Float My Concrete Boat

That is floating concrete. It is amazingly light. While making the roof of the dog house, details to come, I saved a blob of the light weight concrete and let it harden so I would have a sample for density tests. In that photo above it is floating in a bucket of water. If you press it to the bottom and release it the concrete blob jumps back to the surface. It doesn’t just barely float, it is up above the surface – as can be seen in the photo 90% of the blob is above the water. No iceberg is it.

So what is the use of floating concrete? Well, I suppose you could make a dock out of it for the lake. I’m not sure if over time it would soak up water and sink. My purpose was to create roof insulation for the test roof we put onto the dog house. Below that layer is the hard structural shell of the barrel vault. Above the light weight concrete is another hard layer. This produces an insulated roof with greater structural strength than had it been just the two layers or a single thick layer of hard concrete. It’s a laminated beam effect.

The formula we worked out in parts by volume is: 1.5 water with dish soap for a surfactant a.k.a. water reducer, 2 portland cement, 7 vermiculite (popped rock), 1 small handful PVA fibers.

Outdoors: 74°F/43°F Sunny
Farm House: 69°F/60°F
Tiny Cottage: 71°F/63°F Bathroom vent work & cleanup, Sump forming

About Walter Jeffries

Tinker, Tailor...
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48 Responses to Float My Concrete Boat

  1. pablo says:

    Floating concrete. Is this your own recipe or is such a thing commonplace outside the mad inventor’s lair up there in Vermont?

  2. It’s one of my own recipes. Or I should say one of them. I’ve developed a bunch of different ways of making ultra light weight concrete. One of them is like whipped cream utilizing egg whites. But I don’t have enough eggs to do the whole roof of the house. Other recipes utilized various other foaming agents I had on hand around the house as well as light aggregates such as packing peanuts, styrofoam beads, foam board, glass spheres, perlite, vermiculite (this one) and other stuff.

    I’m sure other crazed inventors have come up with similar things in their mad scientist laboratories on farms and mountain top castles all around the world. :)

    You should see my recipe for ‘chocolate’ concrete cake. Light and fluffy on the inside, hard and crunch on the outside. And it floats too! :)

  3. Anonymous says:

    Be careful Walter. Huge amounts of Vermiculite contain asbestos, which has a nasty habit of killing people.


  4. Hmm… My understanding is that the vermiculite with asbestos came from the Libby mine in Montana which was shut down seventeen years ago (1990). The vast majority of vermiculite does not contain asbestos.

    However, vermiculite is dusty stuff and we are careful not to breath it – treating it properly like any other dust such as the lime, cement and sand dusts we use.

    Once the building is done the vermiculite is encapsulated inside cement and the insulating layer containing the vermiculite is between two hard layers of sand concrete so there is no exposure.

    Still, thanks for the cautionary note and we’ll continue to be careful with dusty things and chemicals. As much as I prefer to avoid them, some are.

  5. Richard says:

    Youmade concrete float with egg whites would you be willing to share the recipe?

  6. Whip up the egg whites, add cement, keep whipping until stiff peaks. Like cooking. Experiment. Have fun. Don’t eat it. :)

  7. RedRussell says:


    Now you may be able to start building a community capturing all that drifting styrofoam and plastic in the oceans. Have you let it sit to see if it water logs(like pumice?) And if it does, could you keep adding packing peanuts to increase water proof air pockets?

    You got me thinking..

    but, I was typing to get in the mix and shed my pedals off the wall, and was wondering if you could suggest a mix for a small trench that I have laid river rock into to catch and direct runoff.<---Urban and Subruban water hog waste) I have stacked the stone on a steep grade at times and need to pretty much apply a permiable concrete or mortor. As I had the creative juices flowing, my baking experience kicked in and realized I needed to fill a gallon plastic bag- cut the corner- and apply as one would when decorating a cake with frosting. You seem to have some concrete pasties in your repritoir, and was wondering, what may you suggest? Any advice would be great.

