Most hardwood floors consist of tongue-and-groove boards, but not all of them. Some people prefer the more primitive look of straight-edge and shiplap boards. Instead of a tongue and a groove, shiplap boards have an undercut on one edge and an overcut on the other, and you assemble the boards by matching the edges. Because the boards don't interlock like tongue-and-groove ones, you need some method of affixing the overlapping edge to the subfloor -- nails, screws and glue are all possibilities. Shiplap milling is usually reserved for softwood planks that are 6 inches or more wide.
Unpack the flooring planks and allow them to acclimate in the installation space for three to five days before installation. This prevents shrinking, swelling and warping after installation.
Check the floor with a moisture meter and proceed with installation if the moisture content is 10 percent or below. This is especially important if you plan to glue the floor to a concrete subfloor. If the moisture content is too high, lower humidity in the installation space with a heater or dehumidifier.
Level the subfloor with floor leveling compound and a belt sander, then vacuum and tack it with a damp rag. If the floor is wood, cover it with a moisture barrier, such as 6 mil plastic sheeting. Staple the barrier to the floor with a staple gun. If the subfloor is concrete, prime it with two coats of waterproofing primer and let the primer dry for at least 24 hours.
Start installing the flooring along the most prominent wall in the room. Rip the undercut edges off the planks in the first row, using a table saw. If you're installing on a wood subfloor, butt the boards together end to end, leaving a 1/4-inch expansion gap between the floor and the wall. If you're installing on concrete, spread flooring adhesive on the floor with a notched trowel before setting the boards in this row.
Affix planks to a wood subfloor with trim screws -- which have undersized heads -- or finish nails. Because you don't sand shiplap floors in the same way as tongue-and-groove ones, the nails can have rounded, decorative heads. Space the fasteners by 8 to 10 inches; avoid driving any into the uncovered groove on the outside edges of the planks.
Lay the next row of planks with their undercut edges covering the overcut edges of the planks in the first row. Stagger the end joints at least 6 inches from the joints in the first row. If gluing, spread glue before laying the planks. If nailing or screwing, drive fasteners into the shiplap joint as well into the ends of each board.
Continue laying the floor in this manner, using a jigsaw to cut notches around obstructions such as door frames and cabinets. Rip the planks in the last row to the proper widths with a table saw, and screw, nail or glue them in place.
Sand the end joints flush with a hand sander and 120-grit sandpaper before finishing. Shiplap floors are supposed to look rustic, so there is no need to sand with regular floor sanders.
Apply stain with a rag, rubbing with the grain of the wood. Allow the stain to penetrate for about five minutes, then wipe off the excess with another rag.
Finish with two or three coats of clear polyurethane. Spread the finish with a paint brush, allow it to dry, then scuff-sand with a floor buffer equipped with a 120-grit sanding screen. Vacuum the floor and tack it with a damp rag to remove sanding dust, then apply another coat. Repeat if necessary.