Because that is how the expanding process forms them, and flattening them costs an extra operation for benefits that most applications do not need.

As the die slits and stretches the sheet, the metal between the slits is pulled down and forward, which rotates each strand out of the original plane. Every strand ends up inclined, and the panel has a distinct top and bottom face with a raised, textured character.

Leaving them that way is not laziness on the manufacturer's part - the angled strand is doing real work. It provides the grip and it provides the stiffness, both of which are lost to some degree when the panel is flattened.

So standard expanded metal is the base product, and flattened material is standard material that has had a further process applied to it. That is also why standard costs less: fewer operations, and no second pass through a mill.

Enough that the heavier grades are used as walking surfaces in industrial and external locations, and enough that it is specified where wet or oily conditions rule out a flat panel.

The mechanism is that each angled strand presents an edge standing proud of the surface. A sole coming into contact with the panel meets a series of these edges rather than a smooth plane, and they bite through the film of water, oil or dust that causes slips on a flat surface. Because the diamond pattern repeats in two directions, the edges face several ways at once, so grip is retained when someone changes direction rather than only when walking straight.

That is why standard expanded metal is the form chosen for catwalks, platform infill and access walkways, and flattened material is not normally used underfoot. If the application involves feet, specify standard.

Because the strands stand out of the plane, which gives the panel effective depth.

The resistance of any panel to bending depends heavily on how far its material sits from the neutral axis. In a flat sheet everything is concentrated in one thin plane, so there is very little depth to work with. In standard expanded metal the strands are inclined, so material sits above and below the mid-plane, and the panel behaves as if it were considerably thicker than the gauge it was made from.

This is the same principle that makes a corrugated sheet stiffer than a flat one, achieved here as a by-product of the expanding process rather than by a separate forming operation.

The practical benefit is that a light gauge covers a large panel without sagging, which reduces both the material cost and the amount of supporting framework needed behind it. Flattening the panel removes much of this advantage.

Wherever grip, stiffness or cost per square metre matters more than a flat face.

The heavier grades go into walkway and catwalk flooring, platform and mezzanine infill, and stair landings, where the raised diamond provides the anti-slip surface. Lighter grades are used extensively for machine and conveyor guarding, fencing and security screens, ventilation panels in enclosures, shelving, and infill panels in balustrades and gates.

It is also the standard product for plaster and render reinforcement lath, where the raised strands key into the render and hold it mechanically - a job the flattened form does noticeably worse.

It is the wrong choice where the panel must sit flush against a backing, where a smooth surface is needed for cleaning or for handling product, and where a refined flat appearance is part of the design intent.

It does, and on site this is one of the easiest details to get wrong.

The diamond has a short axis and a long axis, described as the short way of the diamond and the long way of the diamond. The panel is significantly stiffer across the short way, because the strand geometry presents more resistance to bending in that direction.

The rule is that the short way of the diamond should run across the span - that is, perpendicular to the supports the panel bridges between. Fitted that way, the panel performs as the load tables predict. Rotated ninety degrees to make it fit an opening, the same panel is substantially less stiff, and a panel that was adequate becomes springy or unsafe.

Mark the orientation on panels before they go to site if they are being cut, and check it on delivery. It is not visible at a glance to someone unfamiliar with the product.

Yes, and it handles site cutting better than most open flooring products because the panel is one continuous piece.

Cut with an abrasive chop saw, a bandsaw or plasma. Because the junctions between strands are uncut parent metal throughout the panel, a cut does not release anything - the remaining panel keeps its integrity and the offcut is usable. There is no structural banding requirement as there is with welded bar grating.

Two practices improve the result. Cut along a line of strand junctions rather than through the middle of strands, which leaves a stronger, tidier edge with fewer sharp points. And preserve the diamond orientation relative to the span when planning how panels are cut from stock.

Cut edges leave sharp strand ends and, on galvanised material, bare steel. Trim or edge any cut within reach, and treat cut edges on panels going into wet or external service.

Self-colour carbon steel, pre-galvanised sheet, hot dip galvanised after expanding, and stainless steel.

For external and wet applications, specify hot dip galvanising after expanding. The expanding process cuts through the sheet along every strand, so a pre-galvanised panel has bare steel along an enormous total edge length - far more than any solid sheet product - and it will corrode along every strand rather than at a few edges.

Dipping the panel after expanding coats the strands and their cut edges together, which is why it is the standard external specification despite the extra handling. Stainless is used in food and pharmaceutical areas, coastal and chemical environments, and where the panel is visible and must not stain.

Self-colour and pre-galvanised material are appropriate for internal dry applications and for panels that will be painted or powder coated as part of a wider assembly.