Where the chimney passes through the roof the construction should provide
space for expansion due to temperature changes, settlement, or slight
movement of the chimney during heavy winds. (See fig. 7.) Copper is the
best material for flashings. It is easier to handle than galvanized
sheet-metal, which is more often used because of its lesser cost, but
which will corrode in time, both from inside and outside exposure. Tin or
black iron are cheaper but will rust quickly unless frequently painted.
Lead and zinc are expensive and should not be used for chimney flashings,
for in case of fire under the roof they will melt and leave an opening to
create a draft by which the intensity of the fire will be increased.
[Illustration: Fig. 8.--A. Wrong connection, producing interference and a
poor draft. B. Correct construction, producing a good draft by providing
a free passage for the gases.]