How to Make Fake Rock

Building Materials and Methods

- Second hand bricks from demolished buildings or crushed concrete.

- Recycled real rocks

- Polystyrene

Alternate methods to make fake rock involve using some way of giving an approximate shape to the finished rock before the application of the mortar.

- Forming the shape with chicken wire is a common technique for smaller type rocks.

- The general shape can be formed with carboard boxes cut to size.

- Reinforced metal bar material or expanded metal.

For repetition of the same design the use of moulds is recommended.

- Casting from a mold

The fastest and most inexpensive way to produce the finished item will determine which method and material you choose to make fake rock. The time the job takes is based on how much detail is required. The more definition and finish you give to each rock the more time is involved.

Garden boulders and other larger rocks usually look best with a smoother and less detailed finish compared to smaller decorative rocks and require less work and material.

To ensure the finished result of a faux rock landscaping job looks natural, it is often required to use more than one method. The final result will be much better if you know how to use most of the methods to make fake rock.

Solid Fill Method

This is the most widely used method and also the slowest and most expensive. This method produces the closest representation and feel to natural rock as it is not of hollow construction like other methods to make fake rock.

Materials which are required for this method include:

Building rubble.

This is usually free or is already on site ready to be disposed of. To ensure the rubble bonds together it is advisable to hose it down to remove dust and grime.

Recycled bricks and blocks of concrete

Even if purchased these materials are usually cheaper than using concrete. Due to the regularity in shape of the material it often makes it easier to construct complicated shapes and designs such as waterfalls.

Polystyrene

This is a waste material used for packing and readily available free from your local stores. It can be cut to shape very easily and lowers the weight of the finished faux rock. It holds the shape of the rock and does not slump with the weight of the mortar, unlike some of the other filler materials.

Recycled rock and other natural materials

Natural shapes can be easily achieved with this material which is often found on site. It is important to wash the stone before use.

- To aid in assembly of the faux rock, the material can be placed in plastic bags if the pieces are small enough.

- Cut open and expose the filler material

- An acrylic polymer stabilizer mix can then be applied to the rock in two coats. This will prepare the structure for the final mortar mix by stabilizing it.

- Apply the mortar mix in two 40mm coats.

- Complete the process as described with the other techniques.

This type of faux rock is relatively easy to make. It also uses plain concrete and a basic mortar mix for the final shaping to further save any complications. You first need to build up a base for the rock in concrete, then build up the shape a bit with some rubble, add more concrete, more rubble again and so on until you have achieved the finished shape.

To make fake rock with this method it is a good idea to be doing several rocks at the same time. As the concrete sets on one piece of rock you can be creating another piece. Once the rough shape is finished and set up, the application of mortar can commence.

Throw the mortar on rather than trowelling it as it tends to adhere better. Additional layers of mortar also adhere better to the rough texture left by the throwing action. The finishing coats should start from the top and work downwards. If you are working on two rock shapes side by side be careful to emphasise the join lines between the rocks. The definition of these seems are very important to ensure a natural appearance.

Hollow Rock Construction Methods

Chicken Wire

This method is suitable for making hollow rocks of a smaller size.

- Trim and form the chicken wire into roughly the finished shape required.

- Bend all sharp edges of wire inwards using pliers.

- Soak old newspaper in a 50/50 dilution of woodworking glue and wrap around the chicken wire form.

- Now cover the wire form with several layers of Plaster bandage that has been quickly dipped in water and then allow to dry.

- Decorate with diluted acrylic paint mixed 50/50 with sand to give a rock like texture.

Re-bar and Expanded Metal Construction

This method is popular to make fake rock which is hollow. This method can create rocks and boulders which can effectively camouflage garden utilities such as garden taps and pool pumps. This method can produce waterfalls to garden arches due to its ability to be easily shaped.

When you have set up the shape with the rebar, apply the expanded metal to the structure. Secure it to the rebar at intervals of 100 to 150mm. Make sure it is not loose. Coat the structure with two 3 to 4cm coats of mortar mix leaving a minimum of an hour or overnight between coats, depending on the temperature of the day. Using similar techniques to the solid fill method, add some texture to the structure.

Cardboard

Using a mix of large and small cardboard boxes cut to shape it is possible to build up a firm structure very close to the desired rock shape. Using old boxes from your local store, it is easy to create faux rocks of all shapes and sizes and at low cost.

- To create a firm structure in the form of the final rock shape you can screw, staple, glue or wire the cut cardboard boxes together.

- Apply two coats of acrylic polymer stabilizer to the cardboard structure in a throwing action with the brush. If too much is applied at once there is a danger that the cardboard will be soaked and collapse. Allow the cardboard to dry between coats to give it the required strength to support the finishing mortar mix coat.

- The mortar mix can now be applied in two coats of 80mm thickness.

- The structure can now be textured, stained and sealed.

Casting from a Mould.

Using a mould can produce the most accurate representation from natural stone of all the methods. They are perfect reproductions of stones in nature. The exact same stone design can be manufactured over and over again. Moulds can be made using latex or silicone usually with a plaster back up mould to retain the original shape.

The process of mould making for use with faux rocks will be the subject of another article and will deal with the use of texture moulds.

Copyright © 2010 Stan Alderson

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