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I'm trying to build several 24"x 24" square planters with each wall 1.5" thick, similar to the following picture.

The issue is that each one of these planters will weigh at least 200 pounds (without soil), and they will be used in a rooftop. This means that five planters will be 1,000 pounds at least, and our rooftop isn't designed for so much weight.

I used concrete because it's cheap (at around $4/60-lb bag) and it's easily manageable.

Two questions:

1) Is there any building material, similar to concrete, that doesn't weigh that much?

2) Is there an easy way to build 24"x 24" pots? I'm sort of a DYIer, but I'm a beginner with all of this.

enter image description here

  • why are you putting planters on the roof? Do they have to be square planters? if so why not wood with a liner inside? if not why not plastic (can be painted/textured many ways)? – depperm Aug 26 at 15:52
  • I put them in my rooftop/terrace because it's the only space I have for plants. – rbhat Aug 26 at 16:33
  • They don't have to be square, but for a DYIer with vey limited skills, the square planters were the least-hard to build. – rbhat Aug 26 at 16:34
  • All large planters (plastic being the cheapest) are over $35 each. I figured I could build something more unique for much less than that. – rbhat Aug 26 at 16:36
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    Why not those strong plastic buckets painted green? – Solar Mike Aug 26 at 16:51
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Concrete is a mix of four things:

  • Water
  • Portland cement
  • Sand
  • Gravel

The magic happens between the first two. When the portland cement gets wet, it has a chemical reaction (which makes heat) that turns it from a powder into stone.

The last two are just filler materials. It improves strength, reduces cost, and greatly reduces the heat of curing, since sand and gravel don't get hot. Without this, the heat would be unmanageable.

Nowhere is it written you must use sand and gravel. They make a variety of alternate filler materials where weight matters. First on my mind: crushed polystyrene.

Obviously if you are replacing sand and gravel with something else, that means you can't use pre-mix concrete. Pre-mix concrete is a bag with the portland cement, sand and gravel pre-mixed. Since you don't want the sand and gravel, this won't do.

For that matter, nowhere is it written you must use Portland cement. For instance you can also use epoxy or a variety of other plastics (see TAP Plastics' website for quite the variety) -- either thick with a variety of fillers.

Or you can work "thin", with fiberglass as a very thin but structural material. You can also do "thick" fiberglass, where you do a skin of fiberglass over a polystyrene block "core" - the styrene provides form and compressive strength, and the fiberglass provides tensile strength and a strong skin. Fiberglass boats are built this way.

No matter what you do, you'll need a form and a release agent to get the form to let go of the finished work.

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Add vermiculite to the concrete mix. The process is explained here:

How to make Vermiculite and Contrete Planters

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