Introduction: TRASH ROCKS -- Eliminate Unrecyclable Trash

Picture of TRASH ROCKS -- Eliminate Unrecyclable Trash

To make a trash rock, a sack is first sewn out of fishnet.  It is filled with trash and plastered with cement.  The resulting shells are unique in shape and look very natural.  Trash rocks are an esthetically pleasing and constructive way to eliminate trash.

Trash rocks can be used as benches, tables, sculpture bases, landscape accents, and walls. A family living in one location over time could build a castle out of their trash. I would expect trash rocks to have good thermal insulation, useful in both hot and cold climates.

I'm big into recycling and built my whole house out of recycled nylon fishnet and cement, a material I call nylon-cement. For many years I eliminated all my trash right at home using trash rocks. 

Ideally, I would like to see a chemist develop a way to recycle some of our plastic trash and make a mesh material like fishnet out of it that could be plastered with cement. 

Recycling is all about mining trash; converting waste into something useful. If we separate our trash first and put it into separate trash rocks we would know where to look for specific recyclable materials in the future when we need them.  In the meantime, why not enjoy living around all the trash we generate? 

Step 1: Scoring the Fishnet

Picture of Scoring the Fishnet

Many years ago I bought new fishnet by mail from a fishnet manufacturer. That seemed fairly expensive at the time; $6 a pound, I believe. I wouldn't be surprised if it is double that now, or more.

Then I found the free fishnet mother load of all time right under my nose, the StarKist tuna factory. They were very helpful to me in saving used fishnet that the boats wanted to get rid of. Discarded netting is a trash disposal problem for the factory, so we helped each other out.

After getting it home, the fishnet was opened out, cleaned off, rolled up and stored outdoors. It smelled "fishy". Given a month or two of exposure to rain and air it was completely user-friendly. Fortunately, I live in the country, where I can do this without offending the noses of neighbors.

Good luck finding a source of your own. Fishing ports and fish farms are good places to start looking for used fishnet. Ready-made trash sacks that can be plastered with cement should be available for this idea to really take off. Sewing your own sacks allows you to make different sizes of trash rocks, but ready-made sacks would save some time and effort. This is a link to manufacturers of fishnet. Most are in the Orient. A search for "netting" on Thomasnet can come up with U.S. manufacturers.

Step 2: Sewing the Sacks

Picture of Sewing the Sacks

Sew the sacks however big you want your trash rocks to be. I made my own super-sized curved sewing needle out of heavy wire. Hammer one end of the wire flat and drill a hole in it to make the eye of the needle. Make a rounded point at the other end.

I use nylon twine to do the stitching with. When protected from sunlight by the cement, nylon will last a long time.

Step 3: Fill the Sack

Picture of Fill the Sack

Fill the sack with your trash that nobody wants to recycle. Use twine and the big sewing needle to sew the mouth of the sack closed.

Step 4: Site Preparation

Picture of Site Preparation

Depending on where your trash rock is going, you may have to prepare the site. In order for new cement to stick well to old cement, the old cement has to be clean. A pressure washer works great for cleaning cement. Throw down some cement before placing the trash-filled sack in order to adhere the new rock to the old.

If you are putting the trash rock on the ground, you might want to dig a little nest for the trash rock to sit in. Throw some cement in the hole before setting the sack of trash in it. That gives you some foundation, and might prevent animals from burrowing up into the trash from underneath.

As with building any rock wall, keep in mind how the next row of rocks will sit on the row you are working on. Plan ahead to avoid problems.

Step 5: Mixing the Cement

Picture of Mixing the Cement

The normal cement mix for plastering is one part cement to three parts sand. 1, 2, 3 is an easy way to remember it.

(The same 1, 2, 3 will help you remember how to mix concrete: one part cement, two parts sand, three parts gravel. )

You can mix small amounts of cement in containers, or in a wheelbarrow. I usually mix a sack at a time on a patio area with a square end shovel.

To do that, mix the dry materials first. Then shape the pile like a volcano and add water to the hole in the middle. Mix it and add more water if needed. Try not to add too much water. It is easier to add more water later than it is to patch up a too juicy mix with more dry material.

Depending on the size of the mesh of the fishnet you are using, you might want to have the mix be drier or wetter. Larger mesh might accept a drier mix, where a thinner mix would fall through the holes. With fine mesh, you might want a wetter mix for better penetration.

Step 6: Plastering the Sack

Picture of Plastering the Sack

After dragging the sack of trash to the location you want your rock, and setting it in a bit of cement, the best way to plaster it is usually from the bottom up, with vertical strokes of the trowel, but there are exceptions to every rule.

