-1-4 or ASCE 7. Both codes have provisions for porous and partially porous elements. Use them. Assuming a perforated screen is lightly loaded because it has holes in it is how panels come off buildings.

Privacy and view — asymmetric, and it flips

A perforated screen is not equally transparent in both directions. You see through it much better from the darker side towards the brighter side. In daylight that means occupants see out well and passers-by see in poorly — which is the usual brief.

At night, with the interior lit, it reverses completely. Inside becomes the bright side. The screen that gave privacy all day gives none after dark. If privacy is a requirement rather than a preference, it has to be stated for both conditions, and the answer is usually internal blinds rather than a different open area.

Acoustics — perforation alone does nothing

This is worth saying plainly because it is widely assumed otherwise. A perforated sheet on its own is not an acoustic element; sound passes through the holes.

Perforated metal becomes an absorber only in combination with something behind it — an absorptive backing and an air gap, so that the assembly works as a resonant absorber tuned by hole size, open area and cavity depth. That is a different specification with a different build-up. If acoustic performance is in the brief, say so at design stage, because the screen alone will not deliver it.

Air flow

The straightforward one, and why the “Where it works” list includes plant and equipment screening. The screen hides a chiller yard, a generator enclosure or a car park while letting it breathe. Required free area comes from the mechanical or ventilation requirement, not from the elevation, and it sets a minimum open area that the appearance then has to live with.

The stiffness you removed

Design note 02: perforation removes material and stiffness; folded returns or stiffeners are usually required at larger formats.

Punching holes in a sheet takes material out of the section, and bending stiffness depends on how material is distributed about the bending axis. The loss is significant and it is not a simple proportion of open area, because it depends on how the pattern sits relative to the axis the panel bends about. A staggered pattern removes material in a different distribution from a straight one at the same open area.

So a perforated panel is designed as a perforated panel, with a reduced effective section, not as a solid panel with a discount.

The published answers:

• Folded return 20 – 30 mm on every edge. It turns a flat sheet into a shallow tray and puts material where the bending stiffness comes from. It is the single most effective thing done to a perforated panel and it is why panels are cassettes rather than flat sheets.
• Stiffeners at larger formats — a hat section or a flat bar bonded or fixed to the back, behind a panel that is otherwise too flexible for its size or its wind load.
• The format range, up to 1,500 × 3,000 mm, is where those two stop being enough without disproportionate effort.

• Flat sheetWith material punched out, a large flat sheet ripples under wind and heat — an appearance failure.

• Folded returnThe fold turns the sheet into a shallow tray and puts material where stiffness comes from.

• Stiffened trayAt larger formats, a hat section or flat bar behind the panel as well.

Oblique sections through a panel, waviness exaggerated.

The failure mode when this is under-done is not usually breakage. It is oil-canning: visible waviness across the panel face under load or under temperature, which turns a flat elevation into a rippled one. It is an appearance failure, which on a screen whose whole purpose is appearance is the failure that matters.

Pattern, margin and setting out

Three practical points that decide whether a perforated façade looks deliberate or approximate.

Every panel has an unperforated margin. The edge needs solid material for the fold and for the fixings. That margin is part of the design, not a fabrication leftover: it produces a solid border around every panel, and on an elevation of many panels those borders form a visible grid over the pattern.

The pattern should align across panel joints — and it only will if the panel width is a whole number of pitches. If it is not, the pattern restarts at every panel and the joint is announced by a broken rhythm across the whole elevation. Set the panel width as a whole multiple of the pitch plus two margins, at scheme stage, and the pattern runs continuously across the building. This costs nothing and it is almost never done.

• A whole number of pitchesPanel width is set as pitches plus two margins, so the gap across the joint is one pitch.

• Any other widthThe pattern restarts at every panel, and each joint is announced by a broken rhythm.

Elevations of two adjacent panels.

Decide whether you want the grid or not. Two legitimate designs: the margins read as a deliberate frame around each panel, or the pattern is continuous and the panel joints disappear into it. Both work. What does not work is discovering which one you have after the panels are made.

Image perforation

Design note 03: graded hole sizes can render a pattern or image; this needs the viewing distance agreed at design stage.

The mechanism is halftone printing. Hole size is varied across the panel while the pitch stays constant: larger holes where the image is dark, smaller where it is light. The eye blends them into a continuous tone — if it is far enough away.

• Up closePitch constant, hole size varied: bigger where the image is dark, smaller where it is light.

• At the agreed distanceThe eye blends the holes into tone. The pitch is set from this distance.

• Too farThe image still reads, but the resolution the pitch could carry is wasted.

The same graded field at three viewing distances, simulated.

