This house was damaged by the heavy load of ash that accumulated on its roof during the eruption of Rabaul Caldera on September 19, 1994. Note the thickness of the ash deposits on top of the roof that didn't collapse (right side). Vulcan Volcano, one of two vents that erupted in the caldera, is visible to the left of the house.

Most damage to buildings from ashfall occurs when the load of ash exceeds the strength of either the roof-supporting structures or material used to cover the structure (sheet metal, plywood, etc.). Dry ash has a weight of 400-700 kg/m³ (880-1,545 lb/yd³), and rainwater can increase this by 50-100 percent if the ash becomes saturated. For a dry layer of ash about 10 cm (4 in) thick, the extra load on a building can range 40-70 kg/m² (120 to 200 lb/yd²); a wet layer might reach 100-125 kg/m² (300-350 lb/yd²).

During a heavy ashfall, which might be accompanied by rain and lightning, it is difficult and risky to remove ash from the top of a building or house. The risk to people trying to remove ash from roofs (for example, falling) may be greater than the risk from collapse of the building.