Clearing ash from a roof may prevent collapse, but such a decision must be weighed against the risk of personal injury working in a dark, ash-rich environment—people can easily slip from roofs, fall from ladders, and fall through weak roofs while clearing and removing ash. In addition, if an ashfall is accompanied by rain, the roof may become slippery and the wet ash could be difficult if not impossible to shovel or sweep.
After an ashfall, buildings that have received more than 100-300 mm (4-12 in) of ash and that have not collapsed still run a high risk of load damage, including collapse! The addition of more weight on a roof during cleanup operations can tip the load beyond the structure's load capacity and cause collapse. The ash should be removed, however, as soon as it can be done safely.
It may be advisable to remove ash before it exceeds a thickness of 100-150 mm (4-6 in), but only if the roof is easily accessible and the ash can be removed safely. Because it is often dark or "pitch black" during an ashfall, it may not be possible to safely remove ash until a later time.