All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Firstly, we say that the criterion for determining whether the beard is thin or thick is whether or not it conceals the skin underneath it. If the skin can be seen underneath the beard, then it is thin. If the skin cannot be seen, then it is thick. Ibn ‘Uthaymeen said: “The beard could be thin or thick; the thin beard is the one which does not conceal the skin … the thick beard which conceals the skin ...” [Sharh Az-Zaad]
For more benefit, please refer to Fatwa 46331.
There is no difference with regard to the thick and thin beard in the Ghusl for Janaabah (major ritual impurity); it is obligatory to wash the beard and the skin underneath. This is the opinion of the Hanbalis and others. However, the Hanbalis differentiate between the thick and thin beards in Wudhoo'; they held that if the beard is thick, it is sufficient to wash the outer surface only of the beard and do Takhleel (run the wet fingers through it). If the beard is thin, it is obligatory to wash the skin underneath it. This is the case in Wudhoo', but in the obligatory Ghusl, both the beard and what is underneath it must be washed whether the beard is thick or thin.
Another opinion for the Hanbalis holds that it is not obligatory to wash the lower layer of the thick beard; however, the main opinion of the Hanbali school of Fiqh is that it must be washed. The Hanbali scholar Al-Mardaawi underlined in his book Al-Insaaf: "In case of Ghusl, the whole body should be thoroughly washed, including the hair and the skin underneath it. This is the opinion of the (Hanbali) school…”
Ibn Muflih said: “The meaning of washing the whole body (in Ghusl) is pouring water over all the apparent parts of the body as long as this does not incur any harm on the person... and washing the skin underneath the hair on one's head and beard, even if it is thick...” [Al-Mubdi']
Al-Mawsoo‘ah Al-Fiqhiyyah (Fiqh Encyclopedia) reads: “In case of Ghusl, it is not enough to do Takhleel; rather, it is an obligation to let the water reach the roots of the beard even if it is thick according to the unanimous opinion of the (four main) schools of Fiqh. The Prophet said: “Under every hair there is Janaabah; so, wash the hair and cleanse the skin.” In order to ensure that the water reaches the roots of the hair while avoiding excessiveness in water use, one should reach with all ten of his wet fingers to the roots of the hair, then pour water so as to avoid excessive water use.”
It is enough that one feels that he has most likely fulfilled Isbaagh (that the water has reached all the parts of the body). Ar-Rawdh Al-Murbi' with the Haashiyah of Ibn Qaasim reads, "According to the sound view of the (Hanbali) school, it is enough to pour water over the body until it is likely that water has reached the whole body. This is based on the Hadeeth reported on the authority of ‘Aaishah where she said that whenever the Messenger of Allaah made Ghusl from Janaabah ... he washed his body and did Takhleel of his hair till he felt that the whole skin had become wet...”
Allaah Knows best.