How to Make a Floor for an Inflatable Boat (Using Only a Box Cutter)

My wife and I see eye to eye on many things but we’ve had a long-running joke about a certain habit of mine: reading the comments on YouTube videos.

I have to admit, while I continue to check out the comments, the simple truth is that the vast majority of them aren’t worth a second thought. Why do I bring this up? Because I owe much of this post to a generous comment left on this YouTube video.

Long story short, I have an Intex Seahawk 4 inflatable boat and I’ve long wanted to build a DIY hard floor to add some much needed stability. Doing hours of research on YouTube and Google, I’m pretty sure that every floor I saw plans for used some kind of plywood or OSB sheets. While I’ve seen many floors for inflatable boats made with wood that looked great, I had a few main concerns that caused me to hesitate:

  • Way too heavy — I’ve hauled around a lot of 3/4″ thick sheets of plywood before and it’s never a pleasant experience. Therefore, I’m not too keen on hauling around a plywood base every time I want to take my boat out.
  • Possible water damage — While it’s possible to waterproof OSB or plywood, I knew I could run into some issues years later if the boards get wet (an inevitability on the water).
  • Would be difficult to transport to remote waters — One of my favorite uses of an inflatable boat is fishing remote lakes off the beaten trail in National Forests. Doing that and hauling a 60 lb wood floor? No, thanks.

The Solution: Build the Floor with Lightweight Plastic Cardboard

Wait, what’s plastic cardboard?

OK, so I’m taking a little bit of liberty here but that’s pretty much what it is. Basically, it’s a corrugated plastic, in that there are two sheets of plastic that are held together by plastic fluting (just like cardboard). As with cardboard, the fluting in this corrugated plastic runs in one direction. Here’s a picture that will help make some sense out of this:

showing the side profile of a sheet of corrugated plastic at a store

You can find corrugated plastic under a few different brand names:

  • Corflute
  • Coroplast
  • Polyflute
  • Twinplast

And so on. One of the most common applications you might have seen it with is for plastic campaign signs. Yes, those hideous plastic signs you see piled high in the trash every second Wednesday in November during an election year. How do you turn that kind of plastic into a boat floor? The idea is this:

Creasing this kind of plastic along the direction of the fluting is relatively easy, but it’s more difficult to bend if you try to make a crease that runs perpendicular to the fluting. Therefore, if you take multiple sheets of corrugated plastic and glue them on top of each other with the direction of the fluting alternating, you’ll end up with a much stronger product.

I know this may sound a bit complicated, but stick with me as I provide step-by-step instructions and an exact cutting list below.

An Aside: What If You Still Want a Wood Floor?

Before we get into the tutorial I wan’t to say that I do realize that some may still want to build a traditional wooden floor for their inflatable boats. While that’s not what I’ll be demonstrating, I wanted to quickly share what I learned during research in case it’s helpful to anyone. Here’s what I learned:

  • Plywood would be lighter than OSB, but it is more prone to water damage and you might want to try to waterproof it.
  • Most people covered the plywood/OSB with outdoor carpeting for additional protection and comfort.
  • While some did try 3/8″ or even 1/4″ sheets, most preferred to use 1/2″ and up to 3/4″ sheets of plywood/OSB and that performed well on the water.
  • It’s common sense, but avoid sharp objects like staples, nails, and screws at all cost: there’s no need to test just how puncture-proof your inflatable boat is.

Step-By-Step Plans for Building a Plastic Floor for an Inflatable Boat

I’m going to assume you have a box cutter (utility knife) laying around somewhere in your house or apartment. That’s pretty much the only tool you’ll need to complete this project. Here’s the outline of how this project will work:

  • Buy sheets of corrugated plastic from a big box store (sold usually in 4′ by 8′ sheets).
  • Cut the sheets down to size for transport in the parking lot (assuming you don’t have a truck).
  • Glue the sheets together, alternating the direction of the fluting.
  • Accurately measure your inflatable boat to find the size of your floor.
  • Shape each section to the correct size for your inflatable boat by cutting with a box cutter.
  • Add foam pipe insulation or duct tape around edging for additional protection.
  • BONUS: Cover with outdoor carpeting or paint with textured paint for better performance (not required).

Once complete, you’ll have an extremely light floor for your inflatable boat that is surprisingly strong and waterproof. All without having ever touched a power tool.

