Engineered hardwood floors are made up of layers. The top layer is 100% natural wood, which comes in a variety of species. The bottom layer is also wood. In the middle is a core built from 5 to 7 layers of plywood that crisscross in different directions.
 

Engineered construction creates a highly stable core that is less likely to expand, contract or shift when exposed to moisture, humidity, and temperature. This makes engineered wood flooring a great option in rooms that are subject to moisture (like basements) or over concrete slab and radiant heating systems.