Cob Building Project - Apple and Root Store

Started in autum 2019, this is a new project that is slowly evolving over 2020 lock down period. We have long needed a  rodent proof, frost proof structure that would act as a storage facility over the winter months to enable us to store our crops for longer. Finding a suitable place to keep apples, carrots, potatoes, jerusalem artichokes etc away from little teeth and fluctuating temperatures has been tricky, so this project is an experiment really. The idea was also made in conjunction with a desire to deepen and extend our first wildlife pond (made 16 years ago) into a possible aquaculture carp pond, which I will write about another time. When you start digging soil out - ours is sandy loam over a relatively high sandy-clay sub soil, perfect for cob making - you need somewhere to put it all or else it just soon becomes a nettley mountain! Hence a new structure made of cob, which with its thick insulating walls might become a winter 'cellar' for us. I say might because it is, like all my projects, an ongoing experiment.

First I chose a site that is shaded by a hedge and next door building to reduce heat gain, and is pretty close the the source of the clay to reduce effort transporting down the garden!   

I built a base from second hand car tyres(sourced for free from local garage)  filled with gravel and then filled inside the tyre walls with gravel to give a 20cm depth of compacted gravel to help prevent rodents coming up. This was overlaid with several layers of old aviary wire which I had procurred several years ago when my mother dismantled her aviaries. I overlapped the gaps in the wire to try and again narrow rodent access. If this doesnt work I may have to cement a layer over this, but I'm hoping to avoid that if at all possible.

I then bridged the gaps between tyres with eco grade OSB  smartply, left over from building the roundhouse, in order to give support to cob walls at these points where the tyres meet. You want to avoid cob from ever touching the ground and also you don't want the tyres sticking out further that the walls as this will catch rainwater and feed it back to the cob. The cob walls will evntually be rendered or lime washed, and water needs to be able to drip off at the base and not feed back into the base of the wall.

I then had Stroud PDC group round in October 2019 and they spent a happy afternoon making cob with clay, sand and straw and starting to build the walls up.There was some strange shapes created which I did have to afterwards knock back into alignment, but cob is very forgiving while it is still drying!

The two fence posts were set into the ground with postcrete to  become the support for the eventual door frame. The door was made from an old wooden door that had small panes of glass in it. I removed the glass, adjusted it to fit the narrower gap, infilled the holes with recycled polystyrene (both door and polystyrene hoaded for some years as usual), clad and painted it with several coats of leftover external paint. The doorway was designed to be narrow, and the  frame made so the door fit snugly against it when locked and the door was insulated well,  to avoid CO² loss and to keep cool in summer and frost free in winter.

In the internal corners I  set diagonal timbers to act as shelf supports in the cob wall as it was built up, these enabled several shelves to be rested on them - long lengths of local sourced larch waney board, some left over from the shed build but some new ones too.

An internal  barreled ceiling was made by laying chicken wire over some homegrown poles (willow and hazel) set into the gable end walls and cobbing a thin layer onto this. On top of the ceiling a thick layer of straw was added and then a second set of poles was set into the top of the cob walls to create the actual roof supports. More straw ensured a thick protective layer. 

Around the edges of these roof poles I weaved a 40cm willow edging, just as you would make a basket, to support the edges of the liner. The whole roof was then layered in a recycled geotextile liner used for ponds to protect a roofing butyl liner from being damaged by the sticks and to add more insulation. The butyl roof liner was then secured by cable ties onto the pole ends (passing cable ties through holes drilled in the poles) and around a heavy duty natural rope which would keep the roof liner flat and also look attractive.

The cob was painted with several layers of limewash to give the whole thing a breathable yet waterproof coating. 

This year, 2022, we are storing apples, squash, carrots, beetroot, potatoes and jerusalem artichokes. Last winter the apples did fine but the building was still drying out and squash seemed to get damp.  This year it feels a lot dryer inside, despite heavy rain this autumn, and I have high hopes for my 15 spaghetti squash making it through!  Update - spaghettie squash not long storage crops! They last about 3 months, I am trialing different varieties this year (2024) to get a longer storage.

 There has been no sign of rodent entry except maybe a tiny hole in the ceiling I found in year 1 where a mouse might have entered from under the eaves, I've plugged the little hole with cob and no trace yet of any return. It might just have been a  bit of insecure cob that fell out. Certainly no damage from mice and rats eating apples as there used to be when we stored in the garage.