Independent comparisons. We may earn from shop links. More info

Aluminium box section vs steel for a light UK frame

Updated: October 2026

You are choosing between aluminium and mild steel for a light gate or a garden screen. Aluminium box section vs steel box section comes down to weight, red rust, and how you will join the metal. The five bars on the [aluminium rectangular tube comparison](/hub/rods-and-pipes/aluminium-rectangular-tube) are aluminium only. Listings and public UK sources were set out on 16 October 2026, with no bend test and no working load. A qualifying purchase through our links can earn a commission at no extra cost to you. Choose aluminium when the frame is light and short. Choose coated steel when the same outside size must be stiffer. Do not leave bare steel bolted to aluminium in the wet.

You are choosing between aluminium and mild steel for a light gate or a garden screen. Aluminium box section vs steel box section comes down to weight, red rust, and how you will join the metal.

Our top picks

Fabal Aluminium Fabal 30 × 15 × 1.3 mm, 800 mm

Best overall

Fabal Aluminium Fabal 30 × 15 × 1.3 mm, 800 mm
6.6/10about £12.53
View offer
Fabal Aluminium Fabal 30 × 15 × 1.3 mm, 1 m

Rank 2

Fabal Aluminium Fabal 30 × 15 × 1.3 mm, 1 m
7.8/10about £13.15
View offer
Aluminium Online Aluminium Online 30 × 20 × 2 mm

Best value

Aluminium Online Aluminium Online 30 × 20 × 2 mm
5.2/10about £11.00
View offer
On this page
  1. 1.Same outside size is not the same frame
  2. 2.Rust, oxide, and a coating
  3. 3.Bolts, welds, and the wet joint
  4. 4.Which to buy for a light UK job

Same outside size is not the same frame

A 40 × 20 mm rectangle in aluminium and the same size in steel are not the same beam. Steel is stiffer and heavier. Waldenhaus states about 2.70 g/cm³ for aluminium and about 7.85 g/cm³ for steel, and a stiffness summary of about three to one. Treat that as their summary, not as a test of these amazon bars. ClickMetal’s guide points steel hollow sections at BS EN 10210-2 and BS EN 10219-2, and says 1.5–2 mm is usual for light frames while 3 mm and above is for heavier work. The only 3 mm title, 60 × 40 × 3 mm at £21.59, also stores item form Sheet. The Fabal bars are 1.3 mm 6063-T6, a light rail, not a loft beam. If people will stand on the span, Approved Document A is the question.

Rust, oxide, and a coating

Aluminium in a UK garden does not form the red rust of bare mild steel. It forms an oxide, and mill finish dulls as that film grows. Chlorides, a wet steel bracket, and copper against the bar can pit the film. Bare mild steel rusts outdoors. ClickMetal’s line is primer, paint, or galvanising before the frame sees rain. A cut end or a drilled hole can still open a galvanised coat. White powder, end caps, and a bore that holds rain are on the aluminium box section for outdoor frames page once you have chosen aluminium.
Fabal Aluminium Fabal 30 × 15 × 1.3 mm, 800 mm

Testsieger

Fabal Aluminium Fabal 30 × 15 × 1.3 mm, 800 mm
6.6/10about £12.53
View offer

Bolts, welds, and the wet joint

A steel bracket on wet aluminium is a galvanic couple. JTH, a UK horsebox builder, describes bare steel bolts that attacked aluminium at the mounts. The British Stainless Steel Association notes that a small stainless fastener in a large aluminium part is often acceptable, and that a marine site pitted where insulation was missing. A wet steel bolt still marks the metal. Isolate the faces with a plastic washer or a pad, or stay with one metal. The Health and Safety Executive treats welding fume as a hazard to control either way. If you do not weld aluminium, do not buy a Fabal bar because a bullet says it can be welded.

Which to buy for a light UK job

Buy aluminium from this set when the piece is under 1 m, the load is a screen or a shelf you can lift, and mill finish that dulls is acceptable. The size to write down is on the aluminium rectangular tube sizes page. The 1.3 mm Fabal wall is the thin end of that choice. The £11.00 bar gives you 2 mm and no stated alloy. Buy coated or galvanised steel when the same outside size must be stiffer, or when you already weld mild steel. There is no steel listing here. Buy neither when the bar will carry a person, a roof, a stair, or a floor. If you stay with aluminium, the conditional pick is on the best aluminium rectangular tube page.

Common questions

Is aluminium box section as strong as steel of the same size?
No. At the same outside size, steel is stiffer and heavier. Waldenhaus states about 2.70 g/cm³ for aluminium and about 7.85 g/cm³ for steel, and a stiffness summary of about three to one. That is their article, not a bend test of these 1 m bars. A 1.3 mm aluminium wall is a light rail, not a substitute for a steel box chosen because the span has to stay straight.
Will aluminium rust in a UK garden?
It will not form red rust the way bare steel does. Mill finish dulls as oxide forms, and it can pit where salt, a wet steel fixing, or copper stays against the metal. A review line that the bar should not rust is a comparison with steel, not a promise of a bright face. Caps and isolation are a separate job from choosing the metal.
Will bare steel rust?
Yes, if you leave mild steel outside with no coating. Primer and paint, or galvanising, slow that. A cut end and a drilled hole can open a galvanised coat, so those spots need the same attention as the faces. Galvanised steel is still the stiffer metal. It is not aluminium in another colour.
Can I bolt a steel bracket to an aluminium tube?
You can, and in the wet you should isolate the joint. Bare steel against aluminium is a galvanic couple. JTH reports attack at steel bolts on aluminium mounts, and BSSA notes pitting against stainless steel where insulation was missing on a marine site. A plastic washer or a pad breaks the wet contact. If you will not isolate the faces, stay with one metal.
Which is easier to weld at home?
Mild steel, if you already weld it. Aluminium is a different process, and HSE treats the fume as a hazard to control either way. A bullet that says an aluminium bar can be welded is not a lesson. If you do not weld aluminium with extraction, bolt or rivet the light frame, or buy steel and coat it.
When is neither the right buy?
When the span carries a person, a roof, a stair, or a floor, or when you need a certificate and a stated load. Approved Document A is not settled by picking the lighter metal. A 1 m amazon cut and a guess at a steel price are both the wrong shop for that job.

Related pages

How to cut aluminium box section

Aluminium rectangular tube

Best aluminium rectangular tube for a short light frame

Aluminium rectangular tube sizes, wall and cut length

Aluminium box section for outdoor frames in the UK

Sources

Public UK sources and amazon.co.uk listings from the category digest, checked on 16 October 2026. Shelf prices and star lines can change after that date.

  1. Consumer Rights Act 2015 (retrieved 16 October 2026)
  2. Consumer Rights Act 2015, section 9 (satisfactory quality) (retrieved 16 October 2026)
  3. HSE: welding (retrieved 16 October 2026)
  4. HSE: COSHH basics (retrieved 16 October 2026)
  5. Approved Document A: structure (retrieved 16 October 2026)
  6. Using the UKCA marking (retrieved 16 October 2026)
  7. BS EN 755-2: mechanical properties of extruded aluminium (retrieved 16 October 2026)