When you stroke upward, the trowel makes a sort of "V" shaped pocket in which the cement on the trowel rests. If you stroke downward the "V" is upside-down and the cement tends to fall on the ground. After the cement is up, it can be stroked in different directions without much problem.

You might want to brush the surface of the cement after it starts to harden up some, in order to knock off any sharp bumps that might interfere with plastering the next layer, or applying the color coat. Quite often, to get adequate thickness, you have to plaster the rock twice. The second coat is easier, because the first coat is solid at that time.

Step 7: Colorizing

Picture of Colorizing

Cement can be painted, or colorized by adding pigments to the cement. Paint tends to weather away, chip, and blister. When the pigments are part of the cement, colors are more permanent.  Powdered pigments for colorizing cement are available in hardware stores. 

Pigments, powdered or liquid, can be added to the cement used in plastering the trash rocks. Pigments cost money, though. Less pigment is used if plastering is done with uncolored cement and a thin layer of colorized cement is then brushed onto the surface. That is what I usually do.

When exposed to sun and rain cement will slowly wear away. The thicker the pigmented layer, the longer it will last. Colorized cement can be brushed on with a broom head or big house brush. One can also fling cement from a brush to create irregular spotty effects.

Exposed to the weather, even un-pigmented cement can become beautifully colorized by nature as algae grows on it.

I haven't colorized any trash rocks yet, but this photo shows colored cement effects on the wall of a house.  I added some concrete acrylic fortifier to the cement to hopefully make it more weather resistant. 

Step 8: Trash Rocks I Have Known

Picture of Trash Rocks I Have Known

These are some trash rocks I have made over the years. 


DanielS396 (author)2016-03-10

Hey! That's a whole house built with waste rocks of glass botles. Located in Morretes, Parana State, Brazil. "Pousada Itupava" for net searching engine... It pays a visit. Bottles were collected from waste thrown, by tourists, into Nhundiaquara River.
A place to stay some days and look the landscapes, and get into the rivers... But the waste rocks house is particularly beautiful to stay into, make some pics, and speak about its building with Mister Ibrahim, the owner. Fantastic!

DIY-Guy (author)2014-03-14

Great details!

For me personally I wonder if I'd just be passing off the problem to my children's children, just like the national debt. I watched the movie Wall-E and saw mountains of trash bricks in that vision of the future. A 3D printer seems a better way to use up trash plastic and reduce the purchase of new plastic products at the same time. It's not a solution for everybody though, so make- make- MAKE what you can!

Thinkenstein (author)DIY-Guy2014-03-14

How does a 3D printer use up trash plastic? I got one recently and it uses new rolls of plastic filament to make things. What I need is a home machine that makes filament rolls from trash plastic, and I don't see that on the market yet.

My trash rock idea does put the problem on hold, hopefully indefinitely in permanent construction. If we are smart, we would separate our trash first, so our children's children know which rocks to mine for clear glass, for example, or various plastics in the future when we are more hard up for used materials.

I mostly see the printers making novelty toys, or decorative oddities; things we don't really need -- generating more mountains of plastic to get rid of eventually. They can maybe make a cell phone case, but they don't include the electronics that go in it.

DIY-Guy (author)Thinkenstein2015-12-22

Sorry to get back to you so late Thinkenstein:
The 3D printer using recycled plastic solution to (some) trash is based on the make your own filament machine plans on the net. Depending on the grade of plastic, there are grinder/extruder machines to be built which recycle trash plastics into filament for 3D printing. Hope this helps, I apologize for not having a nice Google search string ready for you.

Thinkenstein (author)DIY-Guy2015-12-23

The make your own filament idea is good. I wish one could make plastic rebar and plasterable mesh material for cement work, too.

melliott11 (author)DIY-Guy2014-03-19

Don't we just keep building over urban areas anyway. What is the difference between building over this and great urban areas built over older layers. I really like this idea. Save the gas to haul it to the dump, the landfill space itself, and it isn't like these materials couldn't be reclaimed as I am sure the elements will break down the and maybe by then we will have better recycling programs in place. We have none where I live.

luckypalm (author)2015-02-07

Not a castle, but I have an insulated workshop in mind. Outer wall and inner wall 8" apart made of steel mesh to be plastered in cement and use trash for the fill. The fill isn't load bearing but I'm going for thermal mass and it's got to be cheaper to use cardboard and plastic refuse rather than portland cement.

radznjason2011 (author)2014-03-10

But isn't this just hiding the trash for later?

diy_bloke (author)radznjason20112014-04-13

isn't putting it in a landfill the same?

