"The objective of this overall project was to develop and test-radiological countermeasures that are applicable to post-nuclear attack recovery operations. The specific objective of this phase of the project was to conduct an exploratory experiment on the possible effectiveness of passive roof decontamination, by weather induced migration, in reducing the potential exposure rate in the basement shelter area of a small dwelling having a sloped roof. For the structure utilized and incident weather encountered: 1. Contrary to expectations that migration would cause dose rates to decrease in basement shelter areas, the actual migration of fallout particles from a sloped roof may cause such dose rates to either increase or decrease with time. 2. The presence of gutters can effect a dose increase during early time. The same effect may be expected in some, but not all shelter space if the fallout fell in a line under the roof eaves. 3. Even mild weather conditions can have significant effect on the movement of fallout particles on a sloped roof."
Ballistic Research Laboratory Report No. 1476
United Stated Federal Emergency Management Agency, Learning Resource Center: http://www.lrc.fema.gov/