Praise be to Allaah.
When doing ghusl from janaabah you have to wash the entire body, including the hair. Ghusl means letting water run over every part of the body. Al-Sharh al-Mumti', 1/128.
It is essential to pour water on the hair and make it reach what is beneath the hair. Simply wiping over the head is not sufficient because it is not regarded as ghusl (washing); what is required is washing not wiping.
It was narrated that Aa'ishah said: When the Messenger of Allaah (peace and blessings of Allaah be upon him) did ghusl from janaabah, he would start by washing his two hands, then he would pour water into his left hand with his right hand and wash his private part, then do wudoo' as for prayer, then he would take the water and, using his fingers, make it reach the roots of his hair, until he was certain that the water had reached the roots of his hair, then he would pour three scoops of water over his head, then he would pour water over the rest of his body, then he would wash his feet.
Narrated by Muslim, 474.
Shaykh Ibn Uthaymeen said in his commentary on this hadeeth: It is essential when doing ghusl from janaabah to make sure the water reaches what is under the hair, even if the hair is thick.
This is understood from the fact that he (peace and blessings of Allaah be upon him) put his fingers in his hair and poured water over it three times after he made sure that the water was reaching the roots of his hair.
Sharh Buloogh al-Maraam, p. 399
Shaykh Abd al-Azeez ibn Baaz (may Allaah have mercy on him) said: Women who find it difficult to wash their heads when doing ghusl from janaabah should be told that it is sufficient to pour water over their heads three times so that the water reaches every part of their heads, with no need to undo their braids or change anything of their hairstyle which they find it hard to change. They should also be reminded of the great reward they will have with Allaah, the good consequences and the everlasting good life that they will have in the abode of honour if they are patient in adhering to the rulings of sharee'ah.
Majmoo' Fataawa al-Shaykh Abd al-Azeez ibn Baaz (may Allaah have mercy on him), 6/236
See also question no. 34776.