Why do hardwood flooring planks have grooves?

Why do hardwood flooring planks have grooves milled onto their bottom faces?
—Joe Madison, Fergus Falls, Minn.

There are several reasons for those grooves, Joe. Brett Miller of the National Wood Flooring Association explains:

1. The grooves provide a "relief area" for splinters to break into when the planks get nailed in place. Without this, splinters and bulges could create raised "pivot points" where the installed flooring meets the subfloor, which could result in squeaks.

2. Many manufacturers of solid-wood flooring cut grooves into the back face to reduce overall shipping weight. This does not negatively affect the performance of the flooring.

3. With engineered flooring, the grooves become part of the "balancing" of the multiple layers within the final product. Similar to plywood, engineered flooring is manufactured by sandwiching top and bottom layers of hardwood around interior-core layers of lesser-priced processed composites. To help counter the differing degrees of shrinking and swelling among these layers of unlike materials, the bottom layer gets kerf-cut.

4. The last (and most humorous) purpose of these grooves ensures that the installer knows which side of the flooring faces up.

Although many people believe the grooves are meant to deter cupping by relieving internal stress, Miller says this is not the case in most solid-wood flooring (typically 3–4" wide). Cupping usually results from a moisture imbalance in the wood, not stress. But, he adds, grooves can help ward off cupping in flooring 5" or wider, especially in flatsawn planks.