What's the word for the sound of snow sliding down a roof?

Solution 1:

The sound is whoosh and the action is slide.

The lull of the winter night was broken by the intermittent whoosh of snow sliding off the tin roofs.

From the Oxford English Dictionary (login required):

whoosh, v.

2. transitive. To cause to move rapidly with a rushing sound. Also figurative. Const. up, to enliven.

DERIVATIVES

whoosh n. a sound of this nature (also reduplicated); also, an exclamation ‘whoosh!’; a movement accompanied by a rushing sound; a gushing or ‘whooshing’ style.

From Billy at billy.com:

Is a Metal Roof Right For You?
. . . metal roof owners will be pleased when, as the sun heats the roof up, they hear the whoosh of a small avalanche as the snow slides off the house.

I can’t vouch for the notability of these writers, but . . .

Every noise outside the house distracted her, the whoosh of the snow sliding off the roof, the crack of a branch.
_{Christmas In Delaney Mountain}

The truth hit me with a whoosh, a heap of snow sliding off the roof.
_{North by Night: A Story of the Underground Railroad}

. . . a place where at night you will listen to the whoosh of the snow sliding off the roof.
_{The Mountaineer}

More at Google; search ”whoosh” “snow” “sliding” “roof”.

Solution 2:

A Canadian's perspective: It depends on how you want to convey the image of the quantity of sliding snow. Having frequently heard the sound (and on occasions been frightened by it - especially in the middle of the night), I like the "the grumble of snow sliding off the roof" (relatively small quantity, slow movement) or, with a thaw in process, "the roar of the accumulated winter's snow being dumped from the roof" (huge quantity, rapid movement, nightmares).

David