container floor construction

My last post on container wall construction seemed to be very popular.  Thanks to everyone who clicked, read and offered comments.  I hope everyone enjoys this post on how to construct the floor of a shipping container.

First a little review:  As you’ll remember from last time, containers are not initially intended to house humans….or dogs, cats…really anything with skin and/or fur is not meant to be housed in these things.  The paint is toxic and needs to be either sandblasted and coated with impervious paint, or (if you take the short cut) simply painted with impervious paint (this is really not recommended).  The floor, also, is not friendly to us or anything else as it is typically either OSB or standard plywood soaked in formaldehyde so that it doesn’t rot as it travels across the globe being beaten to hell by God and Creation.  So, we need to get rid of it, again, safely.  Contact your local EPA to find out how to properly dispose of this crap – believe me you don’t want to hang on to it.

Once you’ve removed the plywood floor, you’ll see the structural steel ribs below that give the container it’s horizontal support.  Now, we’ve got a couple of options on how we can build our floor.  It all depends on your foundation type: whether your containers are off grade on a stem wall or sitting on a slab.

The easy option is to screw down a new subfloor (either plywood or OSB) and, while you’re spraying the exterior of the container with SPF, simply spray the underside of the floor as well, thus insulating and sealing the underside of the container.  Then install whatever floor finish you choose – tile, vct, carpet, hardwood, engineered wood, cork, bamboo…you could even leave the plywood exposed and finish it with a polyurethane for that “industrial” look.

Another option for the floors, one that is much more sturdy and permanent, in my opinion, is to use concrete.  A typical concrete floor with deck is approximately 4″ thick (2 1/2″ of corrugated metal deck and 1 1/2″ of lightweight concrete fill).  If poured on grade, you’ll also have about 2″ of foam insulation underneath with a vapor barrier to prevent  water intrusion through the slab.  For a container, what we do is first lay in exterior grade plywood (5/8″ min.) between all of those steel joists, followed by 2″ of rigid foam insulation (you can find this stuff everywhere – even for free).  On top of the foam you pour your concrete (3″ – 4″) with a medium gauge welded wire fabric.  The concrete is leveled, smoothed and finished in any number of ways – saw cut, stained, brushed, etc.  Also, don’t forget to add in at least 2 expansion joints across the length of the container.  Steel and concrete move in very different ways and the last thing you want is a huge crack from expansion and contraction of the steel.  Essentially what we’ve created is a reinforced waffle slab that is insulated between the steel joists.  Any thermal bridging at the joists will be minimal.

Whichever option you choose for the floor of your new container home, be sure to consult with a local structural engineer and your local building inspector/plans examiner to make sure that you are building to proper code.  While containers provide an inexpensive building unit with which to construct a home, remember there are important steps that need to be taken in order to make them habitable and safe according to modern building codes.


7 thoughts on “container floor construction

  1. Detail #1 calls for closed cell spray foam insulation, which is R-6 per inch according to product data. So, 2″-3″ will give you between R-12 and R-18 plus whatever minor R-value might be associated with your sub floor and finish.

    Detail #2 is rigid foam insulation and concrete. 2″-3″ of rigid insulation (extruded polystyrene) will give you approx. R-4 per inch, so around R-8 to R-12. Add in the concrete and you get a max of around R-13. Structural lightweight concrete is approx. R-0.19 per inch. Using non structural lightweight concrete, around say 40 or 50 pcf, and you get between R-0.7 and R-0.8 per inch.

    Radiant floor heating could certainly be added in either scenario. For me personally, I’d go with a combination of the 2 above examples, i.e. concrete floors with closed cell SPF underneath on a stem wall foundation with an insulated crawl space (makes running plumbing and electrical easier).

    Thanks for the comments Marjorie!

    • aside from the loss of space do you find there to be any benefit in building a 2×6 floor joist ontop of the containers preexisting 3/4″ plywood floor for insulating and plumbing? Or do you prefer to use the steel joists of the container as the only floor and space for insulation and plumbing? Thanks!


      • If you put a 2×6 floor over the existing floor you would reduce the overall interior height to less than what is allowed by code. That is not an option.

  2. Awesome! Cheers for your info, building a prefab ‘container’ out of freezer panel from scratch for storage shed to go with our housebus. Mostly because the second-hand containers available are now too expensive (post-quake) Christchurch and even before the quake the quality of the containers meant weeks of sandblasting, patching & painting before insulating could even be looked at. Making my storage shed/workshop/art studio cotainers (three containers eventually) sized to be equivalent to 20′ containers means transport is very easy. Your information will be of great help!

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