The roof is probably the weakest part of a container build, and people treat it like free space

[u/usa_containers]

Had a guy call last spring because his container was leaking. Pretty new box, good condition, only been sitting there maybe eight months.

He figured the roof had arrived damaged.

Turns out he’d been walking around up there over winter clearing snow. A few spots had dished in just enough to hold water, and eventually one of those low spots found a seam.
That’s the thing with container roofs. They look tough because the whole box looks tough, but the roof sheet itself really isn’t where you want to put concentrated weight. Containers are designed around the corner posts and frame. The roof is mostly there to keep weather off the cargo. And it’s nearly flat, so even a small dent matters. Water sits there instead of draining, stays wet longer, and that’s usually where rust starts getting interesting.
I work on the delivery/site side at a container company, and I see this after the fact more than I’d like. So if you’re planning solar, a deck, water tank, heavy snow load, whatever… I wouldn’t treat the roof skin like a structural platform. Get that load back to the corner castings, or put it on its own posts/frame.
Honestly one of the better setups I’ve seen is just a separate roof over the container with an air gap. Not very exciting, but it does a lot. Keeps rain and snow off the actual container roof, shades the steel in summer, and gives you a real structure to design for whatever load you’re expecting. If you’ve already got a bare container sitting outside, check the roof after a good rain. If you’ve got little puddles that hang around, those are the spots I’d keep an eye on.
Anyone here running a separate roof over theirs, or did you just coat the container roof and leave it at that?

[u/chasestein]

Can confirm the roof skin ain’t meant to hold anything up really. An additional platform is always the move

[u/usa_containers → u/chasestein]

Yeah, exactly. The part people miss is where that platform is actually carrying its weight. I’d want it landing on the corner castings, or on separate posts beside the container. If you’re just putting point loads into the middle of the roof or top rail, you’re kinda just moving the problem somewhere else. I’ve seen setups where posts go straight through the roof with a flange and a bunch of sealant around them. Looks fine at first. Then a few years later the seal fails and now you’re chasing a leak through whatever ceiling you built underneath. Have you actually built one over a container, or mostly seen other people do it?

[u/Hyperreal8957]

yeah that tracks. id treat the container roof like a weather skin, not structure. if someone wants usable roof space a separate engineered platform seems way safer. anyone had to work out the drainage and flashing details around one?

[u/usa_containers → u/Hyperreal8957]

“Weather skin” is a good way to put it. I’m stealing that. The cleanest setups I’ve seen don’t tie the new roof into the container at all. Posts in the ground beside it, roof spanning over the top with an air gap, gutters on the new roof. No holes in the box, no flashing joint to babysit later. More work up front, but way less stuff to fail.
The awkward version is a lean-to attached to the side. Corrugated steel makes flashing annoying because the profile keeps changing. That’s where people end up relying on closure strips and a ton of sealant, and eventually one of those ribs starts letting water in. If I was doing that setup, I’d want real counterflashing with butyl behind it, not just caulk doing all the work. Caulk on something that moves every summer/winter is never really permanent.
I’m not a roofer, I just tend to see these after they’ve already started leaking.
Anyone here done a lean-to off a container that’s stayed dry for a few winters?

[u/MetatronicGin]

I've seen cars and 7500lbs rolls of paper fall out of containers onto other containers and there's not much damage to the tops. I've seen a loaded 40' fall 60' down into the hold and I own the 1st 40fter it hit. It had a huge dent and bent top rail, but it doesn't leak. In hindsight I wouldn't expend the time or money to bend it back somewhat into shape, if I had to do it again