Is it obligatory for a woman to wet all of her hair, or is it sufficient to run her wet fingers through its roots when performing ghusl for janabah?

The obligation in ghusl from janabah (ritual impurity) is to wash the hair with water until it reaches the skin, not merely to wet it. This is because the obligation is to pour water over the hair, cover it completely with washing, and saturate its roots. Ibn Qudamah said: "Washing the skin of the head is obligatory, whether the hair is thick or thin, and likewise everything beneath the hair... because Asma' narrated that she asked the Prophet, peace and blessings be upon him, about ghusl from janabah, and he said: «She takes water and purifies herself, performing the purification well, or perfecting the purification, then she pours it over her head, and rubs it until it reaches the roots of her hair, then she pours water over her entire body»." The jurists have agreed on the obligation of covering the entire hair of the head with water—its outer and inner parts—for both males and females, due to the Prophet's saying, peace and blessings be upon him: «Indeed, there is janabah under every hair; so wash the hair and purify the skin».