Wattle and Daub

Humans have built with earth for more than 10,000 years, and earth buildings are found on every continent except Antarctica. Here in Britain, earth building usually takes the form of wattle and daub, clay lump or cob. All of these techniques rely on clay to bind together different subsoils such as sand and gravel. In cob, the subsoil is sculpted directly into walls; clay lump is a a method of making subsoil into unfired bricks, and wattle and daub uses small timbers (wattles) to support the subsoil mix, known as daub.


There was plenty of ash coppice available on site, so we used this to make the wattles. We sprung ash uprights into pockets cut into the oak frame, and then wove smaller ash poles between the uprights. Hazel is the more traditional material for wattling, but ash works fine too.


The mix for cob, clay lump and daub is basically the same: clay, aggregates such as sand and gravel, and straw. The proportions can be varied according to the subsoil available. The soil beneath our foundations was almost pure clay, so we had put this aside while digging the foundations to use for daub. We mixed clay and sand 50:50, and added about one bale of wheat straw per tonne. This gave us a mix which clung to the wattles, didn't shrink or crack too much while drying, and set very hard.

We trod the mix underfoot, turning it on a tarp. Our building needed a relatively small amount of earth, since the walls are only about 6" thick. For buildings which need more earth, such as cob buildings whose walls can be up to 2' thick, it would be more practical to use a minidigger to mix the subsoil.


Applying daub to wattle is easy and fun, especially when lots of people get involved.


Earth building lends itself to sculptural creativity. Here, a niche has been built directly into the wall.


We chose an earth walled building for several reasons. Firstly, we wanted to use as many materials from the site as possible - the subsoil came from under the foundations and the straw from a couple of fields away. Secondly, we wanted to involve volunteers in the construction, and daubing is something that anyone can join in with; it's arduous but fun, and many hands make light work. Third, we wanted to minimize the environmental impact of the building. Although earth is not a great insulator, this is a building that will be used in the summer and doesn't need to retain lots of heat. Using earth means that the building has a very low embodied energy - little energy is expended in the extraction, processing and application of the materials. We just dug up the earth and mixed and applied it by hand. Bricks, on the other hand, are mined, fired and transported large distances, all of which is very energy intensive.
An earth wall also has the advantage of being breathable; it can absorb and release humidity to create a stable environment which doesn't suffer from dampness - this is useful in a shower block.

Roof

We framed the roof from a mixture of sweet chestnut and larch. Because the oak frame does the job of holding the roof together, we were able to make a roof space uninterrupted by ties.


Green oak batten is laid over the rafters. Using oak batten means that it can be untreated, and it looks good from below.



The roof covering is oak shakes. These have been split along the grain. Although they look delicate, they have a life expectancy of 70 years.


The shakes are dressed with an axe on the roof to cut them to the right shape, especially around details such as hips and valleys. They're fixed with stainless steel nails.


Our roof has no membrane so that the shakes can be seen from below, but it is still perfectly watertight.

Wall frame

We sourced some green oak from just over the border in Essex, then we built an octagonal frame.


The joints in the frame are held together with tapered oak pegs. As the peg is knocked in, it draws the joints ever more tightly together. Below you can see a diagonal brace with the peg ready to be knocked.



As the frame dries, it will change from a blond colour to a silvery-grey. Oak frames are extremely durable and resist rot because of the wood's high tannin content.

Plinth

The oak frame needs to be separated from the ground to protect it from damp. The usual way of doing this is with a masonry plinth.

We chose reclaimed Suffolk red bricks from a local reclaim yard for our plinth.


The brick plinth starts below ground on top of the limecrete footing, and rises to at least 1 foot above ground level. The bricks are bedded in a mortar made from sharp sand and natural hydraulic lime, no cement. As with the limecrete, the lime mortar translates to an 80% reduction in carbon emissions.

Traditional lime mortar is also much more flexible than cement mortar. It can accomodate the tiny movements in the building over time without cracking, making it ultimately more durable than cement. Unlike cement mortar, lime mortar is breathable. It encourages moisture to evaporate away from the plinth, rather than being retained in the wall.



The lime mortar will weather naturally over time. It gives a soft, attractive result which complements the tradition Suffolk red bricks.

Foundations

The site for the building is on heavy clay. We removed the clay and put it aside to use later for making the daub.


We filled the lower part of the foundations with drainage gravel with a drainage pipe bedded into it. The drainage pipe leads to a deep soakaway below the clay layer. These "self-draining" foundations encourage moisture to move away from under the foundations and not rise up the wall.


On top of the drainage layer, we layed a limecrete footing. Limecrete works like concrete, but instead of cement, natural hydraulic lime is used. Lime production releases 80% less CO2 than cement production, so reduces the environmental impact of the building. Limecrete is also more flexible and breathable than cement, so is more appropriate for a building which uses natural materials.