Aluminium Extrusion Profiles NZ: Which Shape for Which Job

Most people arrive at an extrusion supplier knowing the length they need and not much else. The shape question is the one that decides whether the finished thing is stiff enough, easy to join, and worth the material. There are only a handful of profiles in common use, and each one is good at something specific.

Start with how it will be loaded

Before shape, work out what the piece is actually doing. Three cases cover most jobs. Spanning. The piece bridges a gap and something sits on it, so it wants to resist bending. Depth in the direction of the load is what buys stiffness, far more than adding wall thickness. Holding position. The piece is a frame member, a bracket or an edge, and the load is modest. Here workability and how it joins matter more than raw strength. Taking impact or wear. Edges, thresholds, guards. Section shape matters less than having enough material where it gets hit. Getting this straight first eliminates most of the catalogue in one step.

• Spanning: prioritise depth, look at hollow sections
• Positioning: prioritise ease of fixing, look at angle and flat bar
• Wear: prioritise material where the contact happens

The profiles, and what each is for

Equal angle. Two legs at ninety degrees. The workhorse for frames, edges, brackets and corners, because the two faces give you something to fix through in both planes. Reach for equal angle when you need a corner or an edge that also needs fixing points. Unequal angle. Same idea with one leg longer, for when one face needs more fixing room or more cover than the other. Flat bar. Simple rectangular strip. Straps, packers, backing plates, edge trims and cover pieces. Not stiff on its own across a span, but easy to drill, cut and bend, which is often the point. See flat bar. Rectangular hollow section. Box. The one to use when something has to span, because a hollow box puts the material where bending resists it. Posts, frames, rails and structures generally. Rectangular hollow section covers a wide range of sizes. Square hollow section. Same logic, equal in both directions, which suits posts and anything loaded from more than one side. Round tube. Handrails, rollers, guards, and anywhere the piece is handled or has to look resolved from all angles. Joins differently to box, since it needs either a socket, a swage or a purpose-made fitting rather than a simple flat plate. Solid round and solid square. Pins, shafts, spacers, machined parts. Heavy for their size, so used where the section is small and the duty is concentrated. Moulding and trim profiles. Finishing pieces rather than structural ones, for edges, transitions and covering joints. The full standard extrusions range shows the sizes available in each.

How it joins is usually the deciding factor

This is where a profile choice quietly succeeds or fails. Angle and flat bar give you flat faces to bolt or rivet through, so they suit anything you want to assemble with ordinary fixings. Box section gives you flat faces too, but access to the inside is limited, so blind fixings, self-drilling screws or welding come into it. Round tube needs a fitting, a socket or a welded joint, which is fine when planned and awkward when discovered late. If the assembly needs to come apart again, favour profiles that bolt. If it is permanent and you have welding available, the shape options widen.

• Bolted assembly: angle and flat bar are the easy path
• Box section: plan for blind fixings or welding
• Round tube: plan the joint before you choose the size

Bare or powder coated

Two things to weigh, and neither is only about appearance. Bare aluminium forms its own oxide layer and holds up well in most inland conditions. It is the straightforward choice where the piece is hidden, industrial, or will be machined further. Powder coated extrusion adds a durable finish that suits visible work and gives you colour matching. It is also the usual choice where the piece sits in a coastal or marine environment, since the coating adds a barrier over the metal. Worth planning around: cutting and drilling after coating exposes bare edges. Where it matters, cut first and coat after, or accept that cut ends will need attention.

Aluminium or steel for this piece

Sometimes the honest answer is steel, and it is better to know early. Aluminium is roughly a third the weight of steel, does not rust, and is easy to cut and machine on site. That combination is why it wins for anything handled, lifted, or exposed without painting. Steel is stiffer for the same section and cheaper per kilo, so for a heavily loaded span where weight and corrosion are not concerns, it can be the better engineering answer. We cover the comparison in detail in our box section guide. Mixing the two brings its own consideration, since dissimilar metals in contact in a wet environment need isolating.

Where people usually get it wrong

Three patterns come up often enough to be worth naming. Choosing thickness instead of depth. When something sags, the instinct is to order a thicker wall. On a span, depth does far more than thickness for the same weight of metal, so a deeper section is usually the cheaper fix as well as the better one. Designing the shape before the joint. A profile that suits the load but cannot be fixed the way the assembly needs turns into extra brackets, extra fabrication, or a redesign. Work out how it connects at the same time as what it is. Ordering to a drawing rather than to stock. A section that is close to a stocked size is much easier to get and often just as suitable. Checking what is stocked before finalising sizes tends to save both time and money.

A quick way to narrow it down

Run these in order and the shortlist gets short fast:

• What is the load, and which direction does it act
• Does it span, or does it just hold position
• How will it be joined, and does it need to come apart later
• Will it be seen, and does it need colour
• What environment: inland, coastal, wet, or industrial
• What lengths and quantities, since that affects what is practical to order

Lengths, cutting and what is practical to order

Two practicalities that shape the order more than people expect. Stock lengths. Extrusion is made and stocked in set lengths. Designing a job around pieces that divide neatly into a stock length wastes far less material than designing to a number and working out the offcuts afterwards. On a large job that difference is real money. Cutting. Aluminium cuts cleanly with the right blade and is straightforward to work on site, which is one of its practical advantages over steel. Where the cut face will be visible or exposed, plan whether it gets deburred, capped or coated, because a raw cut end reads differently to the finished face beside it. If a job needs many identical pieces, it is worth asking what can be cut to length before delivery rather than handling full lengths on site.

Frequently asked questions

Getting the right profile

If you can describe what the piece has to do, what it attaches to, and where it lives, the profile choice is usually quick. Talk to our team with those details and we can point you to the section and the sizes that suit.