Building a root cellar means creating an underground space where you keep potatoes, carrots, apples, wine and preserves cool and humid without plugging anything into the mains. You use the earth itself as a cool box: at roughly one to two metres deep, the soil temperature stays fairly constant all year round. A good root cellar aims for about 4 to 10 degrees and a high humidity of around 85 to 95 percent, which is exactly what most fruit and vegetables like. Two things make or break the project: the water table on your plot and the ventilation. In this article I’ll walk you through all of it: types, choosing a spot, the build step by step, the ventilation system, costs, permission and the mistakes you really want to avoid.
What is a root cellar and why build one?
A root cellar, sometimes called a food store or potato cellar, is a largely buried space that draws its coolness and moisture from the surrounding ground. The principle is centuries old and simple: deeper in the ground, the temperature swings far less than it does at the surface. Where it can be 30 degrees outside in summer and below freezing in winter, a well-built cellar stays moderately cool down at depth.
So why go to the trouble?
- Storage without electricity. Your fridge is full, but a root cellar gives you cubic metres of cool storage with no energy bill.
- Longer shelf life. At low temperature and high humidity, the respiration and ripening of fruit and vegetables slow down, so they keep for months (source: Practical Self Reliance).
- Somewhere to put a big harvest. If you have a vegetable patch or fruit trees, a root cellar is the obvious home for your autumn crop.
- Self-sufficient and sustainable. No refrigeration unit to break down, just physics.
What are the ideal temperature and humidity?
Most sources on root cellars name a target between 0 and roughly 4.5 degrees with a humidity of 85 to 95 percent for moisture-loving crops such as carrots and apples (source: Ecohome, Practical Self Reliance). In practice, a root cellar without active cooling usually doesn’t get that low, and you’ll sit closer to the 4 to 10 degree band. That’s still perfectly fine for most of your stores.
Important: not everything wants the same climate. Broadly speaking, there are two groups.
| Storage group | Temperature | Humidity | Examples |
|---|---|---|---|
| Cool and damp | approx. 0 to 8 °C | 85 to 95 % | carrots, beetroot, celeriac, apples, pears, potatoes |
| Cool and dry | approx. 10 to 15 °C | 60 to 70 % | onions, garlic, squash |
(Source: Practical Self Reliance, Mother Earth News)
Watch out for a few crop-specific points:
- Potatoes want dark and cool, but not too cold. Below about 4 degrees the starch converts to sugar and they turn sweet (source: NICE Voedingsinfo). Count on around 4 to 8 degrees.
- Carrots and beetroot like it close to freezing and very damp. Pack them in moist sand if you need to.
- Apples and pears give off ethylene gas. That gas makes potatoes sprout and other fruit ripen faster. So keep apples separate from your potatoes (source: Practical Self Reliance).
- Onions, garlic and squash want the opposite: dry conditions. In a damp root cellar they go mouldy, so they don’t really belong in the humid zone.
Always hang up a thermometer and a hygrometer and check them regularly. Without measuring, you have no idea whether your climate is right (source: University of Alaska Fairbanks, Cooperative Extension).
What types of root cellar are there?
You don’t have to build a brick vault straight away. There’s a whole sliding scale from simple and cheap to solid and expensive.
| Type | Advantages | Drawbacks |
|---|---|---|
| Buried barrel or IBC container | Cheap, quick, a nice starter model for testing the idea | Limited space, less robust, insulation and ventilation restricted |
| Fully buried cellar | Best and most stable temperature | Lots of digging, water table critical |
| Cellar in a bank or under a mound | Easier access from the side, natural drainage | You need a slope |
| Cellar under a shed or outbuilding | Sheltered, double use of the space | More complex structurally, foundations of the existing building |
| Partitioned part of an existing cellar | Very little work, uses what’s already there | Only if your cellar stays cool enough |
For a first go, a buried barrel or a cleaned-out IBC container is ideal: you dig a hole, drop the container in with some straw around it as insulation, and you have a mini food store. If you want serious storage, you build a proper buried cellar or make use of a bank.
Choosing your spot: this is where the project is won or lost
Before you put a single spade in the ground, pick the right location. Three things are crucial.
1. Water table. This is the big one. If the groundwater sits high, your cellar floods and you end up with a pond instead of a food store. Dig a test hole to the depth you’re planning and come back a few days later (ideally during a wet spell) to see whether water is standing in it. Ask your local water authority or environment agency for the current and the highest recorded groundwater level too. Always build well above the highest water table.
