Figure 3-30.-Types of built-up roofing.
APPLICATION OF BITUMENS.—The method of
applying roofing depends on the type of roof deck.
Some roof decks are nailable and others are not. Figure
3-30 shows examples of wood deck (nailable), concrete
deck (not nailable), and built-up roof over insulation.
Nailable decks include such materials as wood or
fiberboard, poured or precast units of gypsum, and nail
able lightweight concrete. Non-nailable decks of
concrete or steel require different techniques of roofing.
View A of figure 3-30 shows a three-ply built-up roof
over a nailable deck, with a gravel or slag surface.
View B shows a three-ply built-up roof over a non-
nailable deck with a gravel or slag surface. View C
shows a four-ply built-up roof over insulation, with a
gravel or slag surface.
The temperatures at which bitumens are applied
are very critical. At high temperatures, asphalt is
seriously damaged and its life considerably shortened.
Heating asphalt to over 500°F (260°C) for a prolonged
period may decrease the weather life by as much as 50
percent. Coal-tar pitch should not be heated above
400°F (204°C). Asphalt should be applied to the roof at
an approximate temperature of 375°F to 425°F
(190.6°C to 218.3°C), and coal-tar pitch should be
applied at 275°F to 375°F (135°C to 190°C).
Bitumens are spread between felts at rates of 25 to
35 pounds per square, depending on the type of ply or
roofing felt. An asphalt primer must be used over
concrete before the hot asphalt is applied. It usually is
unnecessary to apply a primer under coal-tar pitch.
With wood and other types of nailable decks, the ply is
nailed to the deck to seal the joints between the units
and prevent dripping of the bitumens through the
deck.
Built-up roofs are classed by the number of plies of
felt that is used in their construction. The roof maybe
three-ply, four-ply, or five-ply, depending on whether
the roofing material can be nailed to the deck whether
insulation is to be applied underneath it, the type of
surfacing desired, the slope of the deck, the climatic
conditions, and the life expectancy of the roofing.
The ply-and-bitumen membrane of a built-up roof must
form a flexible covering that has sufficient strength to
withstand normal structure expansion. Most built-up
roofs have a surfacing over the last felt ply. This
protective surfacing can be applied in several ways.
SURFACING.—Glaze-coat and gravel surfaces are
the most commonly seen bituminous roofs.
Glaze Coat.—A coat of asphalt can be flooded over
the top layer of felt. This glaze coat protects the top
layer of felt from the rays of the sun. The glaze coat is
black, but it maybe coated with white or aluminum
surfacing to provide a reflective surface.
Gravel.—A flood coat of bitumen (60 pounds of
asphalt or 70 pounds of coal-tar pitch per square) is
applied over the top ply. Then a layer of aggregate,
such as rock gravel, slag, or ceramic granules, is
applied while the flood coat is still hot. The gravel
weighs
3-26
