How To Make Garden Boulders

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Making artificial rock can benefit anyone, from the casual garden enthusiast to the landscaping professional who wants to spice up their garden life. Combining basic construction skills and artistic creativity, you can create artificial rocks with concrete that are virtually indistinguishable from naturally occurring stone. Sculpting landscape accents from concrete is an economical and lightweight alternative to large rock installations.

1
Choose a material as a base for your rock's shape. You can use a variety of materials to create the shape of your rock. There are several common items you can choose from:
- Styrofoam
- Cardboard
- Crumpled newspaper
2
Create the rough shape of your rock. Cut the cardboard or styrofoam to the shape you want your rock to be. Combine different materials with glue to create oddly shaped rocks.
- Use a plain cardboard box for a roughly square shaped rock.
- A hot wire foam cutter works well to shape styrofoam.
3
Cover your rock shape in chicken wire or hardware cloth for a better look. Use a metal mesh to wrap the rock shape. The metal gives strength to your artificial rock and provides a structure for the cement mortar mix to adhere to.^[1]
- Use wire twist ties to secure the wire frame to your rock base.
4

Refine the curves of your rock. To make the most natural looking rock, bend and shape the wire form around your rock shape. Natural rocks have dips and creases; simulate these shapes by pushing your wire form in various places to create uneven surfaces.

1
Combine dry ingredients for mortar mix. Mix 3 parts sand with 1 part portland cement. Add all the ingredients to a wheelbarrow or concrete mixer, depending on the size of rock you are creating and amount of mortar you are mixing.
- You can reduce the sand, and add 1 part peat moss to create a more porous artificial rock.^[2]
- If you want to use the fake rocks in an area exposed to water, use a hydraulic cement mix instead.
2
Mix the dry mortar and sand mixture into 1 part of cold water. The exact amount of water depends on humidity and temperature conditions, so you may need to adjust with more or less dry mixture. Slowly add the dry mixture to the water while mixing until both form into a thick paste.
- Stir the mortar mix into the water as you add it.
- Watch closely as you add mortar so your mixture does not get too thick.
3
Stir the mortar mix for several minutes. For small amounts, turn the mixture over in the wheelbarrow repeatedly, or stir with a paddle attached to an electric drill. For larger amounts, use a concrete mixer. You will need to mix the mortar to the consistency of cookie dough.
- Ensure the mixture is fully mixed and uniformly wet.
- Add more water if necessary to get the consistency of a thick paste. You do not want the mixture to be runny.
- Unmixed blobs of sand will cause weak spots in the finished rock; be sure to mix everything completely.
- Keep track of what you added and adjust until you reach the desired consistency. Write down the formula you find that works best. Follow this formula and use the same measuring device for the water each time to keep a consistent mix from batch to batch.

1
Apply the mortar mix to the wire form. Use a flat pointed trowel to apply a 2-3" layer of mortar to the wire frame.
- Build the rock from the bottom up.
- Make a layer of mortar around the base of the rock and work upwards around the wire frame.
2
Add texture to the mortar. Create a realistic looking rock by adding contours and patterns to the surface of the mortar.
- Use your trowel to create dips and creases in the surface of the mortar.
- Press a real rock into the mortar to make imprints of the rock's texture.
- Press a sea sponge or scouring pad into the rock to create pock marked look.
- Wrap a plastic bag around your hand and press it into the mortar to give a wrinkled look.
3
Cure the rock for 30 days in a dry location. The curing process is a result of a chemical reaction, not the cement drying out. Although 75% of the curing is completed after one week, it may take up to a month for cement to cure completely.
- Mist the surface of the rock every few days as it cures.
- Keep the cement out of direct sunlight to prevent cracks.
- Cover the rock with plastic sheeting as it cures.

1
Scrape the rock to smooth the edges. Use a scraping stone or a hard wire bristle brush to rub the surface of the rock. Scrape off any sharp or pointy edges that exist on surface of the rock.
- Allow the rock to cure for a week prior to scraping to prevent crumbling.
2

Wash the rock. Thoroughly rinse the surface of the rock. Brush the surface with a wire brush while washing to remove any loose bits of mortar. Make sure to flush out any creases or dips in the rock to get rid of rock dust.
3
Stain the rock. Use a penetrating concrete stain to cover the surface of the rock in the color of your choice. You can apply multiple colors to provide the most natural look. For a more obvious design element, shiny aggregates or even glow in the dark powders are also available to add to the mix.
- Brush the stain on rock with a paint brush.
- Add depth to the coloring by using more than one color.
- Apply more stain in some areas for darker contrasts.
4
Seal the rock. Use a water or solvent based concrete sealer to protect your artificial rock from the elements. Some sealants provide a glossy finish while others have no sheen but still provided protection.
- Brush on 3 coats of sealant. Wait about 15 minutes between coats.
- Maintain the sealant by reapplying a coat of sealant every 1-2 years.
5

Remove the inner base from the rock. Decide which side is the bottom of the rock, and cut it open so you can remove the inner structure. The rock's shape and strength come from the mortar and wire frame; the inner materials do not provide structure after the concrete has cured. Removing them will prevent them from decomposing.

