Are Shipping Containers Waterproof?

While they are water-resistant, they 100% waterproof shipping containers don’t exist. Because these boxes are designed to store and transport goods in a variety of harsh environments (heavy rain, snow, water shipping), the weathering steel frame is made to withstand wear and tear, including exposure to water. However, worn seals or dented roofs can break the watertight integrity.

In this Midstate guide, we give a comprehensive answer to the question “Are shipping containers waterproof?” and discuss how you can improve your unit's water and weather performance.

Are Shipping Containers Completely Watertight?

When we discuss whether shipping containers are watertight, we use the definition “100% stopping rain and spray from entering through the roof, doors, walls, and floor line.” However, being completely watertight under real conditions is usually impossible, as it depends on such factors as age, repairs, modifications, and the condition of every element of the unit.

In most cases, new shipping units have the best water resistance – they haven’t been exposed to serious wear and tear, so the doors close cleanly, paint doesn’t peel off, and rubber seals haven’t hardened. Therefore, such boxes are often picked for the most demanding tasks, like storage in extreme weather conditions or long-distance maritime transportation.

Still, no need to dismiss used storage units – many still offer reliable watertight storage, suitable for storing tools and equipment. However, be extra cautious with the state of such containers and pay attention to damaged seals, roof dents, rust, and bent doors can make the container less fitting for water-related applications. 

Most water tight shipping containers can last up to 15 (and sometimes even 20) years if properly maintained. However, the state also depends on the heat, salt water, and UV exposure, frequency of use, and harsh weather conditions. Because of that, most containers used for sea shipping or coastal storage tend to age and lose their watertightness faster.

Waterproof vs. Watertight: What's the Difference?

A waterproof container for shipping must be inspected before use – even a slightly deformed door or dented roof could become a problem. Also, poor repairs or leaving it in standing water for a few days is likely to harm a unit of any grade. Make sure to properly maintain the container to prolong its watertightness and extend the overall lifespan.

Need a durable unit for your site or transportation project? Contact Midstate Containers at sales@midstatecontainers.com | (805) 925-4422 or browse our range to get a suitable container today!

The Importance of Proper Maintenance 

Proper maintenance and inspection must be your habit if you want a container to serve long and well. Noticing damage early on helps prevent it from becoming a costly problem

What your inspections must include:

Examining the Roof 

Uneven roofs may cause water pooling, which means rust forming, especially along seams. When checking it, pay extra attention to spots where metal feels weaker, flaking paint, and, most importantly, signs rust is spreading. If you notice any of these, address them immediately – if left unattended, they could compromise the rest of the unit. 

Checking the Walls 

While corrugated steel walls are extremely strong (because they are made from corten steel), bends could still trap dirt and moisture. Make sure to check the lower corners, panel welds, seams, and the area near the floor – these are the biggest risk points. Look for bubbling paint or signs of damage like deep dents or rust. 

Evaluating the Door 

Container doors use rubber gaskets and door seals, forming a tight seal against the frame (which makes some people think that shipping containers are airtight, although most aren’t). These elements tend to wear down with time, so if you notice hard or cracked rubber, it may be time to replace them.

You can also use a simple paper test – close the door on a sheet of paper and pull it gently. If the paper slides out with no resistance, the seal likely needs repairs or replacement.

Ways to Protect Your Shipping Container Long-Term 

Even if you think your shipping container waterproof, it still needs care to serve longer and protect its contents better. 

Inspect Your Container Often  

Inspect the unit at least every 6 months, plus every time after heavy rain, flood, or serious impact. Think about regular maintenance as an investment – spending a couple hundred dollars on repairs is better than buying a brand-new unit.

Treat Rust as Soon as Possible 

Even if it looks like just surface rust, it may have crawled deeper into the steel. To treat rust properly, start with sanding the spot (40-80 Grit sandpaper should help), then remove loose paint and apply a rust converter. In the end, apply the primer, and, once it has dried, repaint the spot. This helps slow down further corrosion or even eliminate it completely.

Ventilate the Shipping Container 

Proper airflow helps avoid condensation, which could lead to water damage. If possible, install ventilation in the unit, especially if you are storing moisture-sensitive goods. Having said that, you can also air out the unit yourself, but it requires manual work, time, and consistency.

Waterproofing a Shipping Container 

To waterproof a shipping container well, think of it as an active, ongoing process rather than passive, occasional maintenance described earlier. What you should do:

• Apply Coatings and Membranes: Adding waterproof membranes and protective coatings (elastomeric roof paint, marine-grade epoxy paint, bitumen-based sealant) to the walls helps the surface of the unit repel water, meaning less of it gets to seams or causes erosion.
• Raise the Structure Above Ground Level: Standing water is probably the biggest risk factor for most shipping units. Elevate the container above ground level to avoid contact with it. 
• Seal Modified Areas: If the container has any modifications, like extra doors, windows, or vents, seal the area around them with waterproofing materials. Installing these often breaks the integrity of the container, which needs to be addressed.

Considering container modifications – keep this in mind: although they help make the unit more suitable for different applications, poorly installed mods are worse than no mods at all. Modified containers with an amateur setup tend to have a lower value, both in the market and in use.

Additional Measures for Waterproofing 

• Protective Roof Coating: Helps shed rain and reduce standing water.
• Foundation or Pile Elevation: Keeps the base away from wet soil.
• Drainage: Moves water away from the bottom rails.
• Sealed Openings: Keeps windows, doors, and vents from becoming leak points.
• Moisture Control: Desiccants, vents, or the manual thermo process can support a drier interior.

Again, it’s impossible to get 100% waterproof containers, but these waterproofing measures can help protect the contents of the unit better.

New vs. Used Containers: Wind & Watertight Grade

Always assess your needs critically and answer these questions: “What am I going to store?” “How dry of an environment does it need?” “Am I going to transport it?” “Are these cargo containers waterproof enough for this project?”, and “Am I willing to do any extra work with the unit?”

Get Your Waterproof Container With Midstate 

Still wondering whether shipping containers are waterproof enough for your project? Contact us at sales@midstatecontainers.com | (805) 925-4422 and let our expert find the right container for your needs.