The snow that had piled up on the roof during the night slid off with a thump.
Literal
Night [gen-の] interval-during roof [on-に] piled-up snow [subj-が] gasatto fell-down-[toward-speaker-てきた].
Complex relative clause: 夜の間に屋根に積もった modifies 雪 — 'the snow that had piled up on the roof during the night.' ガサッ is the onomatopoeia for a sudden rough/loose sliding or falling sound. ~てくる marks the downward motion as coming toward the observer. 積もる ('pile up, accumulate') is specifically used for snow, dust, leaves.