  8. I would suggest clay. Mix it in with the soil and then make a fairly thick layer on top of about 1″ of mostly clay. Cover that with stones to protect the clay from wash. This makes for a self healing stream bed.



  9. Anonymous says:

    Hi Walter, I am an artist in Albany Ny and I was looking into casting some concrete sculptures. I was looking for a lightweight concrete that also has a smooth-ish surface? Do you have any recipes you can share with me that are like that? Thanks in advance for any help.

  10. Anonymous, what sort of mold are you casting into? I have cast in polyethylene and was molds with the styrofoam cement and gotten very smooth results. I suspect that a pumice or vermiculite would also cast very smooth as it is the cement particles filling in along the smooth mold surface that makes it come out so nicely. I never tried casting the whipped protein based cements.

    Do note that the light weight concrete is also correspondingly more fragile. One trick might be to do a mold splash like ceramics using 3,000 psi cement for the first 1/2″ or so and then fill the rest of the hollow with the light weight concrete if necessary.

  11. Anonymous says:

    I am going to be casting large geometric shapes, with either a plywood/MDF mold or foam insulation sheets mold. The pieces will be at most 5 cubic feet in volume. What proportions would you recommend for a pourable Styrofoam concrete? I have also heard that adding some fiberglass fibers to light concrete gives it a bit more strength.

  12. See these posts about the PVA fibers we used in our projects. Experiment.

  13. Charles says:

    Howdy Walter,

    I found your blog while searching for some info about lightweight concrete.

    I am replacing my floor in my family's 18' fiberglass ski boat. The floor and stringers are rotten. I was considering mixing lightweight concrete using perlite, air entranement admixture, PVA fibers, and maybe an epoxy admixture.

    The mix needs to be light, strong in thin layers (say 1 1/2" thick), and flexible enough not to crack when the boat flexes.

    My idea is to remove the wood floor and stringers. Then leave the existing expandable close-cell foam and cut some additional "trenches" 1 1/2 to 2" wide in the foam to form concrete stringers. Then top this with 1 1/2 to 2" of concrete slab as the new boat floor deck. Then seal this with epoxy paint like I did in my garage.

    What do you think about this idea? Any suggestions?

    Thanks for your thoughts.

    Charles Volz
    San Antonio, Tx

  14. Interesting idea. I know next to nothing about boats. If you try this, let me know how it comes out. Make sure it is strong enough. I imagine a boat takes a lot of beating from the waves. I build things that don't move, or at least not fast, more like glacial. :)

  15. Sir,
    Congratulations for your floating concrete.
    I am a civil engineer. Can I use Extruded Vermiculite with the help of cement and fine sand and give a layer on my terrace to prevent heat? OR Can I make a sheet of vermiculite and lay down on terrace to protect from the heat of the Sun.
    If Yes, What will be the bounding agent shall I use to make that sheet?

    • Yes, I expect you could do that. The cement is the bonding agent. A key is not to over mix it as the vermiculite breaks up from excessive mechanical mixing. I would test with vermiculite vs styrene and also consider an epoxy top coat to keep out water. If you have freezing conditions the water in the matrix could be an issue. You could create sheets, much like tiles, in molds. We did some like this although I don’t think the big ones are shown on my blog. Basically we took 4’x8′ sheets of 2″ thick insulation and then covered them with a thin layer of neat cement. Next we did two thin coats of long fiber cement. This we assembled into a larger structure and then coated again. It is light weight, insulating, very strong and durable. It could be used to make boats. I would add a stainless steel mesh if I were going to do it in salt water as a boat.

  16. Sir,
    Thanks for your prompt reply. If I can use cement as a bonding agent then in India we are getting Pozzolona cement also in addition to Portland cement. Which cement shall I use in my mix? What will be the proportion should I take wrt to vermiculite ? How much quantity of water shall I use ? Any chemical shall I need to add in this ? If yes, how much shall I have to add in it.
    In India people do the brick bat coba as a water proofing on tarrace. So I want to lay these sheets over this water proofed tarrace as a finished product. It means people can easily or feel comfort to walk on it without damaging my product. Please guide.

