The effects of ash loads on buildings vary greatly depending of their design and construction, including roof slope, construction materials, roof span and support system, and age and maintenance of the building. In general, flat roofs are more susceptible to damage and collapse than steeply pitched roofs, and roofs made of smooth materials like sheet metal and glass are more likely to shed volcanic ash than roofs made of rough materials like thatch and asphalt or wood shingles.
Buildings designed to withstand a heavy load of winter snow will clearly support thicker accumulations of ash than buildings not engineered for any type of load or shear stress. Surveys of buildings damaged from the accumulation of ash during the eruptions of Mt. Pinatubo in the Philippines and Rabaul Caldera in Papua New Guinea indicate that roofs with wide spans (for example, warehouses) are more vulnerable to collapse than buildings with short spans typical of small homes.
Complex roof profiles or geometry and obstructions on roofs such as chimneys, parapets, roof tanks or solar panels may lead to a greater accumulation of ash next to these features if the ash is drifting with the wind. Such uneven accumulation of ash on roofs can lead to an unbalanced load on the roof, increasing the potential for roof failure. Also, ash loads against these obstructions may lead to their failure and, indirectly, to failure of the roof.