The Root Cellar Floor
The floor of the structure should be of sand or gravel to help keep things clean and dry. If you have clay soil place a few inches of gravel on top before starting your root cellar.
The structure can be of framed material similar to building a house. The roof can be trussed or timbers laid flat across the walls. You may also build an A-frame type structure although they are not the most practical due to sloping walls.
Having worked with logs I prefer heavier more solid structures. Besides the fact that logs are plentiful in our area they are free for the taking. When it comes to root cellars we need not concern ourselves with having white washed walls or that they are perfectly straight.
What we want to accomplish is to create a 8-10×10 area using uprights on at least two foot centers. If we are using logs then we can go four feet centers. One should also nail logs or boards for bracing one the outside of the structure on all sides. (Bracing always goes from ground to next corner at 45 degree angle).
The Root Cellar Building Construction
The door should be on the south side to permit winter days to provide some warmth. The door of the building should be doubled to prevent cold or hot air entering the building. Build it similar to having a screen door on your house. Each door should be well insulated with about one to two inches of air space in between.
For simplicity one can use normal trusses or they can build strong ones out of smaller saplings.
When one considers the amount of dirt on top of the structure we can visualize how a truss would be stronger than simply lying logs flat across the walls. But if you place 6-8 inch logs or rail way ties on the flat just make sure you place them side by side with little to no space between them.
If you use trusses for the roof then you should also use green treated plywood for the roof to protect it all. If you used logs then simply place a few sheets of black plastic across the top before placing any dirt.
There should be a small 1-2″ vent placed through the roof to allow food gases to escape. You could also place one nearest the peak on the south side instead of through the roof. The choice is yours. If you go through the roof or at the peak place an elbow so rain will not enter. In extreme cold climates you can plug the vents on very cold days.
The walls of the structure should have plywood, slabs, logs or boards along the sides to help keep the dirt from caving in.
*The dirt will cave in and settle and make you miserable if you do not do the above*
One you have it built you should place plastic or tar on the wood all around the building to help keep rainfall from filtering into the root cellar. Gently back fill the dirt against the structure. Filling the south side by the door can be tricky but it only needs to fill as much as possible without falling back into your doorway. If you are using clay loam it should pack fairly easy by walking on it as it goes up in height.
Once you have completely covered the structure with at least one foot of dirt on the roof and banked a few feet along the sides you can let the weeds grow or plant grass or wild flowers to help hold it all together.
You may also want to build bins for potatoes, carrots, onions, and make shelves for other vegetables. If you wanted to put canning in the root cellar I would have a backup heat source to keep from freezing. (Not recommended for solar users)
Enjoy the fruits of your labor.