pouring down the side of a chimney, a dormer window, or any other
vertical wall, to run off in an oblique direction and into cracks that
never thought of being exposed to falling rain. 'Valleys' fail to
carry their own rivers when they are punctured by nails carelessly
driven too far within their borders; when the rust that corrupts the
metal of which they are commonly composed has eaten their substance
from the under side perhaps, their weakness undiscovered till the
torrent breaks through; when they become choked with leaves and dust
and overflow their banks; when they are torn asunder by their efforts
to accommodate themselves to changes of temperature, and when ice cakes
come down from the steep roofs and break holes through them.

"The other danger is peculiar to cold climates, where the roof must
protect not only from driving rain but from snow and ice in all their
moods and tenses. When the higher peaks feel the warmth of the sun or
the internal heat of the building, the lower slopes and valleys being
without such influence, it sometimes happens that the rills will be set
to running by the warmth of the upper portions, while the colder
climate below will stop them in their course, building around the
slate, shingles or tiles an impervious ice dam, from which the
descending streams can find no outlet except by 'setting back' under
the slates and running down inside. Eave spouts and conductors are
especially liable to this climatic influence, for nothing is more
common than to find them freezing in the shade while the roofs above
are basking in the sun. As Jack observes, admitting water above an ice
dam is a different kind of sin in a roof from that which caused the
ruin of my plans last night, but it is no less unpardonable. The same
treatment that will make a roof non-conducting of fire will, to some
extent, overcome this danger, or a double boarding may be laid upon the
rafters, with an air space between. This or the mineral wool packing