2. Aspect and shade. Go for the north side or a shady spot if you can. A north-facing slope is often recommended; cellars that face full east or west are harder to keep cool (source: Duurzaam Vlaanderen).
3. Drainage. Rain and surface water needs to run away from your cellar, not towards it. Don’t put the entrance at the lowest point of your garden, and make sure the ground around it slopes gently away.
Also check whether there are cables and pipes running where you want to dig. More on that further down under permission.
Building a root cellar: step by step
Below is the layout for a fully buried cellar in brick, block or concrete. For a buried barrel you simply skip most of the steps.
Step 1: Excavation
For a proper cellar you won’t get away without a digger or mini excavator; doing it by hand is next to impossible (source: Duurzaam Vlaanderen). Dig wider than the outside dimensions of the cellar, so you have room to work outside the walls and add drainage later. Allow for the thickness of the floor, the walls and the layer of soil that goes on top.
Step 2: Foundation and floor
Lay a stable, level base. That can be a concrete slab or a layer of heavy stone on a well-compacted bed of gravel or sand. Gravel under the floor helps with drainage. If you want maximum humidity, you can deliberately go for a partly open floor or a sand and gravel floor: an earth floor keeps the cellar damp all by itself (source: University of Alaska Fairbanks). With a concrete floor you sometimes have to add moisture back in with trays of water.
Step 3: Walls
Build the walls up in concrete blocks, brick or block work, or pour concrete walls. Masonry is the classic method (source: Duurzaam Vlaanderen). Think about stability: the walls have to cope with the earth pressure pushing in from outside. If you’re digging deeper or the ground is soft, a structural calculation isn’t a luxury, it’s a necessity.
Step 4: Waterproofing
Finish the outside of the walls so they’re watertight, for example with a bitumen or tanking layer and, if needed, a dimpled membrane against the soil. Run a drainage pipe in a gravel trench all around the foot of the walls, so water travelling down the outside is carried away instead of pushing its way in.
Step 5: Roof or vault with a soil layer
The roof can be a concrete slab, a brick vault or an arched structure. A layer of soil goes on top. That soil layer does two jobs at once: it insulates against the outside temperature and works as thermal mass that keeps the temperature inside stable. Plant the soil layer with grass or ground cover so it doesn’t wash away and blends nicely into the garden (source: Duurzaam Vlaanderen).
Step 6: Door and access
Fit a well-sealing door, ideally one you can keep shut in summer and prop open on cool nights for extra cooling. With a bank-built cellar the door goes conveniently into the side. Consider a draught lobby or a double door if you want to keep the cellar really stable.
Ventilation: the most important technical part
If there’s one thing home-built root cellars get wrong, it’s the ventilation. Without air circulation you get mould, condensation and rotting stores. The good news: you don’t need a fan, natural draught does the work.
The principle is called the stack effect. Cool, fresh air comes in low, warms up a little inside the cellar, picks up moisture, rises and leaves the cellar high up. That way the air refreshes itself continuously (source: FarmstandApp).
Here’s how to do it in practice:
- Two pipes. Use two separate pipes, for example PVC of roughly 10 to 15 cm diameter.
- Intake low. The intake pipe ends inside just above the floor, so fresh, cool air flows in at the bottom.
- Exhaust high. The exhaust pipe starts inside just under the ceiling and sticks up well above ground level and the roof outside. The higher the exhaust, the stronger the draught (source: FarmstandApp).
- Place them diagonally. Put the two pipes on opposite sides of the cellar. That forces the air to travel through the whole space instead of short-circuiting straight from one pipe into the other.
- Make them adjustable. Fit a slide or damper on both pipes so you can tune the airflow. In a cellar that’s too damp or too warm you open them further; in a frost you close them (source: FarmstandApp).
- Mesh over the openings. Put fine mesh or an insect screen on the pipe openings so mice, rats and insects can’t crawl in. Bend the outside opening downwards or fit a cowl on top against rain.
Moisture, condensation and pests
A root cellar is allowed to be damp, but not wet. Too much moisture plus stagnant air is the direct route to mould. The ventilation above is your first line of defence; airing the place extra on cool evenings carries excess moisture away.
If you want more humidity instead (with a concrete floor that dries things out too much), put trays of water or damp sand on the floor (source: University of Alaska Fairbanks). Store potatoes in perforated crates, nets or open baskets so the air can get to them, never in a sealed plastic bag.