We are all just waiting to be compost. Our trash is all just waiting to contaminate the environment, since it may outlast us. We encyst nuclear waste in containers to protect us from it -- for a little while at least. Encysting our trash is a similar strategy. The longer it stays encysted, and useful as fill inside our construction the better the situation for us and the environment, I would say.

One problem with recycling is that it is not always profitable for someone to recycle things. In our system, nobody puts more energy into recycling materials than it is worth to them, so we will always have piles of useless crap building up. I take my useless crap and build with it, even if only as fill material.

The city doesn't really want to haul it away. Our dump is full, and we have to pay another city to take it. That city's dump is filling up rapidly. I live on an island and. Islands have limited dump space. Project far enough into the future and we have an archaeologist's paradise, but a bad life support system.

Got any better strategies?

diy_bloke (author)2014-04-13

I live in a neighboorhood were most families put out a 250 liter trash bin every other week and a 250 liter greenbin the other week.
still have the old 140 liter trash bin and I last year I put that out twice rather than 26 times. My greenbin I NEVER put out... I have a garden

Todd Gehris (author)2014-02-17

This is a future archaeologist's dream. These are like time capsules. :)

EngineerJakit (author)2013-02-10

I have found out that my nearby Wal-Mart sells nylon netting at about $1 a yard. Given the fact that my college doesnt recycle a LOT of plastic bottles, i think i might have a way to test my island plans!

ceekay (author)EngineerJakit2013-07-05

Please tell me what department that would be in. Fishing? Fabrics? I want to try making some trash rocks and don't have immediate access to fish nets.
Thank you

I take it you think of making a floating island. I have heard of a guy in Mexico who did that on some lake. I don't know if you could get away with that in the states. I hope the bottles don't just escape into the water eventually.

Thinking of the Pacific garbage patch, it would be nice if there was some sort of floating recycling plant that could turn the trash into plasterable mesh trash bags and plastic boats to float float it back to civilization.

Some chemistry student could do big help in the plastic trash problem if he could figure out how to convert plastic trash into mesh material that could be plastered.

EngineerJakit (author)2012-02-22

Man, this is some neat stuff you've made here! Do you think an island could be made out of this stuff, depending on the methods and materials used?

I made a floating cement trash rock once, filled with sealed plastic bottles. I suppose you could make a floating island, or one that rests on the bottom.

Also regarding the floating trash rock, what would your opinion be on making one thats in the shape of a boat hull? if the right amount of bottles were used, it could be done, yeah?

It would float, like a regular bottle raft. Without a skin over it, though, there would be a lot of turbulence in the surface area -- not like knifing through the water with a solid hull. It would be a boat shaped raft is all.

what if you added a few bottles and some cardboard to give it an angled keel along the bottom of the hull? Depending on how it would be fashioned, the fishnet would have to be cut in a pattern to cover the entirety of the boat itself.

Hm.m.m... a boat made out of soggy cardboard and plastic bottles. That sounds unique. Try it out first in a swimming pool, rather than on the ocean.

Here is an idea that I never tried out:

Shape a boat hull upside-down out of sand, like a sand castle. Sift (flour sifter) dry cement over it and trowel it down while still dry. It then soaks up water from the sand and hardens, making a thin shell of cement on the mold.

Plaster that with nylon-cement and let it harden. Tunnel under the edge and remove the sand. Carefully turn it over, without breaking it, (how, I don't know) and line the inside with aluminum cans, pop top openings taped shut, holes against the hull, cemented in place. That might make a cellular flotation layer. Use light-weight cement, possibly with ground up styrofoam filler, instead of sand, to connect the cans.

If you put cans side by side on the floor, you can actually stand on them without crushing them, so this might strengthen the outer shell considerably.

Experiment small-scale and work up.

mcenerny (author)2011-01-27

terrific "ible" I have been pondering making some lightweight boulders but could not settle on the "filler" - trash bottles, etc are a great idea. I am thinking that in lieu of fishnet, you might use the plastic fencing/trellising that comes in various sizes - or plastic screening??

Thinkenstein (author)mcenerny2011-01-27

Fishnet is great because it stretches in all directions. Plastic screening is good for cylinders, but not for balls.

Ideal would be for somebody to figure out a way to recycle some of our trash plastic into a plasterable mesh material.

