In the previous article Container Houses – Above the Ground Decks, we outlined major challenges you will face when building a rooftop deck on the top of a shipping container house. Below we will try to address practical issues related to the realization of the project.
Shipping Container House with the Rooftop Deck. (Source: Indiamart.com)
1. Does the container need extra reinforcement?
The maximum weight of a fully-loaded 20ft shipping container is 62,150 lbs (about 28,190 kg). It includes the net weight of the container (about 5,000 lbs) and its cargo (up to 57,150 lbs). Such a container must also withstand the weight of 8 fully loaded containers stacked on top of it. It means, that the bottom container must withstand the weight of up to 8 x 62,150 lbs which results in a staggering number of 487,200 lbs (about 225,5 tons). This enormous strength by far exceeds the needs you may face when building your shipping container house. However:
- It applies to an unmodified structure of the shipping container. In other words – its structural integrity is not weakened by removed walls, or cuts in the walls for windows, doors, and in general any modifications necessary to transform the cargo container into a habitable space. It also means that the container’s side doors are closed.
- The load is applied to the container’s corner castings. While the whole frame and walls contribute to the overall strength of the container’s structure, the main elements carrying the load are reinforced corner posts (Castings).
- The roof itself (just to remind you – also made from the corrugated Corten steel) has much lower mechanical strength. To give you an idea – the area of 1 sqr ft can withstand a load of about 227 lbs (103 kg). In practice, if you will pile on the roof heavy items or even walk on it, it may flex, and the deformation (warp) may become permanent. This will have a serious impact on the long-term durability of the roof due to the water ponding effect.
Shipping Container House with the Rooftop Deck. (Source: M-Nade-in-China.com)
In practice, removed sections of walls for windows and doors do not substantially weaken the container’s structure. In such cases, building the rooftop deck should not be a problem. However, when an entire wall is removed, you may have to use extra supporting beams to make it safe.
2. How to safely attach a rooftop deck?
а. Original container’s frame
The safest way is to attach an original container frame to the top of your container house. By the definition (ISO standard), it will exactly match the locations of corner castings, and thanks to built-in corner fittings, it can be easily and securely locked in its permanent position.
In an ideal case, it can be the frame from an old container (provided you can find one in an acceptable shape and buy it for a reasonable price). Note, that you may be also able to buy a new one. They are massively manufactured in China for cargo shipping containers as well as for customized container houses.
New, shipping container frame (Source: Laizhou Dingrong Steel Structure Co. Ltd, China).
You can then use standard twist locks designed to hold stacked containers during the transport on the ship!
Original Twist-locks used to secure containers stacked on top of each other (Source: “Six Practical Shipping Container Accessories” by Paige Welsh, Falcon Structures)
b. Custom Frame
The original shipping container’s top and bottom frames include multiple cross members. They will make an ideal, ready-to-use deck frame, mechanically largely exceeding your current requirement. Remember, that they are designed to support up to about 57,000 lbs of cargo (assuming the container’s net weight of about 5,000lbs).
In practice, however, it may be a “mission impossible” (or too pricey), so you may have to make your own supporting frame. Certainly, the weight of your deck will never be even close to the number above, so a custom frame will be much simpler and most likely less expensive. But it will take time and will need special skills to build one.
Corner fitting (Source: Shipping Container accessories- Lift Point Products, FB)
In theory, it is the most straightforward solution. Welding adequate side profiles to four corner fittings will result in a permanent solution. You can follow by attaching the wooden joists and security barrier to it as well as installing the deck’s floor.
Attaching by screws
You may use holes in the corner fittings to attach the deck’s frame to the container’s structure. This may be a preferred solution if the frame is made from wood (popular 2 x 4 beams).
Using original twist-locks
This may be probably the most elegant and easy DIY type of solution. Originally, ground plates with twist locks are used to secure shipping containers to a foundation. However, in an upside-down orientation, you can use the plates to attach the deck frame to the container’s structure. All you must do is rotate the levers to secure the locks in the fittings.
Ground plate w/twist lock (Source: “Six Practical Shipping Container Accessories” by Paige Welsh, Falcon Structures)
Note: For better strength of the deck’s frame, you may have to add extra support point(s) somewhere at the midpoint of the roof’s sidebars. Depending on the longitudinal strength of the deck’s frame (resistance to bending), this requirement may apply to all mentioned above solutions. Such support point may just be a “separator” (block) firmly attached to the deck’s frame but sitting freely on the container’s frame.
Warning: We strongly discourage drilling holes in the structure of the shipping container to attach the deck. Although it may be a tempting solution, you will expose your container house to water leaks! Remember one of Murphy’s laws:
All sealed joints are leaking.
