Plywood Sub-Floor

Mod: Insulated Floor – Part Six

During this part of the project, I install the plywood sub-floor and seal it to protect it against moisture. As you may notice, I forgot about the staining until after the installation and fastening of the plywood, so only the visible plywood was sealed with Shellac. You may also stain the other side or abstain entirely as sealing is clearly an option.

Project Insulated Floor Content

Tie Downs & Wheel Wells
Paper Plywood Templates
Floor Insulation
Fasten Insulation With Glue
Spray Foam Application
Plywood Installation
Hardware & Soft-Ware

What You’ll Learn:

  • How to cut the plywood sheets by using the paper templates.
  • How to add dadoes to lock-in the plywood.
  • How to fit the plywood sheets.
  • Why to seal the plywood.

What You’ll Use:

  • Table saw.
  • Jigsaw.
  • Small paint brush.
  • Wide paint brush.
  • Tape measure.
  • Marker.
  • Sandpaper.
  • Painter’s tape.
  • Safety glasses.
  • Hearing protection.
  • Vacuum.
  • Optional:
    • Hand saw.
    • Circular saw.
    • Router.

What You’ll Need:

  • Paper templates (made earlier in this project).
  • 3 sheets of bcx plywood (exterior grade) – 4′ x 8′ 15/32” each.
  • 4 oz of Shellac flakes.
  • 32 oz (4 cups) Denatured alcohol.
  • Optional:
    • Marine varnish or
    • Polyurethane.

Approximate Duration For This Project: 4½ hrs. for the plywood and an additional 2 hrs. for the Shellac.

Plywood Sheets


Inside Height
The basic composition of the sub-floor in an RV or van conversion should consist of a bottom layer of one-half to one inch of Poly-Iso insulation and a top layer made of exterior grade plywood. The standard ¾ inch thick plywood is sufficient to securely hold all cabinetry and appliances. Don’t use MDF or Particle board as these products have weight and/or moisture issues.

For practical reasons, I have to deviate from these standards as a result of the roof height. The medium roof Ford Transit has an interior height of about 69 inches between the top of the floor ribs and the ceiling cross members. Even with my personal average height, there is very little space left for floor or ceiling materials, without sacrificing the ability to stand up straight in the vehicle.

If you have a high roof vehicle all of this does not apply, but I decided to only use Poly-Iso insulation strips in between the floor ribs, with only a half inch thick plywood sheet on top of that. That and taking my shoes off, will hopefully solve this issue.

Weight is another issue that may change your choice of materials. These sheathing materials are heavy and make a big impact on the overall weight allowances of the individual vehicle. Dependent on the tonnage and size of the van, you may decide to use relatively lighter or thinner materials. The half inch (actually 15/32 inch) thick plywood that I use, adds another 73.5 lbs to the conversion.

As a sub-floor, I prefer to install the plywood sideways as the finish flooring will be installed lengthwise on top of that. But the size of the floor will probably dictate, how to lay the plywood most efficiently to save a few bucks in materials. I ended up using three sheets of plywood, with lots of scraps leftover.

Step-by-Step Installation

Gather all the tools and materials before proceeding. Regularly fit the materials, to avoid costly mistakes.

  • Tape the paper template to the plywood.
  • Mark the cut-out areas.
  • Cut out the marked areas. Stay within the lines, to slowly ease into the final cut.
  • Check regularly by fitting the ply sheet.
  • Mark and cut any additional areas that need to be removed.
  • While fitting, make sure that a ¼ inch gap exists between plywood and the surrounding areas. Adjust as necessary.
  • Cut the rear plywood sheets width at the center of the central floor rib plus ¼ inch.
  • Cut dadoes at the front & back (from the top, ½ inch wide, ¼ inch deep) and left side (from the bottom, ½ inch wide, ¼ inch deep) of the right, rear plywood sheet.
  • Cut dadoes at the front & back and right side (from the top, ½ inch wide, ¼ inch deep) of the left, rear plywood sheet.
  • Cut a dado at the back (from the bottom, ½ inch wide, ¼ inch deep) of the front plywood sheet.
  • Be sure, the dadoes interlock.

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Sealing the plywood floor has the advantage of protecting it against rot and mold. It also makes these materials more difficult to ‘breathe’, where moisture can evaporate and the materials can dry,

So, optionally you can apply some protection to it with a (Marine) varnish or polyurethane. I love to use Shellac. As a hobbyist woodworker, I use Shellac flakes, mostly because I can store it for an extended period, while the canned version has to be used within a few months.

For this application, I use a 1 pound cut (4 oz of flakes with 32 oz/4 cups of Denatured alcohol) and it takes a day for it to be ready to use. Just add the two together and swirl regularly to help to dissolve the flakes. It has an orange/amber tone when dry and it dries quickly; add Denatured alcohol if you need more time.

Clean up is easy: give brush a swirl in Denatured alcohol and let dry. It will dry stiff, but gets back in shape the next time you use Shellac.

