Old timer is right about wing surface smoothness.
My motorglider would roll to the left when flying in rain and not when dry.
I determined the reason was the difference in the way rain beaded up on the left wing which had been painted and the right wing wing that had old paint and the rain laid smooth. Very minor difference, but it scared me the first time it happened. Both wings painted now and it flies straight and level.
Also, if the root gap tape is missing the aircraft will shake at low speed. The root gap is only about .060" without tape, but it does make a difference!