Air that travels into the attic from the house under carries water vapor. Even after the dust settles, the chemicals produced by the ashes' interplay with water vapor also ­scatter the solar's radiation and forestall it from reaching the Earth's floor. If allowed to proceed, the wet surroundings can create situations conducive to mildew and mold development, and can even rot. The mix of heat from under and snow on high creates conditions that warm the roof sheathing and shingles. Because the ic...