DEFEATING ICE DAMS BY HENRI DE MARNE PROPERLY BALANCED SOFFIT AND RIDGE VENTILATION IS THE ONLY FOOLPROOF APPROACH I ce dams are not peculiar to northern regions:They can and do occur in any area of the U.S.with a total mean snowfall of six or more inches an- nually-nearly three fourths of the con- tinental United States,according to U.S.Weather Bureau data. Most of us are familiar with how ice dams form,and the damage they can cause.But few of us seem to know how to prevent them or,at least,limit their damage. Ice-Dam Basics For those unschooled in the me- chanics of ice dams,a quick review may be helpful.On the roof of an unheated building snow will melt gradually from the perimeter first,while slowly sinking over the entire blanket.On a heated building,the snow