Lightweight Concrete Faux Rock

What do you do when you can’t lift those heavy rocks but you want ‘big’ rocks? You knew I was going to say; ‘make them’! Here’s how to make your own Lightweight Concrete faux Rock.

When you get a ‘new’ TV:

When you get a new appliance and look at that mountain of styrofoam; there must be something I can do with it?! I heave been saving it for a while… and hubby was bugging me to get rid of it, soooo…

I like large rocks; diamonds and the like, the bigger – the better. I know rocks are cumbersome and back-breaking, so light weight ones are a dream. That’s when my problem-solver brain brought a few methods together.

I wanted to keep this as simple as possible. I did not want to fight with that chicken wire that always seems to poke & hook my skin. Simple – just attach a random shape of pieces of styrofoam. Wear gloves so you won’t be sticking to everything. I did not break many pieces as I hate the way it starts to cling and get everywhere.

Styrofoam does not absorb water, It’s light and pretty sturdy as well; so perfect as a form. Many glues will dissolve the styrofoam, so I found a can of ‘Great Stuff’ (as I was also incidentally also doing some window trim repair) It works well to attach the styrofoam as it quickly starts to set and also fills gaps. I was not fussy at all, just stuck random pieces together since I do not like the round rocks; I much prefer rocks with angles, textures and layers. There is another step for more detail so this is just a basic starting shape.

I was amazed at how quickly they all came together. I had 3 big bags full of pieces!

Base forms made!

These range about 18″ to 30″ in size. It’s ok to have a bit of flat sides as natures rocks often break off in layers as well.

Messy hands time!

I have used draping of concrete soaked fabric for a few projects now. The (famous) Spook, The Faux Bois Chair, the Cat and even the Rope Orbs all have some part of this method. So I realized that it would make for a great way to get a base shape, a stiff concrete skin.

My favourite way is to use only portland cement as it gets really smooth and can absorb well into fibres. Since I have a lot of used flannel from Eco printing, I found that it works well. It is a bit thinner but nicley absorbent. No problem to have extra layers or wrinkles either.

To make it less messy (a bit) I used smaller pieces, not really sized at all. ‘Think of it like paper-maché, layer and smooth out.

I found trying to wrap all the way around was a problem, as it would fall off. So I only did the tops and sides and then let cure. Remember, do not let it dry out too fast. Misting with water does not hurt, it helps.

I know they look strange, bit it’s just a base shape… you can be fussier if you like but usually the more contrived then the less it looks like nature..

Once the tops and sides are cured (usually overnight) you can wrap the bottom and even up the sides if it’s weak. There will be another layer so a few weak spots are not really an issue. It all depends also where you plan to put these. Mine are going to make a waterfall structure, not be climbed on. If you want them pretty indestructible, add more layers of the cement soaked fabric. If you choose thicker fabric it tends to get stiffer since more cement is impregnated, but also uses more mix. See my tests here.

These still look a bit boxy but looking pretty good considering how easy they were so far!

Some simple Tools:

To make some rock-like texture some simple tools can be used. An old brush dipped in a bit of fast setting cement or whatever else can make some good pebbly divots. Knives, like putty or pallet knives help spread the next layer, trowels help apply as well.

When adding any new concrete to existing it needs to be wetted, and if you want extra reassurance a bonding layer with a Bonding Agent can be painted on. I generally don’t use it bit did try for a couple to compare in the future.

The Second Concrete Layer

To make the texture and detail layer is made with Rapidset Mortar Mix. I have used their other product for many things so I trusted this one as well. It feels sandier than the Rapidset Cementall which makes better texture for rocks! BEWARE – it does set really fast! So use small quantities.

Add handfulls, some lumps, some divots, some crack lines… Dab with brush for texture. If needed google images of rocks. If you overthink it, they look more contrived.

Even corners are ok, as quarry stone is broken in big blocks.

The palette knife lets you form the concrete like icing a cake. These rocks will become a waterfall so the top layer was to be more like split layered rock. It’s quite fun to make random ledges, cut some cracks and roughen the edges. HOWEVER, do not get too much small detail! You will notice that small detail generally gets lost from a distance.

As it starts to set it can be carved into to make crisp cracks or crumbly edges.

The Question about Colour

As an illustrator I know that rocks are not just grey. They have many undertone colours. It also depends where your rocks will be, are they going with other natural rocks? Even some areas have different tendencies of colour. I knew I may need some adjustment later on (it’s the artist in me) but for now I added some black concrete colour watered down and worked into the areas that would naturally get darker. Vertical areas usually get darker as well as cracks. It is tricky since it will dry different later.

Keep things damp; the spray bottle handy – it allows the colour to flow naturally into crevices.

