Phase I: Subfloor Sandwich System

Todd Miller from the Oregon Cottage Company is building another cottage and is going to be sharing with us a series of the steps involved in building a tiny cottage on wheels. This is the first in the series and I hope I can assemble his information in an easy to understand way. This first phase is about assembling the subfloor sandwich system. I will turn it over to Todd now:

Once you have a design set, received your lumber, windows, doors, fasteners and updated and provided your proper tool maintenance you will be ready to get your hands dirty.

Custom 10,000 lb GVWR Frame

I call this first phase of construction a “sandwich system” because of the way the materials are stacked to get you to the wall framing phase of the project.

Simply put, the “Sandwich” is 3 ½” rigid board insulation layered between two sheets of ¾” T&G plywood.

Bottom slice of bread: The ¾” T&G plywood must be first prepared to make the underside roadworthy. This process involves first applying an oil based primer then a heavy duty epoxy sealer to the finished side of the plywood. This stuff is definitely not butter and I would recommend not eating it. This is the same sealer used under boats to make them watertight and more durable. It may be overkill, but I want to make sure that any rocks or road debris will not penetrate the undercarriage.

Once this is dried I am ready to cut the 4×8 plywood sheets and attach them to the trailer frame. Place the boards so that all the seams are located on the trailers metal cross members. Now you are ready to attach the first boards. There is a right way to do this and the wrong. You know you have done the wrong when all your bits break and screws are stripped. Use the correct fasteners, and spacing, do your research and I guarantee the process will be putting you on the porch swing with a tall glass of lemonade much quicker than the prior.

Level and true

Sandwich filling: After attaching the ¾” plywood, trimming any overhanging edges to square the layout, you will be ready to install the 2×4 edge boards and sleepers. These will provide a surface to attach the ¾” subfloor and enough space between where the rigid board insulation will be installed. I layout my sleepers on edge at 2’-0”centers to accommodate the plywood edges. I install these with ample amount of hardware hold-downs with #10 pan head screws at crucial locations to prevent uplift. I had a count of 70 hold-downs on the Cottage pictured here.

The easiest part of this system to install is the rigid board insulation. I use the rigid board not only for the durability but also for the R-value it offers. I definitely don’t do it for the upfront cost, but the 3 ½ inches provides you with an R value of 23.35 which will pay for itself every minute you are enjoying the warmth and quiet space in the Cottage. I strike a chalk line where I want my cuts and use a small handsaw to make my cuts. Installing the panels in their corresponding bays is done with a 2×4 board laid flat and a framing hammer, so as not to damage the insulation.

3/4" T&G underlayment w/2x4 sleepers

Top Slice of Bread: The top and final portion of our sandwich is the ¾” T&G subfloor.
For me installing these 4×8 sheets is the most satisfying portion of this first phase. I precut any sheets (finished side facing the conditioned space) that will be adjacent to the wheel wells and trim the other sheets once fastened. Again I make sure any plywood edges have a 2×4 sleeper or edge board located behind it to provide for proper nailing. I use a framing nail gun and finish all edges with 2 ½” nails space 6” o.c.. I space my nails at 9” or less in the field. After nailing all the edges and fields I return to the task by screwing the edges with # 8 2” screws. This is not a typical construction practice, but since most homes do not travel down the highway at 55 miles an hour, I want to make sure the materials stay attached to one another.

Rigid insulation w/ floor hatch framed
3/4" T&G subfloor instalation
Sandwich system complete

37 thoughts on “Phase I: Subfloor Sandwich System”

  1. Bravo Mr. Miller, It’s nice to see someone else who’s conservative in their building practices and for all the right reasons. Put your money where it counts the most and you’ll significantly reduce problems down the the road, in this case literally.

  2. Nicely done and nice pictures to go along with the descriptions. Was the plywood under the frame to help protect the frame as well as the underside of the house? It looks like the framing itself was set on top of the trailer frame, so no extra height was gained. That leaves a dead air space within the trailer frame itself is all.

  3. Todd, is there a reason you don’t use construction adhesive between the framing and the T&G subfloor? That typically makes a stronger floor and eliminates possibility of subfloor squeaks.

  4. I agree Bill. The bottom layer of plywood serves no purpose that I see… but it does add weight to a flimsy trailer making it look like this will probably only move once or twice… or is this intended as a slide off?

    • The bottom layer of plywood is the floor to catch the insulation board, you can see in the pics of the naked trailer it has no floor, only cross members.

      • I was just wondering why the planks were removed and the plywood placed underneath the trailer. The planks could have been left on, and the plywood placed on top of the planks instead. I guess it was to get an extra 3 inches of insulation under the floor, for about 7-8 inches total. My trailer I just left the planks in, and used sheet metal over the top of the planks for protection.

        • I think he started with a custom built trailer, no planks.
          The layers from the ground up: Wheels/ metal frame/ 3/4″plywood/3 1/2″framing and insulation/3/4″plywood.

