There are several ways to make fake rocks or fake rocks and boulders, all with good results.
The method you select for making fake rock is usually based on what is the cheapest and most readily available fill material you have available on site. Very realistic imitation rocks or boulders can be made using any of the following fill materials.
Materials and construction methods
Construction debris such as second-hand bricks or crushed concrete
recycled real rocks
Polystyrene
Alternative methods for making faux rock involve using some form of roughly shaping the finished rock prior to mortar application.
Chicken wire bent to shape is widely used for smaller rocks.
Cardboard boxes of all sizes can be cut to shape.
Reinforcing bars and expanded metal
For the repetition of the same design, the use of molds is recommended.
Casting a mold
The material and method you use to make fake rock will be the one that you consider to be the fastest and cheapest to give you the best final result. The time the job takes is based on the amount of detail that is required. The more definition and finish you give each rock, the more time it takes.
For larger proportions of faux rocks, such as garden boulders, a smoother, less detailed finish looks better, while smaller decorative rocks require more work and materials.
A garden landscaping job will often require the use of several of these different finishes to give the required natural look. The more methods you have for making fake rock, the better the final quality of work you’ll be able to produce.
Solid fill method
While this is possibly the slowest and most expensive method of making fake rock, it is still the most widely used due to the availability of the building material used in most jobs. Not being hollow in construction, it also provides the closest sound and feel to natural rock.
The materials used in this type of construction are:
Construction debris.
It’s usually free or already on the site ready to be disposed of. Hosing down before use is essential to ensure a proper bond.
Second-hand concrete bricks and blocks
Even if you have to buy them, it’s still cheaper than using concrete. Difficult shapes, like waterfalls, are also easier to build due to the regularity of their shape.
Polystyrene
This is a waste material that is used for packaging and is readily available for free at your local stores. It can be cut to shape very easily and reduces the weight of the finished fake rock. It maintains the shape of the rock and does not collapse under the weight of the mortar, unlike other fill materials.
Recycled rock and other natural materials
They are usually on the site and are easy to use giving very natural shapes. Wash before use.
If the material being used is in small pieces, it can be placed in plastic bags and stacked for easy assembly.
Cut and expose fill material
Apply two coats of acrylic polymer stabilizer mix. This must be applied by brush and is designed to ensure the rigidity of the structure before applying the finishing mortar mix.
Apply two coats of 40 mm mortar mix.
· Finish as with other techniques.
This type of rock is very easy to build. It also uses mass concrete and a basic mortar mix for the final shaping to save even more hassle. First build a small concrete base for the rock, then build the shape with some of the rubble, then more concrete, then more rubble and so on until you have a rough finished shape.
To make fake rock using this method, it’s a good idea to be making multiple rocks at the same time. While you allow one batch of concrete to set, you may be working on another rock. Once the rough shape is finished and set, the application of the mortar can begin.
Use a throwing action for the mortar as it sticks better than just smoothing it out. Additional layers of mortar also better adhere to the rough texture left by the action of pouring. Start the finishing coats at the top and work your way down. If you are working on two rock shapes side by side, be careful to emphasize the joint lines between the rocks. Definition is very important with these seams to give the rocks a natural look.
Hollow rock construction methods
chicken wire
This method is useful for smaller hollow rocks.
Cut and wrap chicken wire to the rough shape of the finished rock.
· Bend the sharp ends of the cable inward with a pair of pliers.
Now cover the wire with some newspaper soaked in 50/50 diluted white wood glue and let it dry.
· Cover the wire with several layers of plaster bandage that has been briefly soaked in water and allow to dry.
Decorate with diluted acrylic paint mixed 50/50 with sand to give a rock texture.
Rebar and Expanded Metal Construction
This is the most commonly used method of making fake rock that is hollow. Using this method, rocks can be shaped to cover faucets, pool pumps, and other equipment. Due to its ability to be easily shaped and retain structural integrity, it can be used to make anything from a waterfall to a garden arch.
Once you have set the shape with the rebar, the expanded metal can be attached. Tie it to the rebar every 100 to 150mm. Make sure it’s not loose. Cover the structure with two 3 to 4 cm layers of mortar mixture, leaving a minimum of one hour or overnight between layers, depending on the temperature of the day. Add texture to the project using methods similar to the solid fill method.
Cardboard
Using a mixture of large and small cardboard boxes cut to shape, it is possible to build a firm structure very close to the desired rock shape. Used boxes are available free of charge at local stores and provide an easy and inexpensive way to make fake rocks of all shapes and sizes.
Make the structure rigid by stapling, wiring, screwing, or gluing the cut box shapes to form the rough outline of the desired rock shape.
Cover the boxes with two coats of acrylic polymer stabilizer mix, using a brush to brush the mix directly onto the cardboard. If too much is applied at once, there is a danger that the cardboard will become soggy and collapse. Allow the cardboard to dry between coats to give it the strength needed to support the layer of finishing mortar mix.
· The cardboard can now be covered with an 80mm layer of mortar mix applied in two 40mm coats.
The structure can now be textured, stained and sealed.
Casting from a mould.
To produce the most accurate reproductions of natural stone, there is nothing better than imitation rock castings from a mold. They are exact duplicates of what you have seen in the wild. The same rock can be produced over and over again. Casts can be made from latex or silicone, usually with a plaster backing cast to retain the original shape.
The complete process for making faux rocks with mold casting and decoration will be the subject of a future article along with the use of texture molds in faux rock making.