Figure 3-14.-Roofing underlayment: A. Single coverage;B. Double coverage.the roof with atop lap of at least 2 inches at all horizontalpoints and a 4-inch side lap at all end joints (fig. 3-14,view A). Lap the underlayment over all hips and ridges6 inches on each side. A double underpayment can bestarted with two layers at the cave line, flush with thefascia board or molding. The second and remainingstrips have 19-inch head laps with 17-inch exposures(fig. 3-14, view B). Cover the entire roof in this manner.Make sure that all surfaces have double coverage. Useonly enough fasteners to hold the underpayment in placeuntil the shingles are applied. Do not apply shingles overwet underpayment.In areas where moderate-to-severe snowfall iscommon and ice dams occur, melting snow refreezes atthe cave line (fig. 3-15, view A). It is a good practice toapply one course of 55-pound smooth-surface rollroofing as a flashing at the eaves. It should be wideFigure 3-15.-Protection from ice dams A. Refreezing snowand ice; B. Cornice ventilation.enough to extend from the roof edge to between 12 and24 inches inside the wall line. The roll roofing shouldbe installed over the underpayment and metal drip edge.This will lessen the chance of melting snow to back upunder the shingles and fascia board of closed cornices.Damage to interior ceilings and walls results from thiswater seepage. Protection from ice dams is provided bycave flashing. Cornice ventilation by means of soffitvents and sufficient insulation will minimize the melting(fig. 3-15, view B).ASPHALT FELT.— Roofing felts are used asunderpayment for shingles, for sheathing paper, and forreinforcements in the construction of built-up roofs.They are made from a combination of shreddedwood fibers, mineral fibers, or glass fibers saturatedwith asphalt or coal-tar pitch. Sheets are usually36 inches wide and available in various weights from 10to 50 pounds. These weights refer to weight per square(100 feet).ORGANIC FELTS.— Asphalt-saturated feltscomposed of a combination of felted papers and organic3-14
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