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Raft Construction

This is the information that I would like to have had when I first constructed my first rafts. Hope it helps someone else.


The rafts need to not:

  • release harmful chemicals
  • sink from the weight of the future plants
  • get “water logged”
  • fall apart
  • break when moved full of plants

The product that does this is Dow Blue Board Styrofoam insulation.

Painted Foam

Painted Foam

Styrofoam Overview

Styrofoam is a polystyrene-based insulation product that comes in two type,  expanded and extruded. Dow Product Summary

Expanded Styrofoam

The most commonly thought of is the white (expanded) Styrofoam we see in cheap ice chests and used as cushioning material in shipping. It is:

  1. composed of thousands of  “beads” which are pressed together (expanded)
  2. brittle and fragile
  3. a poor insulator
  4. absorbs and holds moisture (hygroscopic)

Extruded Styrofoam

This is Styrofoam Square Edge (extruded) often called “blueboard” due to its light blue color. Dow calls it STYROFOAM™ Brand Square Edge Insulation Extruded Polystyrene. It is:

  1. one piece with air gaps sealed inside (extruded)
  2. brittle but less fragile
  3. about 20% better as an insulator (modest as an insulator goes)
  4. is completely impervious to moisture (hydrophobic)

Types of Blue Board

STYROFOAM™ Brand Insulation Extruded Polystyrene comes with several options.

  1. Square edge
  2. Tongue and Groove edge
  3. Scoreboard

The Scoreboard has grooves formed in the surface lengthwise to make breaking the board easier during installation. Avoid this as it  encourages you to make smaller pieces of your raft, impromptu.

Square Edge Blue Board

Square Edge Blue Board

Score Board Blue Board

Score Board Blue Board

Tongue and Groove Blue Board

Tongue and Groove Blue Board

Images from Dow website


Dow “Blue Board” is available in many R values, thicknesses and two sheet sizes. Rafts are usually constructed from the 2″ thick, 4 foot by 8 foot sheet.


The UV from the sun eventually cooks everything. To delay that and to reflect heat you should paint the top of the foam with several coats of  a good quality exterior latex white paint. Scrimp here and likely you will get pealing paint mess to deal with later.

Raft Size

Over all raft size:  When floating foam on top of the water in a trough constructed using a pond liner (PVC), care should be give to allow extra space around the edges. As the water level drops the trough size becomes somewhat smaller. Also, the material is thicker on the corners where it is folded to fit the trough box.

I’d recommend you cut the raft smaller than the inside dimensions of  the wooden trough, by 1 and 1/2″ on all sides. That would be 3″ off the width and 3″ off the length as compared to the inside dimensions of the trough box without liner.

Modified Trough Width

Four feet is a long way to reach, so you tend to work from both long sides. More than once I have reached too far and nearly taken a dive onto my rafts! (But hey! my dogs did it on purpose, ONCE!) You might consider building your troughs 36″ wide, with the rafts 33″ wide. That way you  can almost reach all the way across, and if you can’t, then it is likely you will catch your fall on the opposite side wall. This also helps when handling the rafts full of plants.

Raft with Holes

Raft with Holes

Raft Sub-Sizing

Notice my horizontal cuts in the 4’x8′ raft image above.

When you plant in the raft you can take it out and set on a table or sawhorses. When you harvest you can do the same, to avoid stooping and bending a lot. Friendly Aquaponics (FAP) recommends cutting the raft into sub sections, example – “4 foot” across by 2 foot wide, or so.

You might want to consider cutting your raft into equally spaced widths. If you cut them 2′ wide then the last one is usually less than 2′. If your trough length is 96″ ID, your raft might be 3″, say 93″. Four pieces about 23″ wide each would look nice.

Measure / locate your holes for your net pots, before cutting, to make sure you can space them appropriately.

Hole size & spacing

I chose to use all 2″ net pots and therefore 2″ holes in my raft. It made it easier to make spacing look nice, and I figured most of what I grow to start will be smaller plants, IE no Taro, or “Jack in the Beanstalk” type plants.

Net Pot

Net Pot

To space these evenly I made a template to locate each hole. I just used a piece of 1/4″ plywood, drew lines every 3″ horizontally and vertically, then drilled a 1/4″ hole thru the intersections of the line.

Raft Template

Raft Template

Next I laid the template on the Styrofoam and drilled holes. That left location where I would drill holes with my new, sharp 2″ hole saw.

Hole Saw

Hole Saw

My hole saw, and most others, would not drill all the way through the 2″ foam. I just drilled all the holes from the top, then flipped the foam over and drilled into the same 1/4″ holes to complete the plugging operation.



Notice how rough the hole saw saws the Styrofoam edges.

Remember to drill straight down or your holes will have taper or ridges.
Also, save the plugs to fill the holes when you have no plants in them. This keeps out the sunlight and lessens the algae growth.



Here are some examples of the layout patters I came up with. 7 variations for locating the center of your hole. Using a larger hole and net pot give larger plants more support, like Taro. Putting the holes closer together like pattern #6 makes it easier to work with seedlings that require less space until well established. Note: blocks are 1 foot squares, with #3 being 2 foot wide. The overall dimensions assumed were 4′ x 8′.

Plant Spacing Raft

Plant Spacing Raft

Locating Blue Board

I had difficulty locating the Dow Blue Board, in any type. I contacted all the expected sources I could think of, Home Depot, Lowe’s, Lumber yards, 84 Lumber, insulation companies, even one custom safe installer who used it in the past – no luck. And I’m in Dallas, you’d expect I’d at least stumble over it in my hunt!

Note: Avoid the temptation to use other products. They will break easy, get soggy, come apart, or leach bad chemicals, etc.

I emailed the Dow company representative who gave me 3 suspects, non carried the product, at least in stock.

Eventually, I found a vendor that did not stock it anymore and only took orders for truckload quantities. He did call me back when he asked one of his customers where they buy theirs. Amazing! A salesman that is customer focused! Rare indeed. You can bet I’ll shop his store in the future.

In Dallas you can get the scoreboard type at  SSI on Highway 35E near Walnut Hill Lane. Silicon Specialist Inc. It was $50 a sheet, and they were in crisp, clean, new shape. The guy even hand deliver them to me so as to not damage them with the forklift.

I still have no idea how to find Dow 2″ Blue Board Styrofoam Square Edge insulation, without the scores.

The two 4′ x 8′ x 2″ pieces I bought fit in the rear of my Honda minivan, once I remove two rows of seats.

I hope this info expedites your Aquaponics adventure.


To give credit were credit is due, the basis for my trough building experience and research came from the Friendly Aquaponics (FAP) Micro System 64 (/128) plans that I purchased. They are very educational and useful. I’d suggest you purchase them and build the FAP system!

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