Square and rectangular tube is the backbone of most welded structures, from gates and shelving to canopies and machine frames. The sizing system is simple enough; choosing the right section for a given load is where people either pay for metal they do not need or end up with a structure that flexes.
Reading the designation
A marking such as 40×20×2 means 40 mm across the larger face, 20 mm across the smaller, and a 2 mm wall. On square tube the first two figures match: 40×40×2. Length is not part of the marking — stock lengths are usually 6 m, sometimes 3 or 12 m.
Note that the size is the outside dimension. A 40×40×2 tube has a 36×36 mm bore, which matters when one tube has to telescope inside another: 30×30×2 slides into 40×40×2 with clearance.
Commonly stocked sizes
- 20×20, 20×10, 25×25, 1.5–2 mm wall — light frames, shelf rails, decorative frames, internal ribs.
- 40×20, 30×30, 1.5–2 mm — door and wicket frames, furniture frames, shelving rails.
- 40×40, 50×25, 60×30, 2 mm — the all-rounders: shelving uprights, gate frames, canopy frames.
- 60×40, 60×60, 80×40, 2–3 mm — loaded frames, gate leaves, canopy posts.
- 100×100, 120×80, 3–4 mm — gate posts, columns, load-bearing members of steel structures.
This is not the full range but what most suppliers actually hold. Unusual sections exist, though they mean waiting and buying full lengths.
Wall thickness matters more than it looks
40×40×1.5 and 40×40×2 look identical, but the second is nearly a third heavier and noticeably stiffer. The price difference is small; the difference in behaviour is not. Thin-wall tube dents under a clamp, holds a weld less well and distorts more from heat.
Thin walls also weld badly — 1.5 mm is easy to blow through if you are not practised. If you are welding it yourself with limited experience, 2 mm is a sensible minimum.
How the section governs stiffness
The rule for beams: stiffness depends on the height of the section far more than on its width. A 60×30 tube stood on edge (60 mm deep) deflects several times less than the same tube laid flat. Rectangular tube therefore has a right way up — and that is what most often gets confused.
The second rule: doubling the span increases deflection eightfold. A 2 m shelf does not sag twice as much as a 1 m one but far more — and the fix is an intermediate support or a deeper section, not a thicker wall.
What to use where
- Wicket gate — 40×20×2 frame, 20×20 ribs;
- Gate leaf up to 4 m — 60×40×2 frame, 40×20 ribs; wider needs 60×60;
- Gate posts — 100×100×3–4, concreted below the frost line;
- Shelving — 40×40×2 uprights, 40×20×2 rails;
- Carport — 80×80×3 posts, 60×40×2 trusses, 40×20×2 purlins;
- Furniture frames — 20×20 or 30×30, 1.5–2 mm wall.
These are starting points for ordinary domestic structures, not a substitute for calculation. Long-span canopies, mezzanines and anything people walk under need an engineer to size the section: there the cost of an error is collapse, not deflection.
Cutting and welding
Tube can be cut with an angle grinder, a bandsaw or a laser. The difference is not the cut itself but its accuracy: a grinder leaves a few millimetres of error and an out-of-square end, which later shows up as a skewed frame. For structures with many identical members — shelving, trusses, grilles — it pays to order tube cut to length: every part comes out the same and 45° joints meet without fitting.
Tube is welded with MIG; on thin walls use short runs with pauses so it does not distort. A continuous weld all round the joint is only needed at loaded connections — two or three sides is enough on most domestic work.
Corrosion protection
Tube is sold black or galvanised. Galvanised costs more and needs no paint, but the zinc burns away at every weld and rust starts there, so the seams need treating separately. For anything that will be painted anyway it is simpler to buy black tube and apply powder coating after welding — it covers the welds along with everything else.
If the structure is complex or the part count is high, we can build it start to finish — from cutting the tube and welding through to coating. See steel structures or send a sketch through the form.
