Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- Islam gives due care and great interest to cleanliness, either internally or externally.
2- It is each Muslim’s duty to well observe his/her own cleanliness either in one’s body or in the surroundings.
3- It is essential to make the water reach all parts of the body, even the roots of the hair.
4- If a woman has braids, she does not have to undo them unless water does not reach to their roots.
Responding to your question, Dr. Muhammad Salama, PhD in Islamic Studies in English, a lecturer, researcher, and translator, states:
With regard to obligatory ghusl (ritual bath), what is required is to make the water reach every part of the body, even the roots of the hair. Thus, a woman should take care in this regard. If a woman has braids, she does not have to undo them unless water does not reach to their roots.
Umm Salamah (may Allah be pleased with her) asked the Prophet (peace and blessings be upon him), “I am a woman who braids my hair; should I undo it to make ghusl from janabah (major ritual impurity)? He said, “No, it will be sufficient for you to pour three handfuls of water on your head, then pour water over yourself and then you will be purified.”
However, the best way for taking an obligatory ghusl for women, especially after menses, is described in the following hadith. Asma’ bint Shakl asked the Prophet (peace and blessings be upon him) how to perform ghusl after menses. He answered, “One of you should take water and lotus leaves and purify herself well therewith. Then she should pour water over her head and rub it strongly so that it should reach the roots of her hair, and then pour water over herself. Then she should take a piece of cloth that is scented with musk and purify herself with it.” Asma’ said, “How should she purify herself with it?” He said in wonder, ‘Subhan Allah (highly exalted is Allah)! Let her purify herself with it.” `A’ishah said to her, “She should follow the traces of blood.”
Allah Almighty knows best.