I have a painted shipping container steel roof in average condition and plan to pour a 3.5 inch concrete slab on top. My question is " what effect will using red oxide resistant primer, as used in the tropics, bring to the project"?

  • 1
    Any reason for that thick of concrete? There are different types of roof coverings that should work well for leak prevention. In the sun that cement will act to soak up heat.
    – crip659
    Mar 18 at 17:27
  • 2
    Are you going to put reinforcing in the concrete? Without doing calculations, i would be concerned a little bit about the weight. Recall that they are designed to carry weight on the floor. And when they are stacked, the weight is transferred in the corners. Finally, why. The roof will easily last a few decades. Painted properly it is not going to rust. Mar 18 at 19:47
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    Putting concrete over steel may precipitate the failure of the steel. They did that on some museum ships for external decks and found out it was a "really bad idea" afterwards.
    – Kyle B
    Mar 19 at 2:56
  • I'd lay a couple dozen coats of roofing tar over it rather than a layer of primer if you must do this.....
    – Kyle B
    Mar 19 at 2:56
  • Big question is WHY do you feel the need for 3.5" of concrete? Given a reason, we can probably come up with a better solution than all that concrete and work.
    – Tim
    Mar 19 at 15:48

2 Answers 2


Concrete is wet by nature. Wet + steel = fail.

Concrete is porous to water - that is its nature. Champlain Towers South collapsed because they didn't slope or waterproof their pool deck, so rainwater sat there and soaked into the concrete, leaching out the lime that kept the re-bar from rusting. Here, the Building Integrity Youtube channel does a great job of covering this (esp: regarding Champlain Towers), here's a talk on how concrete dooms steel. (And sea salt is less the issue; Josh is in Miami :)

So now you know why concrete decks have been the ruination of many a museum ship. Ryan Samansky has a quick talk on that. Railway cars also once used a poured "lightweight concrete" type product for interior floors, and the the effect this has on the structural steel is ghastly.

On your container the rust will mainly happen around the edges of the roof. You will eventually get a "dotted line" of pinhole rust-throughs along that edge - but really, that is a telltale for the entire edge thinning. I've had the joy of taking a wire wheel to such areas only to watch the steel immediately blow through simply from the few pounds of force of the wire wheel and now I have a hole to patch.

Eventually the roof will collapse along that edge. Note that is the only thing holding the outer wall vertical, so the wall will be able to kick outward.

So your concrete plan is fine for a temporary structure. (no one ever imagined those ships or railroad cars to be in museums for 60 years). After that, you'll need to frequently and carefully inspect the ceiling of the containers for rust-through.

But you were planning to "finish" the ceiling, correct? How will you inspect it then?

There is no substitute for proper prep and paint

Generally, by the time a shipping container dribbles down into the aftermarket, its paint is starting to have issues. The roof, and particular nooks and crannies on the roof, is a top area of concern. When paint fails, rust begins.

Like Cher used to say, "if it came in a bottle, everyone would have a great body". The same applies to paint prep - if there was a product that came in a bottle you could just goob on, nothing would be rusty! So the idea that some magic primer, or tar, or concrete, can be just applied over rust and that will fix it - that's sheer fantasy. That's not me saying that, that's NASA - an organization whose mission is maintaining vast steel structures in a coastal environment, and also has a side gig sending people into space :)

Unfortunately it appears corrosion.ksc.nasa.gov has a new "curator" who has shifted focus from directly sharing their vast research, to simply bragging about their vast research. So I can no longer link the data from which I extracted those conclusions; there's NASA-STD-5008B, but it's a shadow of the riches once found on the site. Shameful. Those assets are property of the American people. Anyway...

What does work is media blast down to SSPC-PC10 near white metal, and again this is NASA talking, followed by appropriate primers. Ideally a mil-spec 2-part epoxy chromate primer, but a quality alkyd primer such as Rustoleum 7769 will do reasonably well in my experience - we have one that lasted 30 years with $11/gallon non-lead primer from a local paint factory because it was blasted to SSPC-PC10.

In my experience, a reasonable second place is achieved with SSPC-SP3 mechanical wire wheel work; I often get lucky and get the rust to stop with that, good surface cleaning, ideal application conditions and Rustoleum 7769 and a topcoat. I consider it worth my time to at least try, when media blast is impractical or too much trouble. But I observe it for a couple years to see if it holds or needs re-treatment.

Do what ships do - wood deck

If it were my project, I would not even bother faffing around with concrete. I would make sure the container roof drainage is set up properly, make sure it is coated correctly, and then, install a wood deck or Trex. I don't think you need teak, though. That's for salt spray.

  • Yeah, plenty of piers are decked with pressure treated timber and last decades, teak is for overpriced pleasurecraft.
    – Jasen
    7 hours ago
  • but I worry about the water pooling under the deck and being unable to evaporate fast enough.
    – Jasen
    7 hours ago

Intermodal shipping containers are constucted with heavy corrugated steel roofs, but concrete is basically made out of rocks, and this makes it heavy.

One reference says that a continer roof can withstand 300 pounds per square foot. which is many times more than a square foot of 4" thick concrete weighs.

so it sounds like this approach is practical.

the next problem is protecting the container skin from rust. that will be harder. the underside of the roof will attract moisture especially during summer morning rains. basically you're going to have to accept that the container roof will rust out leaving the slab sitting on the walls of the container and design for that.

Or just accept that it's going to fail and eventually have to be rebuilt.

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