What is checking?
Checking is small surface cracks in the wood grain.
What happened?
Loss of moisture causes wood fiber to shrink. Wood may check as a result of the drying process that is the beginning of a piece of wood becoming any product.
Why has my floor checked?
Some checking is inevitable and is allowed in every grade of hardwood flooring. Checking is typically not visible at the time flooring is manufactured but is “closed”. It may “open” or exhibit itself after installation, however, in response to changes in the environment. It should not be assumed to be “defective”, nor can anyone predict when/where it will occur.
Additional Resources:
Here is a good article from a leading industry publication with photos and an overview of checking: https://www.woodfloorbusiness.com/troubleshooting/understand-what-causes-checking-in-solid-wood-floors.html