two large-headed slaters’ nails. These are made of hard
copper wire, cut copper, or cut brass. On hips, ridges,
and in other locations where nailing is not possible, the
slates are held in place with waterproof elastic slaters’
cement colored to match the slate. Exposed nail heads
are covered with the same cement.
BITUMENS.— Hot bituminous compounds
(bitumens) are used with several types of roofing
systems. Both asphalt and coal-tar pitch are bitumens.
Although these two materials are similar in appearance,
they have different characteristics. Asphalt is usually a
product of the distillation of petroleum, whereas coal-tar
pitch is a byproduct of the coking process in the
manufacture of steel.
Some asphalts are naturally occurring or are found
in combination with porous rock. However, most
roofing asphalts are manufactured from petroleum
crudes from which the lighter fractions have been
removed. Roofing asphalts are available in a number of
different grades for different roof slopes, climatic
conditions, or installation methods.
Roofing asphalts are graded on the basis of their
softening points, which range from a low of 135°F
(57.2°C) to a high of 225°F (107.2°C). The softening
point is not the point at which the asphalt begins to flow,
but is determined by test procedures established by the
ASTM. Asphalts begin to flow at somewhat lower
temperatures than their softening points, depending on
the slope involved and the weight of the asphalt and
surfacing material.
Generally, the lower the softening point of an
asphalt, the better its self-healing properties and the less
tendency it has to crack. Dead-flat roofs, where water
may stand, or nearly flat roofs, require an asphalt that
has the greatest waterproofing qualities and the
self-healing properties of low-softening asphalts. A
special asphalt known as dead-flat asphalt is used in
such cases. As the slope of the roof increases, the need
for waterproofing is lessened, and an asphalt that will
not flow at expected normal temperatures must be used.
For steeper roofing surfaces, asphalt with a softening
point of 185°F to 205°F (85°C to 96.1°C) is used. This
material is classed as steep asphalt. In hot, dry climates
only the high-temperature asphalts can be used.
The softening point of coal-tar pitch generally
ranges from 140°F to 155°F (60.0°C to 68.3°C). The
softening point of coal-tar pitch limits its usefulness;
however, it has been used successfully for years in the
eastern and middle western parts of the United States on
dead-level or nearly level roofs. In the southwest, where
Figure 3-28.-Finish at the ridge: A. Boston ridge with strip
shingles; B. Boston ridge with wood shingles; C. Metal
ridge.
roof surfaces often reach temperatures of 126°F to
147°F (52.2°C to 63.9°C) in the hot desert sun, the
low-softening point of coal-tar pitch makes it unsuitable
as a roof surfacing material.
When used within its limitations on flat and
low-pitched roofs in suitable climates, coal-tar pitch
provides one of the most durable roofing membranes.
Coal-tar pitch is also reputed to have cold-flow, or
self-healing, qualities. This is because the molecular
structure of pitch is such that individual molecules have
a physical attraction for each other, so self-sealing is not
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