Build a root cellar by digging into a well-drained north-facing slope, shoring or lining the walls as you dig, roofing it with beams sized for the weight of the earth on top, and fitting a low inlet vent, a high outlet vent and a double door. The excavation can bury you before the cellar is finished, and the finished room can hold air that cannot support life.
Part of Root Cellars & Cool Storage
Cellar excavation and construction is the digging, lining, roofing and ventilating of an earth-covered store that stays at 2-10 °C.
Soil Is Heavy, So Both The Hole And The Roof Are Structures
A cubic meter of soil weighs more than a tonne, so an unsupported wall of soil is a danger to the digger and an earth roof is a heavy load on the beams. Most failures of cellars are structural: walls that slump during digging and roofs that sag a few years later. The temperature and humidity are easy once the structure stands. The method for the underground structure itself is in Underground and Earth-Sheltered Building; the roof-beam arithmetic is in Cellar Construction.
What You Need
- A north-facing hillside or a flat site you can dig into and drain, away from tree roots
- Digging tools and a way to remove spoil
- Wall lining: stone, brick, timber or earthbag, so that unlined earth walls don’t cave in
- Roof beams and planks, sized for the soil above
- Ventilation pipes for a low inlet and a high outlet
- An insulated, solid door, preferably a double door, facing north
- Gravel and drain pipe for the floor and surrounding ground
Method: Excavate And Build
Step 1: Test the drainage. Dig a test hole and watch whether water drains. A cellar in a wet site floods.
Step 2: Dig in stages and shore the walls. Dig in stages, and support the walls with boards and props as you go; do not leave a deep face unsupported while you work below it. Keep spoil well back from the edge, since the weight of a pile at the rim adds load to the wall.
Step 3: Line the walls. Dry-stacked stone, brick or timber keep earth walls from caving in later.
Step 4: Lay the roof beams. Size them for the load of earth above; see the arithmetic in Cellar Construction and Earth Load Calculations. Lay planks or poles across the beams before the earth goes on.
Step 5: Fit vents and the door. A low inlet pipe at one end, a high outlet pipe at the opposite end, and a solid, insulated door.
Step 6: Cover and drain. Mound the earth over the roof and slope the ground so rain runs away; see French Drains and Perimeter Drainage.
Common Mistakes
- Digging in the wet season without a drainage test — the hole fills.
- Skipping the lining — unlined walls slump into the room.
- Under-sized roof beams — they sag or break under the earth.
- No outlet vent — damp, stale air and, in a sealed cellar, a gas hazard.
Safety
Trench collapse, roof collapse and bad air
Collapse. Soil is heavy and unsupported excavation walls can fall without warning. As a recalled guide (workplace-safety rules in the United States require protection in trenches from about 1.5 m deep), shore any wall deeper than your waist, never work alone in a deep hole, and keep heavy loads and spoil heaps back from the edge. A collapsed roof of earth is a crushing hazard: do not enter a cellar whose roof is sagging, cracking or dripping soil.
Bad air. A finished cellar full of stored produce can be short of oxygen or rich in carbon dioxide, especially if closed up. Open the door and ventilate before going in, never enter alone, and do not test the air with a flame, which goes out only once the air is already too thin to breathe safely and ignites methane. If someone collapses inside, do not go in after them; rescue from above, on a line.
What’s Next
- Learn next: Temperature & Humidity Zones — arranging the contents
- Combine with: Shelving & Organization — fitting out the room