Birkas Hamazon after drinking a glass of water

Birkas Hamazon after drinking a glass of water

According to R.Ch. Kaniyevsky Shlita, If someone was

thirsty at the end of his meal, and waited to drink

water till after Birkas Hamazon, he must Bentch again

Birkas Hamazon * (D’oryasa)

The Mordechai holds if you are thirsty at the end of

the meal and Bentch then your Benching was only

מדרבנן Therefore after you Benched if you

drink a glass of water you  are now מחויב מן התורה to 

Bench(again) The first Bentching doesn't count

since it was only a דרבנן Bentching.

*see Rema O.Ch. 197:4