Viewing distance sets everything. Too close and the image disappears into a field of differently sized holes. Too far and the resolution is wasted. So the sequence is fixed and it cannot be reordered:

1. Agree the distance the image is meant to read from — the opposite pavement, across a car park, from the motorway.
2. Set the pitch from that distance.
3. Set the image resolution from the pitch, which tells you how much detail the image can carry.
4. Choose or commission an image that survives that resolution.

An image with fine detail, on a façade seen from thirty metres, will not read at any pitch that can actually be punched. High contrast and simple form survive. Photographic subtlety does not.

Two things to settle early. Image perforation means most panels are unique, which changes the fabrication and the cost model completely — see the cost section. And an image-perforated façade must be modelled and approved as a rendered view from the agreed distance, not as a flat elevation, because a flat elevation tells you nothing about whether it reads.

Night appearance: the façade inverts

Design note 04: a perforated screen inverts when backlit; if the façade is lit, design for both conditions.

This is the most interesting behaviour of the system and it is worth the space.

By day, light falls on the front of the sheet. The metal is bright and the holes are dark, because each hole is a small view into a shaded cavity. The façade reads as a surface with dark dots in it, and its overall tone is the tone of the metal.

At night, with light behind the screen, every relationship reverses. The metal is now the dark part, lit only by whatever spills onto its face, and every hole is a point of light. The façade reads as a dark field full of bright points, and its overall tone is the tone of the light behind it.

• By day Light falls on the face. The façade reads as a surface with dark dots, in the tone of the metal.

• By night, lit from behind Every hole is a point of light. The biggest holes are now the brightest, so the image shows its negative.

One graded panel, drawn in both conditions.

Three consequences.

• An image-perforated façade shows its negative at night. Dark areas are where the holes are biggest, so at night they are the brightest areas. The night image is the photographic negative of the day image. This can be spectacular and it can be unrecognisable. Either way it should be a decision, not a discovery.
• Open area reads differently in the two conditions. A screen that looks nearly solid by day can look nearly transparent at night, because a small bright point against a dark field carries far more visual weight than a small dark point against a bright one. Screens are routinely under-specified on open area for their night appearance.
• The light behind the screen becomes the façade. What is lit, what colour it is, and whether it is uniform now determine what the building looks like after dark. That belongs in the lighting design and in the façade design at the same time — and a backlit screen also becomes a source of light spill towards neighbours, which is worth checking before it is built.

Material, finish, and the sequence that gets it wrong

The published options: aluminium 3003 / 5005, galvanised steel, stainless 1.4301, weathering steel. Finishes: powder coat to EN 12206; anodised 20 – 25 µm to EN 12373.

Aluminium is the default. 5005 is specified where the panel is to be anodised, because it anodises to a clean, consistent finish; 3003 is the general-purpose choice where the panel is powder coated.

Galvanised steel is stronger for its thickness and cheaper, with one caveat specific to this system: perforation cuts through the zinc coating at every single hole. The cut edges are protected only by the zinc’s own sacrificial action. In an aggressive or coastal environment that is a shorter life than the same coating on an unperforated sheet, and post-perforation treatment or a different material is the answer.

Stainless 1.4301 is the general-purpose stainless. For coastal and chloride-bearing environments — which includes most of the Gulf — 1.4401 is the usual specification, and that is worth confirming project by project rather than assuming.

Weathering steel develops its protective rust patina, and for the first years it runs. On a perforated sheet the run-off comes from every hole edge, which is a very large number of edges, and it will stain whatever sits below the screen. It is a beautiful material used deliberately and a problem used casually. Design the water path below it, or do not use it above anything that matters.

The sequence: perforate first, then finish

A panel at 30% open area with 5 mm holes contains on the order of fifteen thousand holes per square metre, and therefore around 240 metres of cut edge per square metre of panel.

• Coated, then punched Every hole wall is bare metal. On a panel that is mostly edge, corrosion starts inside the holes and stains the face around them.

• Punched, deburred, then coated The powder coat or anodising runs round the wall of every hole — around 240 metres of edge in each square metre at 30% open with 5 mm holes.

Sections through one hole, coating thickness exaggerated.

Punch, deburr, then coat or anodise, so that the finish covers the wall of every hole. Coating first and punching afterwards leaves every one of those edges bare, on a panel whose whole surface is edge. In a humid, coastal or polluted environment the panel starts corroding from the inside of every hole, and it shows as staining around the perforations long before anything structural happens.

This is also why the anodising thickness matters. The published 20 – 25 µm to EN 12373 is the architectural range; thinner finishes belong indoors, and the more aggressive the environment, the more the upper end of that range earns its cost.

Fixing and substructure

The published options are cassette on rail, hook-on, or bolted.

