There are several ways to make fake rock or faux rock and boulders all yielding good results.
The method you select to make fake rock is usually based on what is the cheapest and most readily available filler material you have available on site. Very realistic faux rocks or boulders can be made using any of the following filler materials.
Building Materials and Methods
· Building rubble such as second hand bricks 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.
· Chicken wire bent to shape is widely used for the smaller rocks
· Cardboard boxes of all sizes can be cut to shape
· Rebar and expanded metal
For repetition of the same design the use of moulds is recommended.
· Casting from a mould
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 finished result. 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.
To make fake rock of larger proportions such as garden boulders, a smoother, low detail finish looks best 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 are experienced in using to make fake rock the better the finished quality of work you will be capable of producing.
Solid Fill Method
While this is possibly the slowest and most expensive method to make fake rock it is still the most widely used due to the availability of used building material on most jobs. Not being of hollow construction it also gives the closest sound and feel to natural rock.
The materials used in this type of construction are:
Building rubble.
This is usually free or is already on site ready to be disposed of. It is essential to hose it down before use to ensure proper bonding.
Second hand bricks and concrete blocks
Even if you have to purchase these it is still cheaper than using concrete. Difficult shapes such as waterfalls are also easier to construct due to the regularity of their shape.
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
These are often on site and 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 ease of assembly.
· Cut open and expose the filler material
· Apply two coats of the acrylic polymer stabilizer mix. This should be thrown on using a brush and is designed to ensure the structure is rigid before applying the finishing mortar mix.
· Apply two 40mm coats of mortar mix.
· Finish as with other techniques.
This type of rock is very easy to construct. It also uses plain concrete and a basic mortar mix for the final shaping to further save any complications. First build a small concrete base for the rock, then build up the shape with some of the rubble, then more concrete, then more rubble and so on until you have the rough finished shape.
To make fake rock with this method it is a good idea to be doing several rocks at the same time. While allowing one lot of concrete to set up you can be working on another rock. Once the rough shape is finished and set up, the application of mortar can commence.
Use a throwing action for the mortar as it adheres better than merely trowelling it on. Additional layers of mortar also adhere better to the rough texture left by the throwing action. Start the finishing coats from the top and work your way down. If you are working on two rock shapes side by side be careful to emphasise the join lines between the rocks. Definition is very important with these seams to give the rocks a natural appearance.
Hollow Rock Construction Methods
Chicken Wire
This method is of use for smaller hollow rocks.
· Cut and wrap the chicken wire to the rough shape of the finished rock.
· Bend any sharp ends of the wire inwards with a pair of pliers.
· Now cover the wire with some newspaper soaked in 50/50 diluted white woodworking glue and allow to dry.
· Cover the wire with several layers of plaster of paris bandage which has been briefly soaked in water and 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 is the most widely used method to make fake rock which is hollow. Using this method rocks can be shaped to cover taps, 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 the shape set up with the re-bar, the expanded metal can be attached. Tie it to the re-bar every 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. Add texture to the project using similar methods to the solid fill method.
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. Used boxes are readily available free from local stores and provide an easy and economic way to make fake rock of all shapes and sizes.
· Make the structure rigid by stapling, wiring, screwing or gluing the cut box shapes together to form the rough outline of the desired rock shape.
· Coat the boxes with two coats of acrylic polymer stabilizer mix, using a brush to throw the mix directly onto the cardboard. 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 cardboard can now be coated with a 80mm coat of mortar mix applied in two 40mm coats.
· The structure can now be textured, stained and sealed.
Casting from a Mould.
For producing the most accurate reproductions of natural stone there is nothing to beat the castings of faux rock from a mould. They are exact duplicates of what you have seen in nature. The same rock can be produced over and over. Moulds can be made using latex or silicone usually with a plaster back up mould to retain the original shape.
The full process to make fake rock with mould casting and decoration will be the subject of a forthcoming article together with the use of texture moulds in the manufacture of faux rocks.