How to Frame the Main Floor

Check building codes and house plans for specifications on floor joist size and placement, including whether a center support will be required. Determine whether to use solid floor joists, typically 2-by-10-inch or 2-by-12-inch boards, or engineered joists with two horizontal boards, typically 2-by-6-inch or 2-by-8-inch, formed into a truss with some type of internal bracing.

Fasten a sill plate of 2-by-8-inch lumber to the perimeter foundation walls, typically with bolts set in a concrete wall. Drill holes in the board with a power drill at each bolt location and secure the plates with washers and nuts tightened with a wrench. Use a level to make sure the sill is level on all perimeters. Install 4-by-4-inch center posts with a 2-by-4-inch sill plate on top for any floor too wide for a single joist to span the opening.

Mark the rim bands for joists, typically 16 inches apart, measuring with a tape measure down the long side of the walls so that the floor joists will span the narrow part of the foundation. Stagger the spaces if a center support is used so that the joist ends will overlap on the center support. Lay the joists in place on the sill plates to be rolled into place erect and fastened one at a time.

Secure the joists to rim bands either by nailing through the rims into the joist ends or by using metal joist hangers -- brackets with a u-shaped support -- to hold the joists. Toenail solid joists or bottom chords of engineered joists to the sill plate. Use two nails per joist through the bands. Nail hangers to the bands first, then lay joists into them from the top and nail them through holes in the sides of the brackets. Nail the joist ends together where they are spliced on a center beam.

Cross-brace the joists with 2-by-4-inch boards with 45-degree angle ends. Nail an end to the top of one joist and the bottom of the next one. Put a second brace going the opposite angle next to the first one. Continue this X pattern of cross bracing the width of the joists. Cross brace every joist to another one. Space braces according to building code requirements.