The central visual theme for our building at Amarula Campsite are Arches and Domes.
In my deliberations about a suitable roof structure that doesn’t involve wood (there are no large trees in the area that can produce lumber or long, straight structural pieces) I explored the idea of domes and vaults. It made sense: Handmade, fired bricks are available in the village, clay for mortar is on site – all we were missing was the expertise to build.
After some searching I realized that dome-building is part of biogas digester construction which is quite common here. So contacting one of the biogas companies in Arusha lead to the contact with our “fundi” Petro Omala and his son Goodluck Omala- both experienced and open to the challenge of our project.
First we needed a support structure that was able to span the 2+m opening between the kitchen and sitting room: the Arch. This arch had to be strong enough to carry one side of the sitting room dome and the south edge of the kitchen Vault. Petro recommended using cement mortar for its higher compressive strength, and suggested pointing with clay later.

Completed central Arch

Our layout includes several part circles, so my question was all along how to go from that to a circular shape for the dome: turns out the shape does not need to be a perfect circle. It is important that there are no straight edges though so that the forces can always press toward the center.
The sleeping room dome was built first- a radius of about 1.4 m.
The bricks were laid on edge- this reduces the overall weight of the dome. Layer after layer gets built with ever increasing tilt to the center. The clay mortar forms a strong bond with the bricks after a few seconds.

And this is the secret to the building of a dome: Hold the bricks in place with special hooks until the mortar dries a little or until the layer is complete.

stabilizing a brick in space

hanging the 'hook'

Let me mention here that cement mortar would not work well because it is not sticky.
Attention must be paid to the edges of the bricks, and it is important to have mortar in all joints

building the dome

We chose to finish each dome with a bottle at the center, allowing light to come through and light up the ceiling a little.

bottle at the center

The larger dome of the sitting room was also built “free”- meaning without the guide of a radial stick. This was necessary because of the irregular shape of the plan, and made it difficult for our learning builders to build more than the first rows of the dome.
The following pictures show some of the scenes at work:

Master and student on the dome

working together on the dome

domes complete, preparing for the vault

With the domes closed our final task was to build the Nubian Vault over the kitchen. The technique of this type of vault relies on a strong end wall on which the first courses of the vault lean. This allows the builders to construct the entire vault without formwork.

Laying out the Nubian Vault's curve on the supporting wall

Start of the Nubian Vault

For the vault the bricks are laid “standing”. Two people work on either side and build the following bottom courses while the complete course is left to dry. Then both work together to complete each arch, supporting bricks for each other.

the opening of the Nubian Vault

The West end of the Vault is met by a half dome: note the changing direction of the bricks in the following picture:

closing from Vault to Half Dome at the West end

Nubian Vault and small dome

To complete the roof we now have to plaster all surfaces and then build gutters and good drainage. Watch for follow up posts in the near future.