Build a root cellar by digging into a north-facing hillside, or straight down at least 2 m below grade, to a size of about 2 m by 3 m by 2 m tall for a small community, lining the walls and ceiling with stone, brick or timber and carrying the roof on beams sized for the earth above. With 0.6 m of earth, 2 m spans, 0.5 m spacing and an assumed 8 MPa bending stress, round beams must be about 15 cm across; at 1 m of earth, about 18 cm. Treat the finished cellar as a confined space, because the air at the floor can be unfit to breathe.
Part of Ice Houses and Cold Storage
Cellar construction is the digging, lining, roofing and ventilating of an underground store that keeps vegetables cool without ice, using the stable temperature of the ground.
The Roof Carries The Hill, And The Air Inside Can Hurt You
A cellar is a hole with a lid. The lid is the problem: it carries the weight of the earth above, which is tonnes per square metre, and it must do so for decades while the timber stays damp. The hole is a second problem: a sealed space full of respiring produce and rotting vegetable matter, with a ventilation system that works backwards in summer.
Root Cellar Build and Root Cellars describe cellar building in more general terms. This page works the arithmetic of the roof and the ventilation.
What You Need
- A north-facing hillside with good drainage (ideal), or flat ground dug straight down at least 2 m below grade. Test the drainage first; see Soil Drainage Capacity.
- Wall lining — stone, brick or timber. The floor can be packed earth or gravel.
- Roof beams, planks and earth — sized as below.
- Two ventilation pipes of 10-15 cm diameter and an insulated, solid door facing north if possible.
Method: Build The Cellar
Step 1: Dig to 2 m by 3 m by 2 m tall. That is the size the parent article gives as sufficient for a small community. Keep walls vertical and shore them as you dig.
Step 2: Line the walls and ceiling. Use dry-stacked stone, brick or timber. Unlined earth walls cave in.
Step 3: Size and lay the roof beams. Use the arithmetic below, and lay planks or poles across the beams before the earth goes on.
Step 4: Place the door and vestibule. Fit a solid insulated door, facing north if possible. A double door keeps temperature stable.
Step 5: Fit the ventilation pipes. A low inlet pipe near the floor at one end and a high outlet near the ceiling at the opposite end, each 10-15 cm across.
Step 6: Cover with earth and drain the surface. Slope the ground so rain runs away from the cellar.
Worked Example: Beam Size For The Roof
Inputs, with the assumptions marked: beams span the 2 m width; beams 0.5 m apart; soil of 1,800 kg/m3 (assumed); an allowable bending stress in sound dry timber of 8 MPa (assumed, so check it against your species); beams treated as simply supported; the weight of the planks, the beams and any snow ignored.
With 0.6 m of earth.
- Earth pressure on the roof: 0.6 m x 1,800 kg/m3 x 9.81 m/s2 = 10,595 Pa, about 10.6 kPa.
- Load on one beam per metre: 10,595 x 0.5 = 5,297 N/m.
- Bending moment: M = w x L^2 / 8 = 5,297 x 2^2 / 8 = 2,649 N.m.
- Section modulus needed: Z = M / stress = 2,649 / 8,000,000 = 3.31 x 10^-4 m3.
- For a round beam, Z = pi x d^3 / 32, so d^3 = 32 x 3.31 x 10^-4 / pi = 3.37 x 10^-3 and d = 0.150 m.
With 1.0 m of earth.
- Pressure: 1.0 x 1,800 x 9.81 = 17,658 Pa. Load per beam: 8,829 N/m. M = 8,829 x 4 / 8 = 4,414 N.m.
- Z = 4,414 / 8,000,000 = 5.52 x 10^-4 m3. d^3 = 32 x 5.52 x 10^-4 / pi = 5.62 x 10^-3, d = 0.178 m.
So the beam grows from 15 cm to about 18 cm when the earth is 1 m rather than 0.6 m. The 15 cm beams at 50 cm spacing in Root Cellar Build match the first case; for more cover, choose larger beams or a closer spacing. When in doubt, overbuild.
The Two-Pipe Ventilation Works Backwards In Summer
The usual explanation is that warm air rises out of the high pipe and draws cool air in through the low one. That drive exists only when the cellar is warmer than the outside, as in winter. In summer the cellar is cooler and its air is denser.
- Air density = pressure / (287 x temperature in kelvin). At 101,325 Pa: inside at 5 degrees C (278.15 K), 1.269 kg/m3; outside at 25 degrees C (298.15 K), 1.184 kg/m3.
- The cellar air is 0.085 kg/m3 heavier. Over 2 m of height between the pipe openings, the pressure difference is 0.085 x 9.81 x 2 = 1.7 Pa.
That small pressure acts in the reverse direction: cool cellar air tends to sink out of the low pipe while warm, humid air enters at the top, where it can condense on the cold walls. Even in the right direction, 1.7 Pa is weak: the dynamic pressure of a 3 m/s breeze is 0.5 x 1.2 x 3^2 = 5.4 Pa, so wind often decides the flow. Practical rules: ventilate on cool mornings and nights, close the pipes against hot humid air, and never rely on the pipes alone to clear the air before you enter.
Common Mistakes
- Under-sized roof beams — they sag, crack and fall. Use the calculation.
- Beams of wet or rotten timber — their strength is far below the assumption.
- No drainage — the cellar floods and the walls slump.
- Trusting the pipes to keep the air safe — they can be reversed, blocked or stagnant.
- Skipping the lining — earth walls collapse.
Safety
A cellar can have too little oxygen and too much carbon dioxide
Produce respires and rotting vegetable matter consumes oxygen, so in a well-sealed cellar the oxygen falls and carbon dioxide builds up. Carbon dioxide is heavier than air and pools at the floor, so the air at the top step can seem fine while the air at the bottom cannot support life. A person who goes down can collapse in a few breaths, and the second person who goes in after them can too. Workplace confined-space rules in the United States class air with less than 19.5 percent oxygen as oxygen-deficient. Never lower a flame, candle or lantern to test the air. Open the door and force fresh air to the floor with a long duct and a bellows or fan before anyone descends, keep ventilating while anyone is below, tie a line to the person going in, and keep an attendant at the top who hauls the line and never climbs in. If the person below stumbles, sits down or goes quiet, pull them out.
The roof is heavy
A roof with 1 m of earth on it carries about 17.7 kPa, roughly 1.8 tonnes on every square metre. Do not remove props, or lay the earth on, until the beams are in place and checked, and keep people out of the cellar while the earth is going on.
What’s Next
- Learn next: Temperature and Humidity — keep the cellar at the right conditions
- Combine with: What to Store — match the food to the conditions