If your shoe molding is separating from the baseboard — bowing outward or pulling away at certain points — it is a common problem with a few possible causes. Understanding the cause will help you choose the right fix.
Wood shoe molding, especially softwoods like pine, can warp as it dries out or as humidity levels change. A warped piece will pull away from the baseboard at its bowed sections. This is most common with pieces that were stored improperly or installed while still wet.
Shoe molding should be nailed approximately every 12 to 16 inches. If nails are spaced too far apart, the molding can pull away between fastener points, especially if the wood has any natural bow to it.
If the nails are too short to reach the baseboard or wall plate, they may not have enough grip to hold the molding tight. Similarly, if nails were aimed at the baseboard but missed, they are just going through the shoe molding into empty space.
Sometimes the baseboard moves as the house settles, pulling the shoe molding with it or creating a gap between the two.