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Table 3-5.-Underlayment Recommendations for Shingle Roofs
Flashing

Builder 3&2 Volume 02 - Construction manual for building structures
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Figure 3-14.-Roofing underlayment: A. Single coverage; B. Double coverage. the roof with atop lap of at least 2 inches at all horizontal points and a 4-inch side lap at all end joints (fig. 3-14, view A). Lap the underlayment over all hips and ridges 6 inches on each side. A double underpayment can be started with two layers at the cave line, flush with the fascia board or molding. The second and remaining strips  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. Use only enough fasteners to hold the underpayment in place until the shingles are applied. Do not apply shingles over wet  underpayment. In  areas  where  moderate-to-severe  snowfall  is common and ice dams occur, melting snow refreezes at the cave line (fig. 3-15, view A). It is a good practice to apply  one  course  of  55-pound  smooth-surface  roll roofing as a flashing at the eaves. It should be wide Figure 3-15.-Protection from ice dams A. Refreezing snow and ice; B. Cornice ventilation. enough to extend from the roof edge to between 12 and 24 inches inside the wall line. The roll roofing should be installed over the underpayment and metal drip edge. This will lessen the chance of melting snow to back up under the shingles and fascia board of closed cornices. Damage to interior ceilings and walls results from this water  seepage.  Protection  from  ice  dams  is  provided  by cave flashing. Cornice ventilation by means of soffit vents and sufficient insulation will minimize the melting (fig. 3-15, view B). ASPHALT  FELT.—  Roofing felts are used as underpayment for shingles, for sheathing paper, and for reinforcements in the construction of built-up roofs. They  are  made  from  a  combination  of  shredded wood fibers, mineral fibers, or glass fibers saturated with  asphalt  or  coal-tar  pitch.  Sheets  are  usually 36 inches wide and available in various weights from 10 to 50 pounds. These weights refer to weight per square (100 feet). ORGANIC  FELTS.—  Asphalt-saturated   felts composed of a combination of felted papers and organic 3-14







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