Install the band joist. The band joist, also called a rim joist, sits vertically on top of the sill plate. Use the same size lumber for the band joist as you will for the main floor joists; 2x10s (50x250 mm) are an appropriate size for most applications. Position each board vertically so that its edge is flush with the outer edge of the sill plate, and then toe-nail the board to the sill plate with framing nails.