What to Buy from a Big Box Store

Everything you’ll need for this project can be found at Home Depot. The most important thing you’ll need is 3 sheets of corrugated plastic, with each sheet being 4′ by 8′ (just like sheets of plywood). I checked at Lowes and Menards, but they didn’t carry the corrugated plastic. Here’s the product I used from Home Depot:

4mm Thick Corrugated Plastic 48″ by 96″ (Coroplast)

I also bought two 20 gram bottles of a cyanoacrylate-based super glue made by Loctite. I’ll discuss this more later, but it’s important to understand this: the type of glue really, really matters here.

What Cuts You’ll Need to Make (and Where to Make Them)

Exactly which cuts you need to make will vary based on a few things. Obviously, the first thing to consider is how big of a floor your inflatable boat will need. This will vary from model to model, but my Seahawk 4 inflatable boat needed a floor that was around 92″ long and 31″ wide (at the widest point). So you’ll need to measure the size of the floor of your inflatable boat to understand how many sheets of corrugated plastic you’ll need.

Once you know the size of the floor you’ll be making, you’ll need to make a decision on how many pieces you would like the floor to be in. Many people make floors that are one piece, and often almost eight feet long. This didn’t make sense in my situation for a few reasons:

  • I wanted to keep my finished floor portable with a normal car.
  • Didn’t want to rent a truck just to transport the sheets home.
  • Multiple pieces make the floor easier to transport into backcountry lakes.

So I ended up deciding to build a two piece floor. Keep in mind that the more pieces you have, the less stable the floor will be as a whole. While a three piece floor would be even more portable, I just thought that it wouldn’t be as steady when standing on it. With all of that in mind, here was the process I used to determine which cuts I should make:

  1. Decide on the size you would like each floor section to be in.
  2. Decide the starting direction you would like the fluting orientated towards.
  3. Figure out how many layers you’ll need for the floor.
  4. Get as many section layers from single pieces, and then use pieces to create your last layer (if possible).

For me, I ended up making a two section floor where each section was 4′ long by 3′ wide. As I was using 4 mm thick corrugated plastic, I wanted a minimum of three layers and I was able to piece together a fourth layer from the scraps. I highly recommend at least three layers of plastic if you’re using the 4mm thick sheets (that’s what’s most commonly available, as far as I can tell).

Also, if you haven’t figured this out yet here’s the deal: you may end up cutting these sheets in the parking lot at Home Depot. At least that’s what I did, as I don’t have a truck to transport the sheets whole. It’s no big deal, just park a little further away and come prepared with the following tools:

  • Utility knife
  • Marker or pen of some sort
  • Measuring tape
  • A straight-edge in the length you need (4′ for me)

This may not be super helpful to you, but here are the cuts I made for the floor for my Seahawk 4:

Plans are not to scale.

Gluing the Corrugated Plastic Sheets Together

Important Note: Gluing the sheets together is definitely the trickiest part of this build. Corrugated plastic is made from a material called polypropylene, and it is very resistant to glue. Even super glues may have some trouble with this material and the glues truly suited for this application are quite expensive.

With that in mind, super glue that is based on cyanoacrylate works reasonably well if you use enough of it. How do you know if your super glue fits that criteria? Most bottles that I saw don’t list the ingredients, but just look for health warnings about cyanoacrylate and that should point you in the right direction.

The actual act of gluing the sheets together is quite simple. Here’s what you do:

  1. Clean off any dust and debris from the sheets (especially for parking lot-cut sheets).
  2. Very generously apply the super glue to one of the sheets (it won’t dry, you have a few minutes here).
  3. Being careful to place it correctly, place the other sheet on top of the sheet with glue.
  4. Use your hands to apply pressure evenly across the sheets to allow for even contact (be careful that they don’t shift).
  5. Repeat this until each layer is glued to its section.
two layers of corrugated plastic stacked on top of each other, with the direction of the fluting alternating
It’s a little hard to see in the above picture, but note that the direction of the fluting for the two layers are alternating.

Once I glued everything together I let everything sit overnight to give the glue time to dry. I’m pretty sure I could have cut the pieces earlier than that but I didn’t feel like rushing it. 

How to Measure Your Inflatable Boat for a Floor

The main idea here is to get a rough idea of the size and shape of your floor. I was fortunate enough to find an Instructables post that outlines how a user built a floor for their Seahawk 4. I confirmed that the numbers matched my boat and then used his diagrams to cut each section out.

If you can’t find dimensions online for your boat, I’d recommend flipping it over so the boat is on its side. The floor of your inflatable boat most likely has a dedicated air chamber, and therefore it likely will be possible to measure the size of the floor by finding the seam for the floor chamber. In the below picture I’ve drawn an arrow that points out the seam for the floor chamber of my inflatable boat:

How to Accurately Mark Up Each Section for Cutting

If you’re like me, the sections after they’ve been glued together weren’t exactly “square” and so it was a little difficult to figure out where to start when marking it up.

