Rammed Earth House, Bonda

Bonda is a small town to the west of the Nyanga National Park in the eastern highlands of Zimbabwe. We were asked to build a rammed earth house on the side of a hill outside the town in a fairly remote place up a long dirt track. The architect was Mick Pearce, as well as his well known work Mick has built a number of rammed earth buildings but this was the first.

The site was steeply sloping, extremely windy in summer and winter and remote. This gave a number of opportunities for the designer and the builder. First we could excavate the site to set the building down onto a level site. This had the effect of taking the building out of the wind. By siting the building carefully it was made a sun trap in the courtyard with its 'back' against the wind.

With the site dug out we also had the material for building the walls, both a clay/silt sand mix and gravel where excavated by hand. The foundations were also dug out and then re-compacted with burnt lime at 5% added. Lime was bought from a steel works were it is a by-product and not well understood. As most of the local building trade didn't use it it was about a quarter of the price of cement. Looking for materials and recognising them gives a market advantage if you are able to do it. With earth building this often comes in handy. The foundations were rammed in layers into foundation trenches.

Once the site was excavated and the foundations filled and compacted the formwork was brought to site. This formwork system was developed in the UK in the 1940's and '50's and was widely used until the 1990's in the UK. In southern Africa it continues to be manufactured and used in a number of countries. The most common use is for flat panels but they also had a curving set mainly used for producing water tanks. It was a set of this which we hired and used.

The foreman on the site Nhamo Chasakara had built a number of rammed earth buildings previously and well understood the formwork and the material mix. He was able to instruct the team and oversee them as they learnt the job. Three circular rooms connected by two straight sections make up the floor plan, easy to draw but requiring some skill to undertake. The bottom of each first lift had lime added and was built to 300mm height where a damp proof course was introduced, just a simple heavy plastic sheet cut to shape. From there on the walls were earth and stones dug and mixed on site.

Although the site is 'off-grid' we put in electrical switch and socket boxes and conduit which came out at the tops of the wall. This meant that should power come to the house in the future the service insets would not require chasing in and the finish of the wall would remain.

We also placed steel strapping into the walls to attach both doors and windows and roof plates. Simple and effective strap banding from containers is also free.

The openings were all arched, cheap to build and termite proof. However the void displacement box for the arches in the circular rooms did require some careful making, all being made of hardboard. This was first laminated into curved sides and then joined with the curved lining. The sides where cut to produce a chamfered edge. The good thing with this approach is the mould can be used again and again so only one was needed.

The roof was built using locally grown gum poles which were cut and then treated with a borax/boric acid treatment. This was effectively free as it was the residue from a previously paid job. At that time we paid $800Zim for the entire previous roof treatment against $4,500Zim for CCA or similar. Borax has the advantage of being both colourless and smell free, something

The total effect was a house which is comfortable throughout the year, out of the wind and with a warm sun trap to sit and work in. The combination of materials and design being the keys to a beautiful house.