Replacing Old Square-Edged Broken Boards
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It is simpler to replace square-edged boards than tongue-and-groove boards, since they do not have interlocking edges. Take care to avoid damaging cables and pipes below floor level.
Mark a pencil line on the damaged board over the nearest joist (Image 1). If the damage is central, mark lines on joists either side of the damage.
Lever up the broken board, using a pry bar (Image 2). Rest the pry bar on a wood offcut to avoid damaging the floor.
Once the board has been raised high enough, place wood offcuts underneath to hold it in a secure position (Image 3).
Saw along the pencil lines to remove the damaged section of board (Image 4). Protect the floor with a spare piece of board.
Using the damaged section of board as a template, mark the new board and cut it to size (Image 5).
Position the new section of board in the gap, and nail it in place (Image 6).