    • From what I have read the Pozzolona is an additive that makes stronger concrete. See here. I would try that if it were readily available. I would use a fiber. We used Nycon fibers and were pleased with them. I’m not familiar with the brick bat coba. Is this it? I’m not quite sure I follow what they described there. It sounds like what we call a adobe parge or something similar. We did this on the outside of our cottage to about 12 mm of thickness and then textured it to look like rock walls. See here. In our case it is to protect the insulation.

  17. R.yamini priya says:

    I have seen your floating concrete, it was great.I am a civil engineer. Now i am doing a project work in canoe , i need your help this is a event which was conducting by a university. Can you tell me the correct proportion of mixing to float a concrete.

    • The last paragraph has the proportions we used but for a canoe I think you might want to do something very different. I’ve read of people building very classy canoes out of ferro cement doing very thin layers. Check out this search pattern on google and also see these photos. There are some ferro cement boats that are over 100 years old. Quite amazing.

  18. ann says:

    hello…..i am wanting to find a lightweight concrete recipe/advice to use in heavy plastic molds. the molds are for stone veneer to be used on my walls in my home. would you mind sharing a good light recipe for this project please? thanks very much! ann

  19. David says:

    I watched a large building put together with Autoclaved aerated concrete blocks. It was a large accommodation block in an oil terminal in the Persian Gulf. Once complete it had fantastic thermal properties. The blocks are quite fragile but once they are coated with a light mortar, they are just as hard as concrete.

  20. Shaun says:


    A quick question. How strong is your floating concrete? Does it crumble easily? Could I make something like a floating dock out of it or would it crumble as I walked and moved equipment on to it?


  21. Mohammed Anas says:

    Hi sir I need to do a floating concrete boat for my project , it should be capable of carrying load also. Can you please guide me

  22. kaveen says:

    hi sir iwant some information about floating concrete .can u please guide me

  23. Mike says:

    Neat. Really neat. You people do a lot of cool things with stone and cement. Love your projects.

  24. Brycorp says:

    Hey ok so i think i want to try this, i want to make 2.6 sq foot hexagon of this stuff. can you tell me what disk soap you used and be a little clearer about the recipe, i.e. how much of each ingredient do i need for an 80 lbs bag of concrete?
    I think this i brilliant, i want to see if i can cast a bunch of little hexagons and lash them together into a small floating island

    • scooby says:

      sir i need to cast a floating concrete slab of the following dimensions:
      50 cm long
      30 cm wide
      6 cm thick
      please suggest me the aggregates and other materials to be used and please give me their proportions too..

      • Generally the term “floating slab” means one that is on the soil without a foundation. Given the context of this post I take it you mean floating in the water? From your specifications it sounds like this may be a school assignment so I hesitate to do your homework for you. The basic principle is figure out what the weight of your bound (cured) cement will be and then figure out how light your aggregate must be for the volume it will occupy in your mix to produce that slab comparing weights with water. This gives you a density to achieve and then you can search for a filler aggregate that will match that need. Enjoy.

  25. scooby says:

    may i use vermiculite for this sir?

  26. scooby says:

    thank u sir… but what about the proportions for the design sir…

  27. scooby says:

    sir please tell me the ratio of aggregates,cement and water to be used for my design…

  28. gilbert says:

    where do you buy the fiber?

  29. phil says:

    Can you pm me on this? I want to pour a fireplace raised hearth in place. Codes on R-value are pretty adamant about using non combustibles. I would then place a granite hearthstone and a stacked stone veneer to finish it. I would like to keep weight down. What is your water to soap ratio and what was your weight savings?

  30. Rob says:

    Hey there, thanks for the information. I’m wondering if I you know or where I can find out which products would best make a light concrete that would be tough enough to handle the sea movement when used as 1 piece for the deck and 1 piece for the rooftop placed in between 2 fibreglass boat hulls please? Each piece would need to handle movement on the sea while under motor/sail. Thanks 😊

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