        • The trailer is a custom build, it never had floor boards on it. One of Todd’s replies to another comment stated that the trailer was a 20′ extra-wide utility trailer with specified (his) extra framing. The boards were never installed.

        • Every inch in height is a HUGE in these. An extra 1/ 1/2″ on not really insulating material that could be cut out. Look at what R-value he has in what height. That is what “designing” & knowage of materiaLS is all about.

      • Scott,
        You are correct. I used the 3/4″ T&G plywood to provide a surface to protect and support the 3.5″ of rigid insulation. Thank you for your observations and response to Peters comments.

        I also chose to eliminate the standard 1 1/2″ decking that usually is installed on flatbed trailers due to the fact that the intended use of those trailers is different than, if I may assume, the one we all have. There was no need for the added height since 3/4″ plywood is designed to span 24″ for residential use and I have no intention of driving a tractor into the Cottage.

    • I just feel I have to reply to this particular comment, I mean no disrespect to Pete at all but I keep coming back to this article and its comment page and the remarks about the trailer being flimsy and pointing out how tippy his leveling job is, this bothers me, as a builder of such units I applaud Todd for showing his construction process and I know he has learned as I have over the years there are many different ways of doing the same job , different variations of achieving the same outcome and just because Joe Q would do it different than Todd doesnt mean Todd is doing it wrong, he has developed a good system to insulate the floor and after willing to share it with all of us he has to read that he leveling job is tippy???? Again I mean no disrespect at all, but from the point of view of another builder that commment is rediculous and Im guessing Todd had quite a smile and chuckle when he read it, as for the trailer being flimsy, come on thats a totally out of line comment, there was obviously a lot of time put into this trailer frame to make it stout and keep it low to the ground.

      Todd , please keep us updated with your projects, I enjoy seeing other builders posts and I like to see the wide range of comments,


  5. If you toe nail the sub floor, you won’t have to worry about it squeaking. All you have to do is drive the nails in at about a 30 degree angle. It will draw the wood down tight and hold it there.

  6. Anybody know what the dimensions are on this trailer? Or did I miss that? I’m planning a vardo project and that trailer looks like a good size.

    • Hi Ginmar – I’ll get Todd to pitch in here on the comment section and give the dimensions, I’m sure he is working on the cottage during daylight hours. Please keep me posted on your vardo project as I am also interested in building something like that down the road.

    • Ginmar,
      The trailer is an 8’x20′ trailer but customized to my specifications. The standard 8′ wide trailer from my supplier has an inside dimension of 83″ from wheel well to wheel well is 83″. I have added a 2×3 tube steel to the outside of the side rails which makes the width 89″ side to side. ODOT’s requirement for a recreational Trailer operational without a special moving permit is 102″ which leaves me 13″ for wall sheathing siding and roof overhang.

      • Thank you! I’m looking to build a vardo with as much reclaimed wood and stuff as I can, some salvaged from my old house, which the city of MInneapolis took via eminent domain. My big splurge will be on the stove, which is going to be one of those Navigator boat stoves I found on this very site.

  7. AH………. Someone else who has done their research. See the RV Trailer stabilizing scissor jacks tucked up on all 4 corners? About $80=100 a pair. Easy to readjust for settling.

    There is also Pressure Treated Plywood out there too for the first layer, for basements walls? HD has it here. If someone doesn’t want to paint with a nasty paint.

    My plan for my floor is on the same idea, except to use a skin of galz. steel on the bottom against chewing little critters of all kinds. My Park trailer had it.

    • Donna,
      My client has requested a floor hatch for access to additional storage from the inside. Once the Cottage is in place he will have the option of wrapping the periphery with a security skirt then access the semi dry protected area under the trailer directly from the inside.

  8. My recommendation to the bottom plywood would be to use Marine Grade Pressure Treated Plywood. On the side facing the ground you can use construction grade adhesive and glue a sheet of stainless steel to protect the bottom of the plywood facing the road if you don’t have a bottom already on the trailer. I know that many here will screw the boards and frame to the trailer but I would go one step further and have metal stock welded to the trailer to act like strapping that is used on homes in areas that require strapping due to hurricane force winds, etc. It just will make the house safer as it rolls down the road in regards to not falling off the trailer.

    Also, everyone on here is using conventional insulation in these. I would recommend using spray foam insulation for the increased insulation / R value. Moreover, when you use spray foam insulation you get increased soundproofing and also a vapor barrier.

    Also, for plumbing I would recommend using PEX plumbing. For an idea of a plumbing system that you can use on the road here is a website that shows how a RV’s plumbing system is set up. Here’s the link:

    I’m in the process of designing a home myself that is going to incorporate some of the new off the grid technologies to include solar cell shingles, etc.

    I want to build a home that can either be set up to run on or off grid and the owner will not have to suffer due to this.

    Also, I haven’t seen it discussed on here but if you are planning on living in one of these houses and keeping it in one place for any length of time you might want to consider using tie downs similar to the ones used on mobile homes to keep your trailer on the ground in the event of a tornado or storm. The best thing is to invest in or build a storm shelter that can be placed in the ground beside your trailer.

