Beavis, I've Got Wood...

So despite a bit of a lull since my last post, I've managed some good progress on the trailer over the past month.  With the frame modification more or less complete, it was time to move on to the wood framing of the trailer.  Because of the over-sized design of the trailer, a good bit of plywood would be needed.  In total, nine sheets of 4'x8' would  be needed- three sheets of 3/4" exterior grade sheathing for the deck, and a total of five (2 1/2 per side) sheets of 3/4" higher grade interior/exterior grade for the walls.  A shopping trip to Home Depot was in order.

Given the size limitations of the Subaru, my dad agreed to meet me at Home Depot, and we loaded everything into his utility trailer to get it back up the mountain to his house.

Also included in the shopping trip were several pieces of PVC moulding, plastic paneling, and FRP adhesive, which is a fiberglass product designed to adhere plastic paneling to wood.  All of this is explained in detail below.

We decided the starting point for the wood construction would be to assemble the deck.  Based on the planned dimensions of 61" x 102" we needed to piece three sections of plywood together.  Also, we needed to find a way to seal the underside of the plywood since it would be exposed to road spray and other moisture.  In researching a solution, I'd read a variety of techniques such as using roofing tar and wood sealer, pressure treated wood, etc.   Each of these seemed to have some serious limitations.  What we came up with was to use the plastic paneling and FRP.  This way, we would a have a completely moisture proof seal on the underside of the trailer.  The way the FRP works is to trowel it onto the plywood, using the teeth of the trowel to ensure a consistent application.

Once the FRP has been evenly spread, the plastic paneling can be applied, and a roller used to remove air pockets.

In anticipation of needing to mount the plywood deck to the trailer frame, we needed to create a level surface on the frame.  Since the Northern Tool trailer is a bolt together design that has bolt heads protruding up from the top of the frame, we decided to use 1/4" PVC trim as essentially furring strips to spacer the deck up from the frame.  Using some all purpose adhesive, we tack glued the strips to the deck, which had been cut to length to span between the bolt heads.

With the furring strips in place, it was time to mount the deck.  After a false start with some Phillips head hardware we decide on using #12 x 2" self-tapping 5/16" hex head metal screws to mount the plywood deck pieces to the frame.  This would allow us (in theory) to drive a screw directly through the plywood, plastic paneling, 1/4" furring, and 1/8" steel frame in a single self-pilot drive.  In reality, what we found was that the self-tap screws had a hard time getting through the steel, and we needed to pilot drill the hole with a bit.  To ensure a flush mount, we using a forstner bit to drill a recess in the plywood to allow the heads of the screws to be flush with the top surface of the deck.  My dad happened to have a nice depth stop attachment for his drill, allowing precise recess holes to be drilled.

When complete, the driven hardware had a nice, smooth, professional look to it.

This part of the process really took some trial and error to get right.  We managed to break a few drill bits, we chewed off a few self-tap heads, and hit parts of the frame where the steel is doubled-up, which the screws could not get through.  A complication of this task was that you needed to drive the screw with consistent speed until it pierced the steel frame.  If you paused the drill when you got to the steel, the head of the screw would bugger and never make it through the steel.  Even worse, the leftovers of the chewed up screw would prevent you from re-drilling the hole, so you would have to start a new recess hole and pilot hole somewhere nearby.  The other problem with this is that excess holes in the paneling were created, which would need to be sealed with silicone.  The other little trick of this technique we figured out was that, to avoid over-driving the screws, you would need to stop the drill after piercing the steel and crank the torque setting down on the drill.  Sometimes this did not work though, and the screw head would bite too far into the plywood.  Oh well, after a few goofs we got this technique worked out and managed to fasten all the screws.

As mentioned, over-sizing the deck included cutting plywood larger than the steel frame by 3/4" on each side to allow for a larger mattress inside.

To create a flush side, a piece of 3/4" PVC trim will be screwed to the side of the frame underneath the plywood overhang.  The plywood wall will then mount up to and bolt through the PVC trim.  This process will be explained in more detail in the next post.


Since the deck would go on in three pieces, we needed to create a seal between each of the pieces of plywood.  We decided to do this by overcutting the plastic paneling.  Specifically, we left about 1 1/2" of paneling overhang on one side of the front two pieces of plywood.  We could then apply a heavy bead of silicone to the paneling which would overlap with the adjacent piece of plywood.

This proved to work very well.  As one last step to finish this technique, we belt sanded the joint between the two plywood pieces where a slight lip was present due to the overlap of the plastic, to create a flush surface.

One last step was needed and the decking would be complete.  Since plywood that has been sealed on only one side is prone to warping due to uneven moisture absorption, we applied a good coat of Minwax Spar Urethane to seal the top side of the deck.

Of course, during the process of applying the urethane, one of Murphy's Laws kicked in and it began to rain.  Thinking quickly, we moved the trailer and hung a tarp over the power supply to my Dad's shed (don't worry- it was a well-insulated 220 extension cord, not an overhead line) to form a temporary shelter while the trailer dried.

After an overnight of drying the deck was finished.   Next up, assembling the walls.  This should be a difficult part of the project where things should really start to take shape, but more importantly this trailer will start to look like a camper.  Until next time...