1
Decide where you want to place your fake rock. Fake rocks can be used as a part of a water feature, lining pathways, or as garden accents. Determine the best place for your rock based on its size and appearance.
- Unless you used hydraulic cement mix, keep the fake rocks away from water. Standing in water or heavy water splashing can cause regular cement to break down.
2

Dig a small indention where the rock will be placed. Place the rock in place and trace the edge of the rock with a stick or shovel. Dig a 1-2" pit in the shape of the rock. Placing the edges of the rock underground will provide a more natural look of a rock outcropping.
3

Place the rock in the hole. Push dirt and other small rocks against the edge of the rock to integrate it with the landscape. Build multiple rocks to create elaborate rock landscapes.

Add New Question

Question

I have a rock that juts out of my fireplace. How can I cut the rock to fit the mantel without removing the whole rock?

Anthony "TC" Williams is a Professional Landscaper in Idaho. He is the President and Founder of Aqua Conservation Landscape & Irrigation, an Idaho Registered Landscape Business Entity. With over 21 years of landscaping experience, TC has worked on projects such as the Idaho Botanical Garden in Boise, Idaho. He is a Idaho Registered Contractor and a previously Licensed Irrigator in the State of Texas.

Experienced Landscaper

Expert Answer

You could use a hand held grinder with a masonry blade and cut the rock or 'score' it and then use a heavy hammer and break it.
Question

What can I use as an alternative to mortar?

Anthony "TC" Williams is a Professional Landscaper in Idaho. He is the President and Founder of Aqua Conservation Landscape & Irrigation, an Idaho Registered Landscape Business Entity. With over 21 years of landscaping experience, TC has worked on projects such as the Idaho Botanical Garden in Boise, Idaho. He is a Idaho Registered Contractor and a previously Licensed Irrigator in the State of Texas.

Experienced Landscaper

Expert Answer

Plaster may work well as an alternative to mortar. However, plaster should not get wet after drying.
Question

How thick do I have to put the cement on the chicken wire to make it withstand walking over them, or my big dog not breaking them?

The strength of your imitation rock will depend more on the underlying structure, and the rigidity of the material that you use to make the shape/structure. Use heavier-gauge wire mesh or combine this with wire fence material, or go big and build a frame out of wire/metal rods with welded contact points. Trying to add thickness to the cement won't achieve the result that you want if you have a weak underlying structure.
Question

Can I follow the article's instructions for a boulder-sized rock, or would it crack from the size?

You can, but you would need to use a heavier wire mesh to keep the rock stiff. This will prevent cracking.
Question

What are the advantages of a fake rock compared to a real one?

A fake rock is easier to move around. You can also make a small opening to store things inside of your rock.
Question

Why would I do all this rather than get an actual rock?

Because carving a hole inside a real rock is hard.
Question

I need to replicate concrete interlocking edging stones that stores no longer carry. How is a mold made from an existing brick?

You can buy molding rubber (which is rather expensive, very easy to use, and works very well), or you can try using wet sand as a molding material. If it's a interlocking stone though, it might be complicated. (Videos on YouTube can help with this though.) The molding rubber is expensive, but if you need to make more than one stone, it might be worth it. If it is one only, try one of the recipes on the internet where you use alternative material to make a cheaper rubber from silicone and soap to make the mold.
Question

Can this be used outside if it isn't supposed to be placed in an area that gets wet consistently?

Yes, my artificial rocks are part of my garden pond and they get wet all the time. My rocks are not opened at the underside; they are hollow and have concrete all around so water can't get inside.
Question

How do I make artificial rocks for a waterfall?

You're going to have an idea in your mind of how you want it to look, so draw it out and build from there. Make sure your materials are pliable, yet durable, and able to withstand water.
Question

How can I fix an imitation rock that has holes in it?

Fill the holes with glue, let the glue harden, and then paint over it.

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Do not attempt to use artificial landscaping rocks as weight bearing installations for swimming pools or hot tubs.
Use caution when working with cement. Lime can cause chemical burns if it gets on your skin or in your lungs. Wear gloves and a mask when mixing cement, as well as proper protective clothing.

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Article SummaryX

To make fake rocks with concrete, start by creating a wire form base with styrofoam or cardboard, then wrap the base in chicken wire. Next, use a flat pointed trowel to apply a layer of mortar to the wire frame, working from the bottom up. You can add realistic looking texture to the wet mortar using a trowel, sponge, or scouring pad, then let your rock cure for 30 days in a dry location! For more tips on sculpting your rocks, read on!

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