Guide · Building
What Are Shipping Containers Made Of?
Yuri Kan Updated
Shipping containers are built from Corten-A weathering steel, or SPA-H, the Japanese (JIS) grade of the same kind — a low-alloy steel with copper, chromium and phosphorus, and up to 0.65% nickel, that forms a protective patina instead of flaking rust. In the 3 published 20 ft specifications we read, the side-wall panels are 1.6 mm thick, the roof 2.0 mm, the door panels 2.0 mm, the sills and headers of the end frames 4.0–4.5 mm and the front corner posts 6 mm; the eight corner castings are cast steel to ISO 1161, and the floor is 28 mm plywood on 18 steel cross-members. The 36 mm deep corrugation, not the gauge, gives the walls their strength.
The shell is Corten-A weathering steel — a low-alloy steel with small additions of copper, chromium and phosphorus, and up to 0.65% nickel, by SSAB’s sheet for COR-TEN A; two of the specifications we read name SPA-H, the Japanese grade of the same kind. The alloying matters: where ordinary steel rusts in flaking layers that keep exposing fresh metal, Corten’s surface oxide forms a dense, adherent patina that slows further corrosion. It is the same steel family as weathering-steel sculpture and bridges, chosen for the same reason: decades outdoors on a few coats of paint.
The steel is thin — thinner than most people guess. What turns 1.6 mm panels into a box that stacks loaded at sea is geometry, not gauge: every surface is corrugated, and every edge is framed.
The numbers in this chart
| Part | Dimension |
|---|---|
| Corrugation depth | ~36 mm |
| Side panel | 1.6 mm |
| Side panel beside the posts, many builds | 2.0 mm |
| Roof | 2.0 mm |
| Door header | 4.0 mm |
| Front corner post | 6 mm |
Steel thickness of a shipping container, panel by panel
| Component | Thickness | Construction |
|---|---|---|
| Side walls | 1.6 mm (2.0 mm beside the posts in 2 of 3) | Corrugated Corten sheet |
| Front wall | 2.0 mm | Corrugated sheet |
| Roof | 2.0 mm | Die-stamped corrugated panels |
| Doors | 2.0 mm | Corrugated sheet in welded frames |
| Sills and headers | 4.0–4.5 mm | Welded sections in the end frames |
| Front corner posts | 6 mm | Formed and welded heavy sections |
| Rear corner posts (outer part) | 4.5–6.0 mm | Pressed section around an inner one |
| Cross-members | 4 mm | Channels under the floor, 18 in a 20 ft |
| Corner castings | — | Cast steel, ISO 1161 |
These are the figures CIMC’s specification S000A25G1, the tender of India’s National Centre for Antarctic and Ocean Research (NCAOR) and Shipped.com’s 20DV print. Other makers and series can differ, because ISO 1496-1 sets strength tests, not sheet thickness.
The eight corner castings are the exception to thin-and-corrugated: solid cast steel fittings standardised by ISO 1161. Their oval apertures are what twist-locks, crane spreaders and lashing gear engage. Every lift, stack and lock in the container’s life passes through these eight castings.
Why corrugation, not steel thickness, gives a container its strength
The corrugations — 36 mm deep in the specifications that print the depth — turn flat sheet into a deep-ribbed panel many times stiffer than the same steel laid flat. The walls act as structural shear webs between the corner posts.
This is the fact every conversion runs into: the walls of a container are not cladding, they are structure. Every opening cut for a door or window removes load path that framing must replace. The engineering consequences for homes are covered in container home dimensions.
Underneath, the steel story continues: 18 steel cross-members in a 20 ft, channels of 4 mm steel, span between the two bottom side rails and carry the 28 mm plywood floor.
Where rust starts on a shipping container, and when it matters
The factory finish of a container is a marine coating system: zinc-rich primer under polyurethane or acrylic topcoats, engineered for salt spray. Corten resists corrosion but is not immune, and rust on containers is predictable. It starts where the coating is broken and water sits:
- dents, which also pool water on the roof;
- welds;
- door hinges;
- the bottom rails.
On a sound unit, surface rust at these spots is cosmetic: a wire brush and zinc primer stop it. Rust that flakes in sheets or perforates is structural and prices the box accordingly.
How long container steel lasts at sea and in static use
A container works about 13 years in ocean service: the useful life over which the lessor Triton writes a dry box off in its annual report. Ocean service means salt spray, stacking loads and twisting in a seaway.
Retired to static duty, the same container relaxes: no stack above it, no salt bath. Its Corten shell goes on for many years more with nothing but drainage and paint touch-ups; no document sets a figure for that second life.
The wear items of a container are not the steel at all: door gaskets and the plywood floor age out first.
Frequently asked questions
What steel are shipping containers made of?
Corten-A weathering steel, or the Japanese grade SPA-H — a low-alloy steel with copper, chromium and phosphorus additions, and up to 0.65% nickel, that forms a protective oxide patina instead of flaking rust. The corner castings are separate cast steel fittings to ISO 1161.
How thick are shipping container walls?
Shipping container walls are 1.6 mm (about 1/16 in) of corrugated steel in all 3 published 20 ft specifications we read, with 2.0 mm in the end panels beside the posts in 2 of them. The roof is 2.0 mm, the door panels 2.0 mm, the sills and headers 4.0–4.5 mm and the front corner posts 6 mm; the 36 mm deep corrugation, not the gauge, provides the stiffness.
Do shipping containers rust?
Yes, but slowly and predictably: Corten steel’s patina resists corrosion, so rust concentrates where paint is broken and water sits — dents, welds, hinges and bottom rails. Surface rust is cosmetic and treatable; sheet-flaking or perforating rust is structural.
What makes Corten steel special?
Its alloying (copper, chromium and phosphorus, with a little nickel) makes the initial rust layer dense and adherent instead of porous and flaking — the patina seals the surface against further attack. That is why a thin-walled steel box survives decades of weather on ordinary paint.