Water is what ends a shipping container, and almost all of it is preventable. Rust needs time and moisture together, and a unit that dries between showers stays sound for decades. Good shipping container drainage is a few hundred euros of groundwork, while a corroded floor is several times that. Here are the four sources and the fix for each.
Why does shipping container drainage decide how long a unit lasts?
Steel rusts where it stays wet. Drainage moves water away from the unit so the surfaces dry between showers and corrosion never takes hold.
Container steel is usually Corten weathering steel, which forms a protective patina only if the surface dries between wettings. Permanently damp steel corrodes like ordinary steel, which is why how long a unit lasts depends more on the site than on the container.
The floor sits on cross members roughly 100 to 150 mm above the bottom rail, and airflow underneath keeps them dry. The load passes through four corner castings specified in ISO 1161, which must stay clear of standing water.
How do you stop water sitting on the container roof?
The roof is corrugated and holds puddles, so set the unit with a small fall to one side and clear leaves and debris twice a year.
A container roof is not flat. It is a shallow corrugated panel, and the corrugations trap water. Standing water is the single biggest cause of roof corrosion.
Pack the pads so the unit has a fall of 1 to 2 percent, which is 10 to 20 mm per metre. That is invisible by eye and moves every litre off the roof.
- Sweep leaves, needles and moss off twice a year, more often under trees.
- Store nothing on the roof, since timber and sheeting hold water against the steel.
- Touch up deep scratches before winter, as the roof is the thinnest panel on the unit.
How do you keep ground water and surface run off away from the unit?
Never site in a dip. Grade the ground to fall away from the doors and add a channel or French drain where run off arrives.
Most of the water that damages a container never falls on it. Shipping container drainage starts at the site boundary, because run off arrives from a yard, a downpipe or a slope above.
Use the same 1 to 2 percent fall across the site. The door end should be the high point, or at least not the low one, because water running to the doors sits at the sill and soaks the first metre of floor.
- Channel drain. A linear channel along one side, for run off arriving off a hard surface.
- French drain. A gravel filled trench with a perforated pipe on the uphill side, for water moving through soft ground.
- Gravel margin. A free draining strip that stops the ground turning to mud.
Rules on where surface water may discharge differ by country and municipality, so check locally before connecting to a public drain. Our guide to preparing the site for delivery covers the groundwork.
Does grass against the container cause rust?
Yes. Grass and weeds hold water against the panels, so keep a 300 mm margin of gravel or hard surface clear all the way round.
Vegetation touching the steel works like a wet sponge and stays damp long after the ground dries. Bare soil is little better, because rain throws grit and water onto the lower panels.
A 300 mm clear margin solves both. Lay a weed membrane under the gravel, and top it up when it thins.
What about condensation inside the container?
Condensation forms when warm damp air meets cold steel. Keep the vents clear, raise the contents off the floor and use desiccant.
Drainage will not cure condensation, but wet ground feeds moisture into the space. Keep the factory vents unblocked, put everything on pallets, use desiccant in units rarely opened, and never load anything damp. Our post on using containers for storage goes further.
What are the four water sources, and what does each fix cost?
Roof water, ground water, splash back and condensation. Each has a clear sign, a simple fix and a cost well below a floor repair.
| Source | Sign | Fix | Typical 2026 European range |
|---|---|---|---|
| Roof water | Puddle rings, flaking paint, moss | Pack the pads for a 1 to 2 percent fall | Shims and labour, under 100 euros |
| Ground water | Standing water after rain, silt on the castings | Channel drain, or a French drain uphill | 200 to 700 euros, or 40 to 90 euros per metre |
| Splash back | Green staining and rust on the bottom 300 mm | 300 mm gravel margin over a membrane | 150 to 400 euros |
| Condensation | Droplets on the ceiling, damp cartons | Clear vents, raised contents, desiccant | 20 to 80 euros a year |
Ranges vary with ground, access and contractor.
What should you check after heavy rain, and once a year?
Walk round within a day of heavy rain and look for standing water. Once a year clear the roof, the drains and the gravel margin.
Anything still holding water a day later is cheap to fix now and expensive to ignore.
- Clear the roof and check for ponding after the next shower.
- Rake out the channel drain and check the French drain outlet.
- Top up the gravel margin and cut back what grows into it.
- Check the cross members with a torch, and the first metre of floor.
What does water damage look like once it has started?
Bubbling paint, brown streaks and soft floorboards. A corroded floor and bearers cost far more to repair than the drain that prevents it.
Early damage shows as flaking paint on the lower 300 mm, brown streaks below the door gear and spongy plywood by the doors. Later come scale on the cross members and pinholes.
Repairing a corroded floor and bearers typically runs 800 to 2,500 euros in 2026, against 200 to 700 euros for a channel drain that would have kept the water away. That is the argument for shipping container drainage before delivery rather than after.
What else do people ask about shipping container drainage?
Three short answers to the questions we are asked most often about keeping water away from a container on a customer site.
Can a container sit on a concrete slab with no drainage?
Only if the slab falls away from the unit. A flat slab holds a film of water under the bottom rail. One laid with 10 to 20 mm per metre of fall is an excellent foundation.
Do I need permission to drain water off my site?
Possibly. Rules on where surface water may discharge, and whether you may connect to a public drain or soakaway, differ by country and municipality. Check locally before you dig.
Does a container roof need a gutter?
Not usually. A gutter helps where roof water falls onto a doorway or scours the gravel margin. Otherwise a fall across the roof and a free draining margin do the job.
Last updated: September 2026
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