You can mix some of the concrete colour into some of the mix as well and spread on. Colours changes are usually not that pronounced, they look more like ‘dirty’ parts of rocks. I bet there will be some natural darkening as mossy areas will develop from being in a woodland setting

Texture is one of the most convincing parts of a rock. Don’t make it look like an ‘iced cake’

Have fun and ‘Rock on’!

Stay tuned to see how the waterfall all came together! There’s also a bit of a surprise since it’s the habitat for a certain ‘friend’ of mine…

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  1. Barb, I have been making faux rocks for a while now and been fighting with the chicken wire, as you said…but that seems to be the standard way in all online tutorials.
    I am thrilled to see your flannel option, but that’s just like you. Always thinking outside the box.
    Like you, I save many things that I think will be useful later…so I have plenty of Styrofoam just waiting for its chance to debut.
    You’re a gem, thank you all your posts, I know it takes a lot of your time to create, take pictures and then post it all…but is greatly appreciated by so many people.
    BTW, I am 76 and still loving to be outside playing in concrete.
    Hugs to you my friend….

    1. I appreciate that you understand the work. My brain and body are somewhat not at the same speed… 76?! I am so impressed! Maybe my new knees will make things better & faster! Thanks my friend!

  2. You have just save me so much time and experimentation. I’ve being trying to use Styrofoam, cement and fabric to make fake rocks. Trouble is foam is hard to manage when tied together with baling twine. The next attempts were going to be with wire or PVA glue ( Aussie for white/wood? glue) My dog found a mouse nest inbetwen the layers when I left it uncemented too long. So gaps, not ideal.
    Thanks heaps for sharing

    1. When attaching styrofoam some glues will dissolve it, others won’t dry as the styrofoam does not let air get to it. I like to use this site for help when attaching odd materials. And those pesky mice are quite resourceful!

  3. Barb, Never tried this but it sounds really interesting. I would like to place vertical shaped
    boulders on a slope area of our property. I only want the face to be showing ,would these
    hold up with sand,dirt & grass on top of them ? Mary

    1. If they are completely well-covered with the concrete cloth then they should be fine. I was worried that I would have some damage as the deer were in my yard again, but I do not see that they stepped on mine or made damage.

  4. Hello. I love these faux stones. Do you think they would work for a backsplash? I am interested in doing a narrow strip of stone above my counter with vertical shiplap above, I don’t have a large area to cover. What do you think?

    1. I’m not quite picturing this? Maybe you could just make some flat stones to apply to a wall like tiles? The Rapidset Cementall ends up quite strong for a thin thickness.

  5. Hi Barb and everyone

    Wow seen so many things I would like to try,being disabled now I miss making things l have visioned in my mind…….which makes for a big Aaarrghhh.
    As l have a small pond with frogs toads newts and a billion tadpoles!
    I would love to do this with a waterfall incorporated and hiding hole’s at the base for toads etc, with holes above for some plants(Think putting bamboo through for drainage?).

    Would the cement need to be sealed once completed and if so with what?

    Many thanks you have my mind ticking again and it’s gonna drive my partner mental hehe.

    1. I do not know what the longevity of the concrete under water would be so that is all a pond liner. (Actually a farm animal water feeding tub) I am not a fan of sealers so I do not use them.As I see all the driveways that are sealed will need reapplication. I would use natural rocks for the pond hiding places as concrete does leach out alkali. Any part of the structure that will hold moisture and freezes will break over a winter. I used the method to make the rocks only. Good luck.

  6. Awesome tutorial Barb, I was looking for a way to make rocks/cliffs for an outdoor model railroad scene. Your idea looks like the perfect fit for my plan. And I now have a use for those huge pieces of styrofoam that my tv was packed in. Thanks again, looking forward to your next “how to”.

  7. Thanks Barb for sharing your ideas and ways to bring our yards a little bit of self expression. My Lady and I have been looking for about a month or so on how to come up with a creative way to make faux rocks and even considered the fiberglass method but didn’t know about that. After seeing this , we are surely about to give this a go. Thanks again

    1. Yay! I have a huge amount of snow right now and they have worked out great. I recall being inside the huge mountain in Canada’s Wonderland; it is sprayed over a form much like these rocks. The draped things I’ve made have lasted quite well. Have fun!

  8. Just love your creativity Barb, and your out of the box thinking. I have made quite a few faux rocks myself. The problem I am encountering with especially the larger ones is that pests are nesting inside them being hollow.
    I am working on a new project, making faux rocks with milk jugs. Do u think cement will stick to them and stay, or draping them with cloth dipped in cement would be better.

    1. I have found that plastic will not hold or stick to concrete. I think you need a good cover with the draped concrete. The other problem is that the jugs are pretty pliable and may need a very strong outer coat, otherwise the concrete skin would break. I completely wrapped my rocks so there should not be an entry part for animals. I do think that there is a chipmunk nest in between the voids of rocks though. It must be quite warm in there…