For storing and aging firewood, a shed is an excellent choice. Set atop a stone foundation it will be sturdy structure capable of lasting for decades. Your choice of wooden flooring in your shed is an important consideration. The foundation material is not as critical as selecting floor planking which is rot resistant. Over time, most wood will decay and eventually rot. Moisture is the enemy, not the foundation stones per se. Here are some ways to give your shed floor a long lease on life.
The higher above the ground line your shed foundation extends, the better it will be able to shield the wood from soil borne moisture, or accumulated snow. Lay the foundation stone atop a one to two foot trench filled with loose washed gravel. This will provide good drainage around the shed perimeter and help prevent spring frost heaves from twisting the structure. A good foundation height would be about 16 inches, roughly the same height as two courses of cinder blocks.
After you have laid the sills, you will be adding floor joists. You might use pressure treated wood. If this is your choice, be sure the planks are thoroughly dried before installing them. Pressure treated wood has rot resistant chemicals injected into it via water. Drying the planks beforehand allows this water to evaporate. As pressure treated wood dries, it will also shrink slightly. Allowing that shrinkage to occur before you build will help prevent the joists from pulling loose from the flooring installed atop them.
Another excellent wood choice would be to use floor joists and planking cut from black locust. The wood is exceptionally strong and naturally moisture resistant. At one time, it was the preferred wood for railroad ties, mine timbers and ship masts.
If your building budget will permit it, you could install redwood joists and flooring. Like black locust, redwood is naturally rot resistant. Unlike locust, redwood is relatively soft wood and will dent and mar quite easily. Redwood is available at many local lumber yards or may be ordered from many home improvement stores.
Black locust can often be obtained from small independent sawmills. They will rough saw the locust for you in any thickness you request. Two inch locust planking will hold up well for decades. Like pressure treated wood, give your locust lumber adequate time to dry.
If your shed design of accommodate it, build an overhanging roof. Rain water and snow melt will drip farther away from the structure helping to keep everything inside nice and dry.
Construct your shed in an open airy location. Good air circulation around the outside of the structure will slow the penetration of moisture into the sill, joists and flooring. Consider adding a window and perhaps a floor vent as well. Anything that encourages dry air to find its way inside, will help prevent the floor from rotting. With proper site selection and rot resistant flooring, your shed will last for many years.