Building a wood floor over dirt can involve several challenges not found when constructing a wood floor over traditional constructed surfaces, such as concrete, sand or gravel. Soil shifts and is affected easily by environmental conditions outside the floor area. Because of this, the dirt floor requires additional preparation not necessary with other floor types. These preparations create issues of their own, forcing you to build a floor on unanchored floor joists. Once completed though, the combination of joists and floorboards holds together the whole in a stable level floor suitable for further covering in anything from carpeting to floor tiles.
Level the floor with a rake and then compress the dirt with a tamp to create a solid surface. Check the level with a carpenter's level before placing the flooring, making certain that its level to keep your floorboards level in turn.
Cover the dirt floor with a layer of 10-mil plastic sheeting to serve as a vapor barrier, which will keep moisture from seeping through the dirt and onto your wood floor. Lay the sheets in rows across the dirt floor. Overlap the bottom of the walls with two inches of the sheet edges raised upward along the wall's surface. Overlap the seams where the sheets join as well, laying the edges of each new row seven inches over the top of the edges of the row before it on the floor. Cut the plastic with a utility knife where necessary, and stagger the joints between rows to add strength.
Secure the plastic sheets to the walls with masking tape and cover each seam between two sheets with masking tape as well.
Place a layer of 1/2-inch polystyrene insulation over the plastic sheeting to protect the plastic barrier from punctures, add insulation to the floor and provide a cushion for the planks. Butt the insulation closely together and then cover the seams with a strip of fiberglass tape. Cut the sheets as needed with the utility knife. Leave a 1/4-inch gap between the sheets and the walls.
Measure the length and width of the room with a tape measure. Divide the length by 16 inches to determine the number of floor joists needed for floor overage. Cut a 2-by-4 plank for each joist location to the measured room width using a circular saw. Make a plan to set the planks along the edges of the wall, and then in 16-inch intervals between the two edge pieces.
Lay the planks in place along the four-inch side to serve as floor joists. Set the first two planks against opposing walls. Measure 16-inches from the center of one of the edge planks, and then place the next plank across the plastic sheeting, centered on the 16-inch mark. Continue across the floor, placing a new plank every 16-inches until you reach the opposing wall.
Lay plywood sheets over the plank, leaving a gap of 1/4-inch along each wall, and 1/8-inch between sheets. Cut the sheets using a circular saw to cover partial areas, and use the unused portion of the last cut sheet in a row to begin new rows with in order to stagger the joints between rows.
Nail the plywood sheets to the 2-by-4-inch floor joists using a hammer and two-inch ring shank nails spaced every 10-inches along the joist. Fill the spaces between the boards with silicone caulking to seal the joints while allowing movement flexibility between sheets.