    • Michael,
      I have considered using Marine grade plywood and metal sheet metal, but I felt that since I was finishing the exposed side of the 3/4″ plywood with a primer and an epoxy sealer it was unnecessary. The epoxy sealer leaves a thick, sheer, tough and watertight layer.

      Thank you for bringing up your concern about the uplift forces that the Cottage might be faced with traveling and resolving it with some sort of tie down. If you will check back for future additions to the construction phase of this cottage you will see how I resolve this issue.

  9. Let’s think more about the environmental impact of the building materials we are using.

    Regarding insulation: sure, spray foam is the poster child of green home efficiency… can’t we come up with a lower impact solution than that?

    I used polyisocyanurate / rigid foam insulation in my trailer home because I found it for free on Craigslist. Next project I do though, even if I found it for free again, I would not use it. I do understand the pros of this product though. Nevertheless, I’m curious about the future in products such as mycelial insulation, as one example.


    • Jenine,
      Thank you for your considerate comment about the impact of the building products being used. I have described my dedication to trying to mitigate the benefits of certain building products with their total embodied energy, environmental impact and the detrimental health effects on my website and I welcome new ideas. I do use ECO batt insulation made out of recycled products and with no formaldehyde in my walls but have yet found a product that can produce an R-value of up to nearly 7 per inch that can perform like the rigid insulation that is being shown here.

  10. If you’re looking to place your structure in a colder climate, I would HIGHLY recommend the following:

    Take a can of PUR FILL 1G spray foam (it’s pretty friendly stuff in comparison to alternatives) and seal around your floor joists/sleepers/etc. At $15/can, it’s a win, win.

    Then find a bag or two of Ultratouch denim cotton insulation–they make an R13 that works well with 2 x 4 framing. Lay this in after the foam has hardened.

    THEN, seek out the appropriate amount of sq footage of LOW-E aluminum staple-flange reflective barrier. LOW-E makes an exceptional line of products that will increase your R-Value, but also serve as a reflective and vapor barrier; it’s the new “no brainer” in terms of heating/cooling efficiency. (In warmer climates, people wrap the exterior of their houses with it and have a 10-15 deg diff since it reflects the solar radiation outwards; in colder, if you wrap the interior, it reflects the heat back in and helps keep the place toasty.) Most places will let you buy the exact amount you need, and for the expense, it’s well, well worth it–even if you’re being cheap about your project. Just staple it a little inside your sleepers/joists so that you have room for air to move between it and your subfloor. Tape the seams with aluminum tape and then you just apply your subfloor directly atop it.

    There’s a lot of youtube videos on the various products; I recommend checking them out. The one with Bill Nye burning pennies on fiberglass insulation vs denim cotton insulation is pretty fabulous…Exceptional stuff.

    • PS–the rigid foam does have the reflective aluminum on it, which makes it a good choice, but it’s all in how much of a tight seal you desire, how picky you are about your project, and how much you’re willing to spend. The rigid foam is cheap, but won’t have the same ‘sealed’ R-Value.

  11. PPS–the other thing to think about is pest control; denim cottom insulation contains borates which serve as a deterrent and have proven HIGHLY effective–unlike the pink formaldyhyde (sp?) fiberglass stuff. Chipmunks in my Dad’s barn tore rigid foam apart for nesting. Regardless, make sure no nooks or crannies are anywhere to be found, if you do find them, the can-o-foam does well to fill them in…

  12. This is a fantastic floor system, I would love to have one of these small homes to move back to Maine with, I will need a very well insulated home for year round living there, this is the type of floor system that will work in that climate! I do have one question, given the high R value of the insulation in the floor, why not use that in the walls/ceiling as well, isnt this a much higher R value then traditional fiberglass insulation? Is it cost prohibitive to use it in the walls/ceilings? Or is there another reason it cant be used there? Thanks for any info you could give me, Tim

    • I bet ya he’s using it there too…. He has gotten that far yet?
      There IS “maybe” a cheap version of the insulation(I can’t find out the costs) I just found a bunch on craig’s list for mine. I google it to see what it was. It is just backed with a fiber type stuff not foil & is used in the Industrial roofing trade. Some have a taper to them to pitch with? but if you can find it to buy maybe cheaper?

  13. Thanks so much for the post! We have decided to use this subfloor design for our tiny house in upstate New York! We have a clarifying question with the “first slice of the sandwich” (the first plywood sheets). I want to be drinking lemonade, so what fasteners does Todd use to fasten the plywood down? Does he drill and use screws right into the steel frame from above? Any recommendations?

    Chris and Michelle

  14. Where did you get your rigid board insulation? I want to use it on my trailer but can’t find the thicker sizes ANYWHERE!

  15. What brand of epoxy sealer did you use and where did you find it? We are in the process of building a tiny house and like thr idea of doing the subfloor like this. Thanks for your posts!!


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