1 kg per cubic meter = 0.06243 lbs per cubic foot
Parameters:
(1) total area of the roof
(2) density of the falling material in pounds per cubic foot
(3) thickness of the debris layer in feet
(4) load capacity for the roof in lbs per square foot (psf)
total weight of fallen debris on the roof in lbs =
= (area of roof in square feet) * (thickness in feet) * (density in lbs per cubic foot)
The density of the debris may range from fairly light (ash, similar to snow) up to solid rock (over 200 lbs per cubic foot).
If the weight of debris per square foot exceeds the load capacity of the roof then it will collapse. A large rock shot out from an eruption is more likely to punch right through.
The presence of rain will significantly increase the weight of volcanic ash.
Sloped roofs are better than flat roofs since some debris will slide off.
If the falling debris is red hot, then fire is another risk to the building.
