Root cellars are natures way of storing fruits and vegetables. And they can be excellent storage areas for other things as well. A hundred years ago root cellars were one of the few ways they had of keeping things cool. People not only put potatoes and carrots in their root cellars, but their preserved meat, milk and cream, fruits and vegetables - literally anything they needed to keep cool. Even though root cellars didnt get nearly as cold as a refrigerator during summer months, root cellars generally were and are 30 to 40 degrees F cooler than daytime summer temperatures.

The first root cellars were usually dug with a pick and shovel. The only wall we had to build was the front wall the door was in. The other three walls were formed by the dirt from the hole we dug. I made the roof with three logs as supports, then I laid 2X10 planks over them and nailed them down (See illustration). Over the top of the roof I put about 2 feet of dirt, with grass eventually growing on top of it. The front wall was also made from 2 inch lumber. Unlike the drawing, this wall extends up another three feet (see photo). The stairs were made with these same planks, as well as the side walls on both sides of the stair case. The planks in the roof, stairs and front wall were all made from rough, unplaned lumber, actually 2 inches thick.

There is an upper door and a lower door. It is important you do this as each door adds greatly to how cool the root cellar will be in the summer time and how warm it will stay in the winter. The lower door on my cellar is constructed with a sheet of 1/4 inch plywood on each side filled with insulation. The upper door is tilted slightly so water will run off when it rains, and so it will be easier to find in the snow. To prevent rain from dripping down between the outside of the cellar and the top of the door, I use a couple of pieces of tin that are wide enough to hang over the top of the closed door after being wedged in between a couple of the planks on the outside of the cellar. This helps a lot. The upper door is constructed from two layers of 3/4 inch rough lumber. There is no insulation in it, and it has a layer of tin nailed to the top of it to keep it water proof. This door is hinged to the stairs side wall on the left side and is hinged so it can swing all the way around and lay on the grass.

Our main reason for having a root cellar is to keep our vegetables from freezing in the winter. We have very cold winters. For example, last winter we had several days when the thermometer dropped down past 40 degrees below zero F. How well has out cellar worked over the years? Very well. Nothing ever froze, except for once, and then it wasnt the cellars fault. On one of the coldest days last winter, I went to get some potatoes and carrots and was surprised to find the lower door open. Even with this, only a small part of the potatoes were frozen. After closing the lower door the temperature rose to above freezing again. I have never tested the temperature in the cellar during the winter time, but in the summers the temperature hovers around 51 degrees F. This is a bit cooler than the temperatures you would expect to find in a root cellar in the warmer parts of the USA. This is because our hottest days in the summer are only around 90 degrees F. And the cold winters tend to keep the ground a bit cooler throughout the year.

Last year my center cross beam in the roof broke right in the middle. The dampness from the earth above it had gradually rotted it over the years. I did not waterproof my roof when I built it, and should have. After it broke, we jacked up the center of the roof and put in a vertical support beam in the middle of the room. This should help the cellar last for several more years.

Temperature is your most important interest: As your root cellar needs to be kept as cool as possible, there are several things you can do to promote this:

First, borrow cold from the ground. Earth, even two feet down, gives a remarkable year wide temperature stability. The further down you go the better it is. You must go down a full 10 feet before complete temperature stability is reached. But for the average builder, how deep you go is limited because of expenses.

You can also borrow cool from the air. Often the nights air temperature will be cooler than the air in your cellar.
And finally, you should do what you can to prevent heat from having access to your cellar. This includes:
Having your root cellar in the shade throughout the day
Building on the north side of hills

Wise use of insulation

Your second most important consideration is humidity.

Even if kept cool, in a low humidity environment, your vegetables will soften and shrivel up. Most vegetables require high humidities. A typical underground root cellar will generally maintain a high humidity all by itself if it has an earth or dirt floor.
Air circulation: The best root cellars have vents (although none of the old cellars here in Southern Alberta I have seen have them). This is because the vegetables in your cellar give off gasses that often are conducive to either spoilage or sprouting. For example, apples naturally give off ethylene gas which makes potatoes sprout prematurely. (This can be used to your advantage if you have potatoes that are slow sprouting. Putem both in a plastic bag.) Good venting fundamentals include:

Have an inlet vent and an outlet vent.
The outlet must always be at the highest level in the cellar with the outlet tube flush with the inner wall.
The inlet should come into the cellar at the bottom. This is easily done if your cellar is built into a hill, but nearly as easy if it is buried in flat ground. With your inlet vent opening on top of the ground near your outlet vent, your inlet vent pipe must go all the way to the floor before opening into your cellar.

Keep shelves a couple of inches away from the walls of the cellar. This will greatly promote circulation around the vegetables stored on these shelves.