• Cassette on rail — the panel is a folded tray that hooks or clips to a horizontal rail. Concealed, demountable, and the usual choice on an architectural elevation.
• Hook-on — similar, with the panel lifted onto formed hooks. Fast to install and fast to remove, which suits plant screening where access behind is routine.
• Bolted — direct through-fixing. Visible, cheapest, and honest on industrial and car park applications where the fixings are part of the language.

Two things specific to a perforated screen:

You can see the substructure. Rails, brackets and fixings are visible through the holes, especially at an angle. They should be set out on a regular grid and finished to match, or deliberately contrasted. What they should not be is unconsidered.

Access behind is usually required. A screen exists in front of something — plant, glazing, a wall — and that something needs cleaning, maintaining or replacing. Which panels come off, how, and with what access, is a design decision made at scheme stage or an argument made on site.

Modelling it so it can be built

Architects model this system before they specify it, and the handover from model to fabrication is where projects lose time.

What a fabricator needs is the pattern as a definition, not as geometry. Hole shape, hole dimension, pitch, arrangement (staggered or straight), margin, and the origin the pattern is set out from. Five numbers and a rule. From that, the panel can be nested, the tooling selected and the programme written.

1. d · hole Shape and size — round, square, slot or hexagonal.
2. p · pitch Centre to centre, and whether the rows are staggered at 60° or straight.
3. margin · and origin The solid border for the fold and fixings, and the point the pattern is set out from.

What is not useful is a model containing every hole as a separate object. A Grasshopper or Revit model that bakes forty thousand circles per panel is slow to open, impossible to revise, and has to be reverse-engineered back into the five numbers before anything can be made. The holes are output, not input.

For image perforation the handover is different again: the source image, the agreed viewing distance, the pitch, the mapping from image tone to hole diameter, and the diameter range available. Those belong in the specification alongside the material.

Panel joint lines and the pattern origin should be in the model. They are what decides whether the pattern runs across the building or restarts at every panel — the setting-out point above.

What to decide, and when

1. 01

   Concept

   What the screen is for — shading, privacy, plant concealment, appearance. That sets the minimum open area before anything is drawn. Whether the façade will be lit at night.

2. 02

   Scheme

   Open area. Then hole size for the grain of the elevation, then pitch from the ratio. Panel size as a whole number of pitches plus margins. Whether the margins read as a grid or the pattern is continuous.

3. 03

   Tender

   Material and finish against the environment. Sheet thickness, against both stiffness and angular cut-off. Fixing type, which follows from the maintenance access required behind.

4. 04

   Production

   Stiffener layout. Substructure setting-out, which is visible. Pattern origin. Perforate-then-finish confirmed in the fabrication sequence.

What drives the cost

1 · Open area — and it runs the opposite way to intuition

A panel at 50% open area costs more to make than the same panel at 10%, even though it contains less metal.

Perforation is priced by the number of holes and the machine time to punch them, not by the material removed. High open area at a given hole size means a tighter pitch, which means far more holes per square metre, which means more tool hits, more machine time and more tool wear.

So the cheapest way to raise open area is to make the holes bigger rather than closer. A 10 mm hole at 15 mm pitch and a 5 mm hole at 7.5 mm pitch are both 40% open — and the second has four times as many holes. If the elevation tolerates the coarser grain, that is a substantial saving available for nothing.

• Bigger holes, wider pitch40% open, a coarser grain, and a quarter of the tool hits.

• Smaller holes, tighter pitchThe same 40% open and the same shading — with four times as many holes to punch.

Two patches of the same size, drawn to one scale.

2 · How many patterns there are

One pattern repeated across the elevation is a production run. Several patterns are several runs. An image-perforated façade, where every panel is unique, is a different cost model altogether — not a percentage above a plain screen, and it needs to be understood as such from the first estimate rather than priced as a variation.

3 · Panel format against the sheet

Standard material widths; panels cut from them. A panel size that nests cleanly into the sheet wastes little, one chosen with no reference to it wastes a lot. Same principle as HPL, same saving, same window of opportunity — scheme stage.

4 · Material and finish

Aluminium powder coated is the economical route. Anodised costs more and lasts longer. Stainless costs more again. Weathering steel is cheap to buy and can be expensive in what it stains.

5 · Stiffening

Folded returns are in the base price. Added stiffeners are not, and they arrive when the panel format or the wind load outgrows the fold. Sizing the panel to what the fold can carry is cheaper than sizing it for appearance and stiffening it afterwards.

6 · The substructure, and the access behind it

As with every ventilated system, the substructure is a large share of the cost. On this one it is also visible, so it cannot be value-engineered purely on price. And the access strategy behind the screen — walkways, removable panels, restraint points — is part of the façade scope or it becomes somebody’s problem later.