Which tool is primarily used for creating volume at the roots?

The round brush is the primary tool used for creating volume at the roots during hairstyling. When blow-drying hair, a round brush can lift the hair away from the scalp. This technique not only adds volume but also helps to shape and curl the ends, enhancing the overall style. The round shape of the brush allows stylists to wrap sections of hair around it, which facilitates the creation of soft waves and volume in addition to the thermal styling from the blow dryer.

In contrast, curling irons are primarily designed to add curls or waves to the hair and, while they can create some volume, they do not achieve the same base lift as a round brush. Straightening irons are intended to smooth hair and can make it appear flatter, thus not suitable for adding volume. Flat combs serve various purposes like detangling or styling but do not provide the lift and volume at the roots that a round brush can achieve when used properly during blow-drying.