To prevent your potatoes from sprouting prematurely, keep your apples above them so the circulating air moves away from your potatoes.

Have a system in place to close your vents in freezing weather. Something as simple as a big sponge can work for this. If you have very cold winters, you may wish to block off both ends of each vent pipe.

How big of a cellar should you build?

A 5 foot by 8 foot root cellar will store 30 bushels of produce.
An 8 foot by 8 foot cellar should hold plenty for the average family.
A 10 foot by 10 foot cellar should take care of everything you can produce.

Shelves:

We have already mentioned shelves should be kept at least a couple of inches away from the walls for increased ventilation. Other things to consider are:

Use rot resistant or pressure treated wood.

After several years they will be less likely to rot and break, tumbling your foods on the floor. (The book gave one example of a person who went down to her cellar one day to find a good share of her canned fruit and vegetables broken on the floor. As the lids on canned goods rust after a couple of years, plan a dryer, cool place for these items.)

Liberal use of shelves will enhance the storage capacity of your cellar considerably.

Build your root cellar into a hill.

You dont have to find a door lying on the ground when it is under 3 feet of snow.

There is less chance of flooding during very wet conditions

Your cellar can be graded so any water that should run or seep in will run out the door.

Can be much more difficult to excavate.

Build your root cellar on flat ground.

Availability: not everyone has a steep hill in their back yard

Easier to excavate

Easier and cheaper to build (you dont have to brace your cellar for all that extra weight from the hill). But that added dirt will keep your cellar cooler!

You can build a vertical door around a staircase if you dont want to be shoveling snow to get at a horizontal door.

Build your cellar as part of your house:

Our house which is only one year old had a root cellar built into it when the house was constructed. Many older houses have a section of the basement that has an earthen floor. Its primary reason was probably for vegetable storage. You can also:

Build and insulate a room in this area.
Dig a cellar next to the house with an entry way to your cellar through the basement.
Put your cellar in an existing underground structure such as a pump house.

Construction methods:

Dugout: The cheapest way to go in stable soil
Wood construction: Be sure to use pressure treated wood.
Cement
Floors
Dirt: the simplest way to go and excellent for humidity control.
Gravel: In a very damp or very dry area you will want to put down three inches of gravel. If your cellar is unusually wet, you may want to even dig a sump in the middle of your cellar floor and fill this with gravel, along with the three inches on the floor. In very dry soil conditions you can sprinkle water on the gravel which will greatly increase the evaporation surface area.
Wood: put gaps in your boards for a higher humidity cellar.
Cement: If you want a storage area that is lower in humidity, this is a good way to go.
You may wish to build two rooms in your cellar. One with a cement floor for lower humidity storage items, and another room with no floor for higher humidity storage items. If you did this, the wall between the rooms should be as air tight as you can make it. If you have a venting system, you should have a separate set of vents for each room. And lastly, the high humidity storage area should be the far room in the cellar.

Using your root cellar:

Keep a thermometer and humidity gauge in your cellar.
Keep the door(s) closed to your cellar as much as possible if it is warm outside.
During the spring and fall of the year, open your vents (and even perhaps the door) at night when the temperature is dropping below the temperature of the air in your cellar. Close them early in the morning before the outside air warms up. (Be careful not to do this if the temperature is expected to drop below freezing.)
If the humidity in your cellar is too low you can raise it by:
Leaving at least the floor of your cellar exposed to the earth (a dirt floor or air gaps in your floor down to the earth).
Sprinkle water on a graveled floor or lay out damp towels or burlap bags.
Pack root vegetables in damp saw dust, sand or moss.
One caution about high humidities: If you get much of a temperature fluctuation in your cellar, humid air as it cools past its dew point will condense on the ceiling, walls, and produce. Excess water on your goods can induce spoilage. Cover vegetables with burlap, towels, etc. to absorb excess condensing moisture. Also, if your air is condensing inside, open your vents if the air outside is cooler than it is inside. Even if it is very humid air, as it warms in the root cellar, its relative humidity will drop. Of course, the opposite can happen. If you let warm damp air in, moisture will condense out as it cools.
During extremely cold weather, if your cellar is threatening to freeze, put a light bulb inside. If you do this, you need to cover your potatoes so they wont turn green. (Do not use a kerosene lantern. Kerosene lanterns produce ethylene, which is a fruit ripener.) Also remember that snow is an excellent insulator. Dont tramp down or remove the snow on top of your root cellar any more than you have to in order to gain entry.
Keep a fairly close eye on your produce and remove any that has begun to spoil. (It is a true axiom that 'one bad apple with spoil the bushel.'