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.
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.
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.
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.