Here’s what I found to be the most helpful method:

  1. Use a straight-edge to draw a line running the length of the section.
  2. Determine the maximum width you’ll need for that section (was 31″ for me).
  3. Mark a few spots that are parallel to the first line and the maximum width away.
  4. Connect those dots with your straight-edge.

You now have two parallel lines on your section that are the maximum width of your floor apart from each other. So, how do you go about drawing the shape of the floor? Assuming that you have a symmetrical floor shape, I’d recommend the following method:

  1. Mark the midpoint between the parallel lines for the top and bottom.
  2. Plot the coordinates for each major point as it relates to the midpoint or sidelines (as in, down 3″ and over 4″; think of Battleship)
  3. Connect your plotted points while trying to replicate the general curvature of the outline of your boat floor.

Cutting Your Sections Down to Size

Now that your boat floor is marked up on your section panels, it’s time to cut it out with your utility knife. Even if you are using four layers of 4mm corrugated plastic like I was, this should be an easy task if your tools are up to the job.

Important Note: I’d recommend you start with a new blade for your utility knife before you start cutting. A fresh blade makes a world of difference and allowed me to make clean cuts with only one or two passes instead of a messy three or four passes. Trust me, it makes a big difference and you’ll be much less frustrated.

Also, I’d recommend that you lay down a layer or two of scrap cardboard to serve as a cutting surface. This will help to minimize how quickly the blade on your utility knife dulls.

Finishing Touches: Adding Trim and Fixing Weak Spots

After cutting out each section I took some time to go around the edge of both panels and look for weak spots that would separate easily. I just added drops of super glue into the cracks and then held them together for 30 seconds or so to form a nice bond.

Once the edges of your pieces are free of cracks, you might want to consider something like duct tape to “seal” up the edges from water and other debris. There are other things you could do here, such as using foam pipe insulation around the edges to create a snug fit with the sides of the boat.

inflatable boat floor for Seahawk 4 fishing boat made of corrugated plastic glued together and laying on ground
How my floor looked before its first test run.

How Much Did Everything Cost?

Here’s the truth: as much as I enjoyed this project, buying all the materials from the store made this inflatable boat floor a bit pricey for my tastes. All in all, it cost me almost $80 to build this floor from entirely purchased materials. While that’s not significantly more expensive than making a wooden boat floor, I was mostly interested in seeing if I could prove the concept laid out in the YouTube comment. You know, for science.

So, giving that I purchased everything from the store, here’s about what this project cost me:

  • 3 Sheets of Corrugated Plastic ($23 each)
  • 2 20-gram bottles of super glue (about $5.50 each)

With that said, if you don’t mind getting creative then there is a free option out there that can significantly reduce the cost of this project.

A Free Alternative: Election Signs

That’s right. All of those annoying campaign yard signs that are cluttering your neighborhood can actually be put to good use. Most of the time those signs are made from the same 4mm corrugated plastic that I used for this floor, they’re just in smaller pieces.

I haven’t done this yet, but I think it would be very doable to build a patchwork floor with a bunch of campaign signs. I know that I’ll be definitely checking around for extra signs after the next big election is up, as I think that might be a cool little project.

What About Performance on the Water? Is it Strong Enough?

My initial tests with this boat floor tell me that it’s definitely strong enough. I was able to easily stand up and fish while on the water, and I didn’t have any problems moving around.

Me standing on the inflatable boat floor and fishing.

I do have some concerns about the strength of the bond between the sheets, as I heard the occasional crack when taking steps. One concern I initially had was whether the 4mm corrugated plastic would collapse when you put your full pressure on it. I haven’t experienced anything in the likes of that, and I’m pretty confident that the plastic will hold up its structure over time.

I’m considering a few modifications to the design of the floor, and I’ll update this post with any changes I make. I’ll also keep it up to date to show how this floor holds up with time.

My Final Thoughts

While I’m very happy with how this boat floor turned out, I realize that it might not be the right fit for everybody. Here’s the deal: if you’re considering the floor to be a more or less “permanent” addition to your inflatable boat, then I think you’re best off with a wood floor. While significantly heavier, you’ll get the best performance and you can spend your extra money buying more trinkets for your inflatable boat.

With that said, a lightweight floor made from corrugated plastic is a wonderful option for those who would like to retain what I consider my favorite feature of inflatable boats: portability. Hauling this floor around is an absolute breeze and the performance on the water is more than enough for my needs. I very much look forward to using this floor on remote lakes that I would never want to haul a sheet a plywood to and for that, I’m grateful.

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