two large-headed slaters’ nails. These are made of hardcopper wire, cut copper, or cut brass. On hips, ridges,and in other locations where nailing is not possible, theslates are held in place with waterproof elastic slaters’cement colored to match the slate. Exposed nail headsare covered with the same cement.BITUMENS.— Hot bituminous compounds(bitumens) are used with several types of roofingsystems. Both asphalt and coal-tar pitch are bitumens.Although these two materials are similar in appearance,they have different characteristics. Asphalt is usually aproduct of the distillation of petroleum, whereas coal-tarpitch is a byproduct of the coking process in themanufacture of steel.Some asphalts are naturally occurring or are foundin combination with porous rock. However, mostroofing asphalts are manufactured from petroleumcrudes from which the lighter fractions have beenremoved. Roofing asphalts are available in a number ofdifferent grades for different roof slopes, climaticconditions, or installation methods.Roofing asphalts are graded on the basis of theirsoftening points, which range from a low of 135°F(57.2°C) to a high of 225°F (107.2°C). The softeningpoint is not the point at which the asphalt begins to flow,but is determined by test procedures established by theASTM. Asphalts begin to flow at somewhat lowertemperatures than their softening points, depending onthe slope involved and the weight of the asphalt andsurfacing material.Generally, the lower the softening point of anasphalt, the better its self-healing properties and the lesstendency it has to crack. Dead-flat roofs, where watermay stand, or nearly flat roofs, require an asphalt thathas the greatest waterproofing qualities and theself-healing properties of low-softening asphalts. Aspecial asphalt known as dead-flat asphalt is used insuch cases. As the slope of the roof increases, the needfor waterproofing is lessened, and an asphalt that willnot flow at expected normal temperatures must be used.For steeper roofing surfaces, asphalt with a softeningpoint of 185°F to 205°F (85°C to 96.1°C) is used. Thismaterial is classed as steep asphalt. In hot, dry climatesonly the high-temperature asphalts can be used.The softening point of coal-tar pitch generallyranges from 140°F to 155°F (60.0°C to 68.3°C). Thesoftening point of coal-tar pitch limits its usefulness;however, it has been used successfully for years in theeastern and middle western parts of the United States ondead-level or nearly level roofs. In the southwest, whereFigure 3-28.-Finish at the ridge: A. Boston ridge with stripshingles; B. Boston ridge with wood shingles; C. Metalridge.roof surfaces often reach temperatures of 126°F to147°F (52.2°C to 63.9°C) in the hot desert sun, thelow-softening point of coal-tar pitch makes it unsuitableas a roof surfacing material.When used within its limitations on flat andlow-pitched roofs in suitable climates, coal-tar pitchprovides one of the most durable roofing membranes.Coal-tar pitch is also reputed to have cold-flow, orself-healing, qualities. This is because the molecularstructure of pitch is such that individual molecules havea physical attraction for each other, so self-sealing is not3-24
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