Stir fish sauce, sugar, juice of 1 1/2 limes, 1 tbs coriander and half the chilli in a bowl until sugar dissolves. Set aside.
Toss crab with remaining lime, chilli and coriander. Season.
Fill a dish with warm water, then dip 1 rice paper in water for 30 seconds. Remove and pat dry with a towel.
Lay paper on a bench and place 2 tbs crab at end nearest you. Top with some carrot, avocado, onion and cress. Fold in sides of paper, then roll up to enclose. Repeat to make 8 rolls.