Against pests: mesh on every opening, a well-sealing door and no gaps. Mice find a food store just as attractive as you do.
Permission: what to watch out for
An underground cellar counts as a structure, and that can need permission. Don’t just assume you’re free to do as you like on your own land.
- Planning permission and building regulations. Building a cellar often needs consent. Whether it does depends on your local authority, the local plan, the depth and the effect on groundwater and nearby foundations. Only small, shallow cellars with no effect on groundwater or foundations sometimes fall under permitted development. Always check your specific situation with your local planning authority and building control before you start.
- Decision times. Count on an application taking weeks rather than days. Ask your local authority how long they normally take, and start early instead of the week you’ve booked the digger.
- Digging near cables and pipes. Before you dig with machinery, run a utility search so you know where services are buried. Contact your utility providers or use a “before you dig” cable and pipe location service, and allow time for the answer to come back. That’s how you avoid dangerous and expensive damage.
In short: sort it out first, then dig. An afternoon on the phone with your council and a utility search are a lot cheaper than a punctured gas main or an enforcement notice after the fact.
Materials list and costs
Costs vary enormously depending on the type you build. A starter model can cost almost nothing; a full masonry cellar runs into the thousands.
Basic materials list for a buried masonry cellar:
- Concrete or stone for the foundation and floor
- Concrete blocks, brick or blockwork for the walls (or formwork and poured concrete)
- Material for the roof or vault
- Waterproofing (bitumen or membrane) and dimpled membrane
- Drainage pipe and gravel
- Two ventilation pipes (PVC) with dampers and mesh
- A well-sealing door
- Soil for the covering layer and planting
- Hire of a mini excavator or digger
Realistic cost range:
| Type | Indicative cost |
|---|---|
| Buried barrel or IBC version | a few tens up to a couple of hundred euros |
| Home-built masonry or concrete root cellar | roughly a few hundred to a few thousand euros in materials and machine hire |
| Professionally installed or large cellar | clearly more, plus permission and structural engineering costs |
These figures are guide prices. The biggest costs are usually the excavation (machine hire) and the concrete or masonry. Any structural calculation and application fees come on top of that.
Common mistakes
- Building where the water table is too high. The number one mistake. Test with a trial hole and check with your water authority before you begin.
- No ventilation, or poor ventilation. Without two pipes (in low, out high) you get mould and condensation. Ventilation isn’t a side issue, it’s the heart of the thing.
- Built too warm. Too little soil on top of the cellar, a sunny aspect or a door that doesn’t seal: then it gets too warm inside and your stores spoil.
- Storing everything together. Apples next to potatoes make those potatoes sprout thanks to the ethylene gas, and onions go mouldy in a damp zone. Separate damp from dry and keep the ethylene producers apart.
- Skipping permission and the utility search. Digging without checking or building without consent can end in damage or trouble with the council.
Frequently asked questions
How deep does a root cellar need to be?
You want enough soil above and around the cellar to keep the temperature stable, which in practice often means around one to two metres deep, as long as the groundwater allows it. At that depth, the soil temperature is reasonably constant all year. The water table on your plot ultimately decides how deep you can responsibly go.
What temperature does a root cellar hold?
A good root cellar stays roughly between 4 and 10 degrees, with a high humidity of about 85 to 95 percent. Root cellars are often quoted at 0 to 4.5 degrees (source: Ecohome, Practical Self Reliance), but without active cooling you’ll usually sit a little higher in our climate.
Do I need permission for a root cellar?
Often, yes. An underground cellar can require consent, depending on your local authority, the local plan and the depth. Check your situation with your local planning authority and building control, and run a utility search before you dig so you know where cables and pipes are.
Can I build a root cellar without a digger?
For a proper buried cellar, a mini excavator or digger is all but essential (source: Duurzaam Vlaanderen). If you want to start without heavy machinery, go for the starter model: a buried barrel or IBC container you can sink by hand.
Why do you need two ventilation pipes?
One pipe low for intake and one pipe high for exhaust together create a stack effect: cool air in, warm damp air out (source: FarmstandApp). That keeps the air moving, carries moisture and ethylene gas away and prevents mould.
Can I store potatoes and apples together?
Better not. Apples release ethylene gas, which makes potatoes sprout faster (source: Practical Self Reliance). Keep them in separate corners or separate rooms of your cellar.
Want to get started right away with professional plans and a step-by-step approach? At Fred’s DIY Plans you’ll find fully worked-out building plans with a materials list, so you can begin without any guesswork.