Roof Loading

The specific weight of dry ash can vary from 400 to 700 kg/m3, and rainwater can increase this by 50-100% or more if the ash becomes saturated by rain, sometimes reaching more than 2,000 kg/m3. The problems of loading by ash are similar to those from loading by snow, but the effects of ash accumulation are much more severe—the load due to ash is typically much greater (see table below), ash doesn't melt, and the ash can clog gutters and cause them to collapse, especially after rainfall. In areas that have snow-loading codes, some protection against ash may result but his is highly dependent on the location of structures because snow load levels vary with altitude and location.

Density & load comparison, 100 mm (4 in) of snow and
100 mm (4 in) volcanic ash
(Johnston, 1997; p. 75)

Description Density kg/m3 Load kPa
New snow 50-70 0.05-0.07
Damp new snow 100-200 0.1-0.2
Settled snow 200-300 0.2-0.3
Dry uncompacted ash 500-1,300 0.5-1.3
Wet compacted ash 1,000-2,000 1.0-2.0

The load on a bulding is given by the equation: