The Floor Plan
To provide a stable base for the dance floor surface, you will need to construct a frame made of 2-by-4 lumber around the dance floor area or the entire room. Measure your room or the space and transfer the measurements to graph paper to scale. Be sure to include any fixed objects in your floor plan, especially any floor drains.
A Level Playing Field
The framing will correct any variations in level in the floor, but it's good to know in advance how much the floor may be out of level. Determine this by measuring 12 inches above the floor on one wall; place a mark on the wall at this height. Repeat on the opposing wall. Using a contractors string level, measure whether the 12-inch mark on one side of the room is level with the 12-inch mark on the other side. If the level indicates the two marks are not the same height, your floor is not level. You will have to shim your frame to create a level surface.
Before constructing the frame, spread a vapor barrier, like plastic sheeting, on the concrete floor surface. Then use high quality 2-by-4 lumber, preferably fir rather than pine, to create a series of frames to support the dance floor surface. The frames will look like wall studs, only they will lay against the floor. A long "stud" should fall on center every 24 inches to 30 inches along the longest side of the room. Insert a cross-supporting 2-by-4 on center every 24 inches along the width of the room. Shim if necessary to achieve a level frame.
Add Some "Spring"
Real dance floors have a "spring" built into them for a purpose: The floor gives and prevents injuries when dancers land hard on their feet. If your dance floor is for a dance company or a dance student, build some spring into your floor by adding strips of 3/4-inch-thick extruded poly bead insulation on top of the 2-by-4 framing. Use construction glue to affix these strips to the frames.
Use "A/C" or better 3/4-inch plywood to create lids for the framing. One side of the plywood should be smooth and relatively flawless, though knots will occur here and there. You can also purchase plywood that is "furniture grade" and will have no knots. Run a bead of wood glue along the top of the 2-by-4 frame (or the spring strips) and lay the plywood sheet down, nicer side up. Countersink and screw the sheet to the frame on 8-inch centers. Neatness counts; make sure there are no gaps between the sheets.
Fill the screw head counter-sink holes with wood plugs or putty and then sand the floor with a floor sander. Test the floor for smoothness by sliding around the floor in stocking feet, "feeling" for any bumps. Remove all the sawdust using a vacuum or tack cloths. Apply several coats of clear polyurethane with a "gloss" finish. An even better, though more expensive, surface treatment is a Marley portable dance floor. Marley floors come in one large roll than can be trimmed to proper size and eliminate all seams.