Used fishnet is a great recycling score, but it's not easy for most people to find. Ideal would be trash sacks all ready for plastering and converting into trash rocks.

uniqueleigh (author)2010-08-12

I am curious to know if you have any ideas on what could be used instead of the recycled fishing nets. I understand that the netting, because it is nylon, would last longer than an organic material. I was wondering if (for smaller rocks) one could get away with using the net bags that come from the grocery store holding fruit, onions, etc. or even maybe the netting that is used in cast nets. The ones used for catching bait fish. My kids go through them quite quickly and I always have useless ones laying around. I live in Texas near the water and so I am hoping to get a chance to catch someone that runs a shrimp boat to check into getting some old nets but, would like to do some smaller projects until then. Make smaller rocks to go around the flower garden, stuff like that. That would be better with the smaller nets and that way I can still make up some trash rocks until I find the other netting.

Thinkenstein (author)uniqueleigh2010-09-15

For some projects, fiberglass window screen material works as a substitute for nylon fishnet. It has a finer mesh, and you can make thinner walls with it. I have used it to make a garden retaining wall, and also some minor wall fill-in at a guest house.

tulekah (author)uniqueleigh2010-09-14

thrift store or dumpster dive for old badminton or volleyball nets.

Thinkenstein (author)tulekah2010-09-14

You need a finer mesh to hold a thin layer of cement. With big holes, the cement falls right through.

Thinkenstein (author)uniqueleigh2010-08-13

Onion sacks don't work very well, because the center part is tightly woven for printing on. The throw nets sound like likely candidates. I prefer multi-filament as opposed to mono filament netting. I don't know how well the mono filament would work. Anyway, it's a good idea to start small and get a feel for it. Play it by feel.

tulekah (author)2010-09-14

1 how about compacting the trash into near solid blocks or shapes! a bone simple press is a long bar fixed at one end with heavy friend on the other.

2 cover the rocks where they are not going to get stepped or set on with hypertufa-moss mix (martha stewert recipe).

3 set flower pots into the rock, or make depressions, for flowers or ground cover.

4 grottos, hobbit homes, walls made of separate blocks leap to mind.

5 take a drive into the country with friend and gather your building materials by cleaning the roadside.

6 now i know how to make the easter island heads for my front yard!

Thinkenstein (author)tulekah2010-09-14

Compressing the trash is a good idea. Black holes come to mind. The idea is to put it to good use, or make it look pretty if you can't make it go away completely.

skiedra (author)2010-07-01

An fine guide, thanks! I stumbled your tutorial to StumbleUpon, hope this will give you more viewers! ;)

Kaiven (author)2010-06-23

Floating rock... now that's an idea :D

scmtngirl (author)2010-05-10

This is a very clever idea indeed. I have an old crumbling retaining wall made out of wood that needs to be replaced - I've been trying to figure out how to do it inexpensively and out of salvaged materials. It would be interesting to rebuild it out of trash rocks. Thank you for the idea and inspiration!

Thinkenstein (author)scmtngirl2010-05-11

I see trash rocks more for free standing walls, benches, and tables.  As retaining walls, plastering the back side of the rock could be awkward.

If you can get the fishnet for the trash rocks, you can do the job with less material just by plastering the earth with a single layer of nylon-cement.

Lean the wall slightly back, so that back pressure from water won't push it outward.  If there is too much back-pressure the nylon-cement will usually crack and leak, thereby relieving the pressure. 

VampiricPie (author)Thinkenstein2010-06-20

I'm surprised you didn't use your zipper stairs in the second picture.

Thinkenstein (author)VampiricPie2010-06-20

Standard stairs are pretty much OK for slopes that are not very steep. The stairs in the photo probably have some shape variations that are just not very visible in that angle. The zipper stairs really come in handy on steeper slopes. My stairs tend to have variable intersection angles depending on the slope. As they get steeper, they approach 90 degree intersections more.

scmtngirl (author)Thinkenstein2010-05-11

Thank you for the lovely pictures. It's hard to believe the first image is not a nature-made rock face! I will post pictures when I find the time to try this out - perhaps sometime this summer. I live in a coastal area (Santa Cruz, CA), so I might be able to score some fishnet from the locals.

rick.leasure (author)2010-05-01

Nifty idea! These look good!
I have a couple of questions, though. First, how thick is the cement plaster by the time you're through? Second, have any broken and revealed their hidden contents? Third, how strong are these?

A few more...
What kind of trash do you usually include?
What is styrofoam concrete?
Have you considered crushing glass to use as a concrete ingredient?

Finally, this is a little off subject, but I would like your feedback on it. A couple of years ago, I had some old latex paint and an idea. I mixed the paint with sand. It had the texture of cement and when it dried, it was hard similar to cement, but pigmented with the color of the paint. Have you had any experience with this?


