A common concern during ashfall is the potential collapse of buildings from the weight of accumulated ash on roofs. However, this is a very rare impact which requires very thick ashfalls (typically >100 mm). Damage due to roof loading is dependent on the load from the accumulated ash and the structural strength of the building.
The load associated with an ashfall can cause the collapse of roofing material (e.g. sheet roofs), the supporting structure (e.g. rafters or walls) or both. Under great enough loads (> 100 mm), the entire building may collapse. Loads depend on the thickness and density of the ash deposit. Wetted ash is up to twice as dense as dry ash thus loading is correspondingly higher.
Non-engineered, long-span and low-pitched roofs are particularly vulnerable to collapse, potentially under thicknesses of around 100 mm or less. Under thinner ashfalls (< 100 mm), structural damage is unlikely although non-structural elements such as gutters and overhangs may suffer damage. Structural damage to engineered buildings with short span roofs is unlikely for ash thicknesses <300 mm.
If very thick ashfall is expected, a survey should be made of the strength of roofs in the area and of the maximum thickness of ash that they will bear without danger of collapse, especially for critical facilities and buildings that are expected to provide refuge for people during ashfall. Such surveys must take into account the density of both dry and wet ash. The effects of volcanic ash on roofs depend primarily on the density and thickness of the ash deposit.
One of the few documented examples was the collapse of roofs from the ash load on roofs during the explosive eruption of Mt. Pinatubo on June 15, 1991, killed nearly 300 people. Historical examples of the effects of ash accumulating on roofs are provided in the case studies section.