Why are most floors made of boards which are 2 1/4” wide?

2 1/4" is the standard, traditional milling for oak (and most other species) of flooring. Departing from the 2 1/4” width generally results in decreased availability and higher pricing. The wider the board, the wider the shrinkage cracks between boards, and this explains why there is not much use of wide plank flooring in our dry climate. Narrower boards, such as 1 1/2”, are often recommended for certain applications such as over radiant heat systems to minimize shrinkage.