Because that Volcanic Ash is just Small Chunks of Rocks if they land in a Roof if that Volcanic Ash Piled up it will be heavy because the Ash will get Heavy and Heavier if more Volcanic ash piled up because that the volcanic ash is small particles of rocks
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So your saying because ash consists to tiny fragments of rocks that becomes heavy as they piled up?
yes because that the Volcanic ash is just tiny fragments like dust if they colide some tiny fragments to it will fall and you thinks its just normal ash if they colide each other again and agian it will get heavy