Ferro-cement is concrete that is heavily reinforced with both wire mesh and rebar. This type of construction is extremely flexible, which seems strange for something as hard and dense as concrete. Because of the reinforcement, ferro-cement does not need to be as thick as concrete, and designers consider it more as a tensile skin that can be made in just about any shape. It is relatively inexpensive, very durable, very strong and requires a bit of skill to get it right. The process is to create an armature out of rebar, then cover it with lathing and wire mesh, and stucco thin layers of a sand/cement mix. Additives are used to give the concrete different added benefits, like quick or slow curing times, added strength or water proofing. As far as concrete is concerned, ferro-cement is one of the most versatile methods available.
Your ferrocement project wants to be anchored to metal that you can cover in concrete. If making a wall, make sure that your foundation has some rebars sticking up out of it every couple of feet that you can tie the wall's frame to. If making a roof, make sure your walls have exposed rebar you can tie to. And so on. This way, your building gets tied together at its basic level.
- Make a basic frame for the wall, roof, bench or whatever it is you are building out of rebar and remesh (6" square floor mesh). You want to put the rebar every few of feet, both horizontally and vertically. When doing a roof, bend the rebar into an arch and criss-cross them. Tie each join of rebar with tie wire. Make sure you have rebar wherever you want to put window or door frames, as well as any other similar features.
- Tie any window or door frames, as well as your wiring conduit and fixtures, to the main rebar and remesh frame.
- Attach a metal mesh to the rebar using tie wire or, better, hognose clips. The amount of layers of mesh needed will depend on the type of mesh used. You want the end result to have holes of no more than 1/4". Lathing often needs no more than 1 layer. When using multiple layers (like with chicken wire or other mesh), make sure you put each layer perpendicular to the last.
- Put hognose clips periodically through the layers of mesh. You want them to be tight against each other all over.
- You need to apply three layers of stucco to each side of the metal framework - scratch coat, brown coat and finish coat. Your first two coats need not be pretty. In fact, it is better if they are a little scratched up, so that the subsequent coat will adhere better.
- Mix your concrete 3 parts sifted sand, 1 part cement and water. It wants to be fairly wet, as far as concrete goes. Experiment with your stucco to see what suits your technique.
- Put some concrete on to a hawk, a flat piece of metal with a handle coming out of the underside of the metal in the center. Hold the hawk against the wall and with a trowel, push the concrete onto the wall. With stucco, always start at the top and work your way down. That way, the concrete can adhere to the stucco above it.
- You will need to apply the first coat fairly slowly, as the wall will move a little. However, once it is on and dried, the process goes faster as the wall will be stiff.
- As with any concrete, do not let it dry out too fast. The longer concrete can cure, the stronger it will be.
- For the final coat, instead of stucco, you can apply a layer of cement, lime and acrylic. This you will be able to paint on and it is much faster and easier, as well as leaving it smoother than stucco, which will use less paint to cover it.
And that's it. It takes a little practise, but once you get going, it is surprising how fast you can get a large area done. Be aware that the hardest stucco to do is overhead (like the underside of a ceiling). If you can avoid this in your design, it will save you a lot of shoulder and neck ache. Once you're a little more experienced, it won't be so bad, and adding acrylic to your mix makes it much easier (as it is stickier).
Ferro cement is a form of building that lets your imagination run wild.