When installing floor joists on top of a concrete slab, consider several factors before proceeding. Because concrete has a corrosive effect on all wood products, including treated wood, you must separate the concrete from the wood. By following a few simple rules for installation, you can be sure that your floor project will be completed successfully.
When installing floor joists on top of a concrete slab, protect the wood from the corrosiveness of the concrete. At each floor joist and around the rim of the floor, install felt paper between the concrete and the wood frame of the floor system. Marks all areas where wood will touch the concrete, and glue 2-inch-wide strips of felt paper to the concrete. Use basic school glue sticks -- contact cement -- to hold the paper down. Avoid using construction adhesive or any other glue that may cause lumps in the paper after it has dried.
Treated wood should be laid flat onto the concrete slab at the point where the floor joists will meet the concrete. Lay the treated 2-by-4s onto the concrete and attach them using concrete nails or screws. The nails should be shot through the 2-by-4s and into the concrete using a concrete nail gun. Rent these tools at your local tool rental store. When using concrete screws to attach the 2-by-4s to the concrete, drill holes through the wood and into the concrete using a hammer drill and concrete bit. Use the appropriate size screws -- corresponding to the drill bit size -- to attach the wood. Countersink the screws by drilling a hole into the wood that is 1/2 inch deep and slightly larger than the head of the concrete screw.
After the treated 2-by-4s are installed, assemble the floor joists on top of the concrete floor.