Perforating removes metal; expanding does not. That is the whole distinction, and everything else follows from it.

A perforating press punches holes through a flat sheet and the punched slugs become scrap. The sheet stays flat, stays at its original thickness, and ends up with precisely repeatable holes of whatever shape the tooling defines. An expanding press slits the sheet in a staggered pattern and simultaneously stretches it open, so the metal that would have been a hole becomes the strand around the hole. Nothing is discarded and the sheet grows substantially in area.

So perforated sheet gives you precision, flatness and free choice of hole shape at a higher material cost. Expanded metal gives you far more covered area per tonne and greater stiffness for its weight, with a raised diamond surface and geometry limited by the tooling. Neither is generally better; they answer different questions.

Expanded metal, usually by a wide margin, and the reason is the scrap rather than the processing.

When a sheet is perforated to fifty per cent open area, half the steel you bought has been punched out and sold as scrap. You paid for a full sheet and received a panel of the same size with half the metal in it. When the same sheet is expanded, none of the metal is lost and the sheet opens out to cover considerably more area than it started with. The cost per square metre of finished panel therefore separates sharply, and the gap widens as open area increases.

That said, cost per square metre is not always the right measure. If the application needs a flat face, a precise aperture or a particular appearance, expanded metal does not do the job at any price. Compare on cost only when both products would actually satisfy the requirement.

For a given weight of steel, expanded metal is generally stiffer; for a given thickness of sheet, perforated is generally stronger.

Expanding leaves the junctions between strands uncut and works the metal as it stretches, so the panel behaves as a continuous network with the strands acting as small members. The raised diamond profile also adds effective depth, which is why a thin expanded panel resists bending much better than a flat sheet of the same mass. Perforated sheet, by contrast, has had material removed from a flat plate, so its stiffness is that of the remaining plate - reduced roughly in proportion to how much was punched out, and reduced further around the holes where stress concentrates.

The practical reading: if the panel must carry load or span between supports, look at expanded metal or at a thicker perforated sheet. If the panel is supported all round and is doing a screening or appearance job, the difference rarely governs.

Expanded metal, in its grating grades, is the normal choice, and the standard perforated sheet range is not intended for walking on.

The raised diamond of standard expanded metal gives multi-directional grip, and the heavier expanded grating grades are made specifically as a walking surface with published load tables. Perforated sheet is flat and, when wet, offers very little grip; it is also weaker in bending for its thickness, so it deflects more between supports.

There are exceptions. Perforated plate with a raised or dimpled anti-slip profile is made for stair treads and walkways, and that is a distinct product from flat perforated sheet. Where a walking surface is the requirement, specify from the grating and stair tread ranges rather than adapting a flat perforated sheet, and check the load table for the actual clear span.

Yes. Carbon steel, hot dip galvanised and pre-galvanised steel, and stainless steel are all standard for both products, and aluminium is widely available too.

One consideration matters more here than on solid sheet: the amount of cut edge. Perforating a sheet creates a cut edge around every single hole, and expanding creates a cut edge along every strand. That is an enormous exposed edge length compared with a plain sheet, and on pre-galvanised material every one of those edges is bare steel. In any wet or external location, specify hot dip galvanising after perforating or expanding, so the coating covers the edges the process created.

Stainless avoids the issue entirely, which is part of why it is specified for architectural facades, food and pharmaceutical areas and coastal installations despite the material cost.

Open area is the proportion of the panel's face that is hole rather than metal, expressed as a percentage. It is the single most useful number for comparing panels, because it governs almost everything the panel is bought for.

Airflow, light transmission, drainage rate, acoustic absorption and visibility all rise with open area. Strength, stiffness, rigidity, privacy and the panel's ability to stop things passing through all fall with it. Cost per square metre of perforated sheet rises with open area, because more metal has been punched away.

So open area is the trade-off dial, and it should be set from the function rather than chosen by eye. A ventilation grille needs it high; a security screen needs it low; an architectural facade is usually chosen on appearance and then checked to make sure the resulting strength is still adequate for the panel size.

Both products are supplied in standard sheet sizes drawn from the mill sizes of the parent material, in a range of gauges from thin sheet through to plate.

For perforated sheet, the thickness is the thickness of the sheet you started with, since punching does not change it. There is a relationship between thickness and the smallest hole that can be punched - as a rule of thumb, a hole much smaller than the material thickness is difficult, so very fine perforations are restricted to thin gauges.

For expanded metal, the parent sheet thickness becomes the strand thickness in standard expanded material, but a flattened panel's overall thickness is different again because the rolling operation changes it. The finished panel size is also larger than the sheet it came from, sometimes considerably.

Quote the finished panel size you need rather than the parent sheet size, and let the supplier work back.

Both cut readily with a shear, bandsaw, abrasive saw or plasma, and both are routinely supplied cut to size, which is usually better value than cutting on site.

Perforated sheet can be folded, rolled and pressed, but the holes affect how it behaves: it prefers to bend along a line of holes, springback differs from solid sheet, and a fold across a row of holes can distort them. Where a bend line is critical, specify a solid margin along that edge so the fold happens in unperforated metal.

Expanded metal cuts cleanly and holds together because it is one continuous piece, but the orientation of the diamond changes its stiffness, so a panel rotated to fit is not the same panel structurally. Cut edges on both products leave sharp points and, on coated material, bare steel - trim or treat them wherever they will be handled or exposed.

A margin is a band of unperforated metal left around the edge of a perforated sheet, or along particular edges of it. It is specified rather than assumed, and it exists to give the panel something solid to work with.

Margins matter in three situations. Where the panel is fixed - a solid margin gives clean material for fixings, which would otherwise have to land on or near a hole. Where the panel is folded - bending through a row of holes distorts them and weakens the fold, so a margin along the bend line keeps the fold in solid metal. And where appearance governs - a consistent margin frames the pattern and prevents partial holes at the sheet edge, which look like a defect.

Unspecified, a supplier will produce whatever margin falls out of the pattern and the sheet size, which frequently means part-holes at the edges. If the edge will be seen or fixed through, state the margin.

Perforated sheet, in most cases, because the specification is being driven by appearance and perforating gives control that expanding does not.

A perforated panel can be made in any hole shape and any arrangement the tooling allows, including graduated patterns where hole size varies across the sheet to create an image or a light gradient. The face is flat, which reads as deliberate on a building, and the margin can be controlled so panels align. Hole size and open area can be tuned to balance daylight, solar shading, views out and privacy in.

Expanded metal is used architecturally too, and its raised diamond gives a strong directional texture that changes with viewing angle - an effect that is sometimes exactly what is wanted. But the geometry is limited to what the expanding tooling produces.

For external facades in either product, specify stainless or hot dip galvanise after fabrication, because the cut edge length is very large.