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Month 5 and 6
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The 5th month, day 29 |
Flashing
Wherever joints occur in which
dissimilar materials come together or wherever there is a possibility of
leakage because of the type of construction, it usually is necessary to
insert flashing‑sheets or membranes of waterproof materials‑to repel the
water.
Although flashing may be composed
of impregnated felt or of combinations of felt and metal, it generally is
composed of one of the following metals: copper, zinc, lead, leaded
copper, tin, galvanized iron or steel, tinplated steel, soft iron and
copper‑bearing or alloy steel. Of these, the pure metals are the best. And
of the pure metals, copper, lead, zinc and leaded copper are the most
commonly used and the most effective because they corrode and deteriorate
the least on exposure to the weather. Coated sheets such as galvanized
iron and tin‑plated steel are satisfactory as long as the coating is
unbroken. Once the coating is broken, however, corrosion may be
accelerated by the electrolytic action set up between dissimilar metals.
No matter what metal is employed,
the installation of flashing is practically identical. Thus, the following
discussion, although based on copper (the metal most widely employed for
flashing), applies to all the other metals that can be used.
When using metal exposed to
changes in temperature found on building exteriors, the first point to
remember is that metals have fairly high coefficients of thermal expansion
and must be free to move with changes in temperature. If they are
confined, the stresses set up in trying to change dimensions eventually
cause fatigue cracks. Short, narrow strips, less than 10 to 12 inches
wide, can be fastened along both edges without much danger; wider sheets,
however, must not be confined along two opposite edges but must be free to
move.
The second point to remember is
that flashing must be installed in such a way that water is shed over any
unsealed joints in the., joints must be so Made that water could work
through them only again9t the force of gravity.
It must be impossible for driving
winds to force water through. 'thus, it usually is necessary that the
joint either provide a tortuous path in which the driving force of the
wind is dissipated or that the laps in the join are long enough that water
cannot possibly be driven through.
Copper flashing generally is
16‑ounce "roofing temper" (R.T.), a soft and pliable material weighing 16
ounces per square foot. Heavier copper is required for heavy‑duty flashing
such as that around certain kinds of tile roofs, whereas lighter copper
(not less than 14‑ounce) can be used in relatively protected points.
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