3. How to prepare the container’s roof for the rooftop deck.
Shipping containers are built from corrosion-resistant Corten steel and are designed for 25 years of service. However, despite their rugged and weather-protected structures, they still need maintenance. It is especially true when you reuse an original cargo unit:
- having years of service in an environment of salty moisture (sea transport),
- carrying potentially toxic and corrosive cargo,
- being mechanically damaged during service,
- or one that underwent modifications during its adaptation to a habitable space.
Installation of the rooftop deck will make future roof inspections and necessary maintenance rather impossible. It’s then crucial to prepare the container roof for long years of service before the rooftop deck is installed.
In contact with oxygen, the Corten steel (similarly to standard steel) is subject to corrosion. However, what makes a difference is that thanks to small quantities of phosphorus, copper, chromium, and nickel-molybdenum added during the production of Corten steel, the corrosion creates a quasi-impermeable surface layer. Known as “patina”, this layer strongly adheres to the surface of the original Corten steel. It prevents deep corrosion, protecting this way the whole structure from gradual destruction. This contrasts with a very porous layer of rust characteristic of the standard steel.
In practice (especially in mechanically damaged areas), the “patina” may lose its integrity allowing the corrosion to progress. Fortunately, small, rusted areas can be cleaned with a wire brush and/or sandpaper, and then painted with marine-grade paints.
However, in the case of a severely corroded container, the sandblasting and subsequent painting of the whole roof may be necessary.
b. Visible mechanical damages (dents, scratches, holes….)
All sharp dents (especially those allowing for accumulation of water) are prone to deep corrosion. They should be cleaned, patched with sealant, and painted.
Warps may not be easily visible, however, due to the water ponding effect, they can have disastrous long-term effects That’s why when building the rooftop deck, we should avoid walking on the container’s roof.
For safety – it’s suggested to run a quick test by spraying water on the container’s roof to see if it tends to accumulate the water. Unfortunately, it may be difficult to eliminate existing surface deformations, without causing even more “damage”. The best we can do is minimize the consequences of water ponding by applying extra preventive measures.
d. Preventive measures
Even if the container’s roof does not show rust or mechanical damages, it is recommended to preventively “reinforce” its inherent Corten rust protection by applying a layer of marine-grade paint.
Before painting it is suggested to get rid of surface dirt, bio-contaminants, etc., preferably by pressure-washing.
Note: If you intend to use the roof (deck) for rainwater harvesting, the materials used for the deck’s construction and protective coating must be selected based on their non-toxicity.
Pro Industrial DTM Acrylic Coating is an interior/exterior, water-based, corrosion-resistant acrylic coating for light to moderate industrial use. It does not require a primer. Source: Sherwin Williams
You may find suggestions to use a heavy-duty pond liner as the first line of defense against water-generated corrosion. Unfortunately, most likely liners will not adhere perfectly to the roof (especially given corrugation). They may leave air pockets that can accumulate water or moisture.
Note that the roof coating may be exposed to a wide range of weather elements. Depending on the climate zone it can be:
- large spectrum of temperatures (from very high to freezing)
- water, humidity, ice….
- Potentially acid rains
Summarizing: Good coating material should be characterized by:
- strong adhesivity to the steel to eliminate potential air-pockets
- basic “elasticity” to accommodate dilatations due to its exposure to a wide range of temperatures
- durability over weather cycles leading to continuous expansions and contractions,
- liquid form when applied to fill all possible irregularities on the roof’s surface.
4. Space between the roof and the deck floor
The deck should be installed “slightly” above the container’s roof. The space in between is necessary to provide:
- Unobstructed drainage of the container roof
- Proper ventilation of the container itself.
The latter is important not only for removing moisture and reducing the probability of mold. It’s also crucial, to the evacuation of heat during hot summer days and so, to minimize the need for forced cooling of the living space.
Note that the rainwater will always infiltrate the space below the deck’s floor. That’s why the deck’s frame cannot act as a barrier for the rainwater, for the flow of the fresh air, and evacuation of the moisture. If that happens, the lifespan of the rooftop deck and that of the container itself will be largely limited.
Spacing comes “naturally” with the use of corner fittings, and twist-locks. (Source: Bremerhaven)
And the practical implementation for a deck by using the standard container frame, corner fittings and twist-locks. (Source: Containers Direct)
How much is “Slightly Above”, largely depends on the structure of the deck itself. The deck’s floor will be always sitting on the wooden joists a few inches above the roof. However, the deck’s frame and joists are usually installed directly on the roof. Thanks to the container’s corrugated structure (it’s about 1” (2.5cm) deep), rainwater will be eventually drained, although with time individual “openings” may become clogged. However, when it comes to proper ventilation, such a structure may not be good enough. Hence the importance of raising the deck above the container’s roof.