I would guess the rock walls are about 1/4 inch thick.  None have ever broken, although occasionally you will find cracks.  The net mesh keeps the pieces in place in case of cracks. 

The strength depends a lot on the shape.  A domed rock might support more weight than a flat top rock would, if one was standing in the center.  If they don't support climbing on, I generally beef them up until they do. 

The trash I usually include:  I burn trash paper.  I don't get newspapers, so the volume is not a lot.  I compost organic scraps.  The city collects empty plastic bottles, but there is a lot of plastic trash nobody wants around here, so that goes into the rocks. 

Aluminum gets recycled.  Iron scraps get warehoused as much as possible, for possible future projects, but the less useful stuff sometimes finds its way into a trash rock. 

Styrofoam concrete is like regular concrete except that rock, and sometimes sand are substituted by different size pieces of Styrofoam.   It is less strong than regular concrete, but also a lot lighter.  It is useful for many projects. 

Although glass could be added to concrete, I hesitate to do that because it might break in the future and be dangerous to work with.  I would rather store it inside trash rocks and mine it at a later date if some good use for it came along.   Perhaps a solar furnace might melt it all down and turn it into glass construction blocks, or skylights. 

Latex paint and cement has been tried, and the resulting material is hard.  I don't have a lot of experience with it, but the sample piece I made was very strong.  There are acrylic fortifiers for cement.   Adding latex may give similar properties to the acrylic.  I'm not familiar with latex and sand, except as a way to make a non-slip painted surface. 

Thank you for your response. So basically, your trash is plastic that you cannot recycle and glass, because you recycle everything else. Makes sense. I was hoping you were able to recycle most of the stuff. Your rocks look good. I'm quite impressed.

Do you have a website that shows other things you have done?

My website is .  I think you'll enjoy what you find.  Glad you like the trash rock idea. 

rhino (author)2010-04-29

Is there a reason I cannot use a burlap bag like the ones you make sandbags with?  After all it will be sealed in cement.

Thinkenstein (author)rhino2010-04-30

Burlap will eventually rot away, so I prefer synthetic fiber material.  People have done experimental architecture with burlap and cement, though, and it lasts a good while, especially if it keeps dry.   I saw a wall with it once, overlapping sacks on horizontal strips of wood.   Toward the ground, where it got rain splash, it had rotted out.  Above, it was still good. 

omnivaal (author)2010-04-28

Love it. The pics of the weathered ones with plants and moss growing on them look great.

nickodemus (author)2010-04-27

This is great, using un-recyclables for landscaping. I like the idea of a seating area made of them, and I absolutely love the floating rock idea (last step)! In my opinion your work is the best on the site! Keep it up!

kelseymh (author)2010-04-22

Just a question.  Since concrete is porous, are there any issues here with the trash inside either rotting, or leaching undesirable solutes out into the local environment?

Thinkenstein (author)kelseymh2010-04-24

I suppose a lot depends on what goes in the trash rock.  Any organics that can rot can probably be composted.  If paper rots inside the trash rocks, that probably won't create any problems. 

The decay of synthetic materials, basically plastics, is reduced by protection from sunlight.

Toxic chemical wastes need special disposal.  Unfortunately, we don't have any municipal toxic waste disposal here where I live.  When I do stick any toxic waste inside trash rocks, I try to package it in glass.  Then I cross my fingers. 

kelseymh (author)Thinkenstein2010-04-24

Thank you for the response!  I wasn't thinking specifically of toxic wastes, but more about things we don't normally consider "toxic," but which can release stuff over time (e.g., phthalates or bisphenols from plastics).  Your comment about plastic degradation being reduced by UV shielding is right on point.

TheHebe (author)2010-04-23

Kelseymh:   Nope.  Common enough mistake though!  Concrete is not the same as Cement  - (search ferro-cement for more info or techie talk).   But think of ferro-cement boats - they actually float even when in water over 100 years!

Thinkenstein:  Spotted this entry and I thought how the years pass!  Nearly 6 years since I stumbled across your website on your place with this very feature as one of the projects you spoke of there and how you presented your progress into inspiring me to think along these lines also!

I was right tickled to see you on instructables -  you do come up with some very practical ideas with whimsies!   


Thinkenstein (author)TheHebe2010-04-24

Thanks TheHebe.   The feedback is the big reward for posting here on Instructables.   So, we go back 6 years.  That's cool. 

About This Instructable




Bio: I'm a refugee from Los Angeles, living in backwoods Puerto Rico for about 35 years now and loving it. I built my own home ... More »
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