Flattening or replacing a cupped board
Warping occurs most frequently in water-damaged or poorly nailed wide boards. Screw down the center of a dome-shaped warp and each side of curled-up edges. Drill a pilot hole for each screw; redrill the part going through the board so that it is large enough to let the screw pass freely and pull the board down. Have people stand on the warp if necessary. Countersink the screws: fill the holes with wood putty mixed with stain to match the floor.
For a stubborn warp, sand or strip off the finish. Put wet towels over the area. Cover them with plastic and weight the board with heavy furniture for a day or two before screwing it down.
To replace a severely warped board, drill overlapping 5 1/4-inch holes across the board; be careful not to go deeply into the subfloor. Then use a wood chisel and a mallet to split out the board and square the edge. If there is no subfloor, cut along a joist edge. Drill a hole and use a keyhole saw. Nail on a cleat to support the new board.
Cut a new board to fit. Trim off the grooved edge’s lower lip. Drill slanting pilot holes for nails. Put carpenter’s glue along the tongue and groove. Nail in the board with annular-ring nails; set them and fill the holes.