Shellac Flakes: The Shellac Shack

Shellac is a resin secreted by the female lac bug, on trees in the forests of India and Thailand and used as a brush-on colorant, food glaze and wood finish.

Step-by-Step Installation

  • Prepare the Shellac solution the day before.
  • If you paint in the right sequence, clean the plywood and apply the Shellac on both sides and edges, if not:
  • Vacuum the floor.
  • Paint the edges.
  • Paint the surface.


This part of the project involves heavier materials and heavier tools, yet is still within the grasp of the ordinary DIY’er. The use of the tablesaw can easily be substituted by a circular saw or a router.

The plywood was acquired locally and the Shellac flakes on-line and the total cost was about $76.00.

Other projects of this Van Conversion:

I’m just a DIY’er with a lot of common sense, but with some of the projects, I use some tools and materials, that require you to really know, what you’re doing. Always read the manual and consult an expert if you’re in doubt.

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  1. Hi!
    I love your site, thank you for sharing. I just got a Med roof Transit, yay! Any thoughts on a lighter wt. floor base? I’d like to apply the peel and stick wood look flooring to a light base.
    Thanks again!

    1. Great to hear from you! And congratulations with your new van.

      Weight in general is a constant issue that I think of during the conversion and is always a compromise between minimalism and practicality. There are the maximum weight limits of the van (GVWR’s as expressed in T-150, T-250 and T-350), but also the fuel efficiency of a lighter vehicle.

      Many thoughts about finish floor materials. The most used materials in RV’s are carpet, vinyl and laminate flooring, but the weight differences between them are not major, although the vinyl is probably the lightest. Exception would be porcelain floor tiles, but these are not really suitable for RV’s anyway. You might want to consider a one piece or tiled rubber flooring material.

      An advantage of carpet and vinyl is, that it is easier applied (at an early stage of the conversion) wall-to-wall, with the added benefit for vinyl that it can keep, the sub-floor better protected from moisture. That however, increases weight again. Much less weight can be gained with the choice of sub-floor: choosing 1/2 inch thickness over 3/4 inch and plywood over particle board.

      I have decided to install a wood floor (towards the end of the project); thin yet much heavier than most other materials. Applied only to the exposed walking area in the van and leaving the bottoms of the cabinets unfinished, thus saving some weight. If less weight is important to you, you might limit your vinyl to the exposed walking area too, but that makes it a bit more complicated to install.

      You really can’t go wrong. It’s more a matter of having too many choices, but that’s a good thing, isn’t it? Choose a material that fits your abilities to install, compromise according to your own desires and try to keep a realistic tally of anything added to the conversion.

      PS. Although warm, good looking and affordable, carpet would be my last choice. The concentrated, intensive use and the heavy exposure to the outside, will bring in lots of dirt and sand, making it next to impossible to keep clean.

      Keep in touch!

      Van Williams

  2. Thank you for your quick reply! Will probably do 1/2 inch plywood and vinyl unless I find a good source for a 1 piece rubber floor mat. Should have squeezed the dealer for it 🙂

  3. Hi there! I’m starting my build with the same Transit van. Super exciting. Do you think the floor would be strong enough with 1/4″ plywood subfloor? Overall it’s cutting weight.

    1. Hey Nhi,

      Congratulations with your Transit van. Being a woodworker, anything that you would walk on should be 3/4 inch thick. You can get away, for the same reasons you already mentioned, with 1/2 inch ply because it’s such a small space with a fairly rigid surface. Don’t even try 1/4 inch; go to a home improvement store and see how flexible these panels are.
      It is a lot of weight, but remember, it’s the base for everything else in your van. Using opposing rabbets where two panels meet, helps the integrity of the floor. And don’t forget appropriate insulation material under the sub-floor.
      Best of luck with your conversion, and please get back to me if you have more questions.

      Van Williams

  4. Like you I am trying to preserve as much height as possible so was thinking just insulating the low ribs in the floor. How has this worked out for you? Do you feel it was adequate?

    1. Hi Blake;
      With a Medium Roof Transit, you probably have not much choice; with a High Roof I would add a 3/4 or one inch Poly-Iso board.
      I haven’t tested it out yet under colder circumstances. I think that before cold becomes an issue, I’ve likely started using the heater, which again negates the problem. Unless you’re camping regularly in colder climates, it should be less of a problem. Cold air sinks, so the bottom of the van is least likely to be highly affected. Insulation in the walls and ceiling are more important. I probably won’t know until the winter of next year.

      Van Williams

  5. Great video! You don’t mention any method for attached the subfloor to the van. Is the idea that it’s a snug enough fit that with the weight of the floor and the cabinetry on to of it you don’t need to actually attach the floor to the van itself?

    1. Hi Phil,
      Some implement it as a sort of ‘floating floor’, but I would secure it to the metal frame of the van. You never know what can happen in an accident!
      I show you in the last of that series of videos how I did it:
      I used a couple of T-Nuts in each plywood panel, with bolts inserted from underneath the vehicle. The lap joints where the plywood panels come together, also help it stiffen up.

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