Volcanic ash is very lightweight and therefore is not much of a threat to buildings.

Question

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Volcanic ash can be a significant threat to buildings as it can accumulate on roofs and cause collapse, block gutters and cause flooding, damage electronic equipment, and clog HVAC systems. Additionally, when mixed with rainwater or melted snow, volcanic ash can form a heavy, concrete-like material that can cause additional weight on structures.

Volcanic Ash Is Very Lightweight and Therefore Is Not Much