shredded wood fibers are considered felts. They areamong the least expensive of roofing felts and arewidely used not only as roofing, but also as water andvapor retarders. Fifteen-pound felt is used under woodsiding and exterior plaster to protect sheathing or woodstuds. It is generally used in roofing for layers or pliesin gravel-surfaced assemblies and is availableperforated. Perforated felts used in built-up roofs allowentrapped moisture to escape during application.Thirty-pound felt requires fewer layers in a built-uproof. It is usually used as underlayment for heavier capsheets or tile on steeper roofs.GLASS-FIBER FELTS.— Sheets of glass fiber,when coated with asphalt, retain a high degree ofporosity, assuring a maximum escape of entrappedmoisture or vapor during application and maximumbond between felts. Melted asphalt is applied so that thefinished built-up roof becomes a monolithic slabreinforced with properly placed layers of glass fibers.The glass fibers, which are inorganic and do not curl,help create a solid mass of reinforced waterproofrooting material.TARRED FELTS.— Coal-tar pitch saturatedorganic felts are available for use with bitumens of thesame composition. Since coal-tar and asphalt are notcompatible, the components in any construction must belimited to one bitumen or the other unless approved bythe felt manufacturer.FlashingThe roof edges along the eaves and rake should havea metal drip edge, or flashing. Flashing is speciallyconstructed pieces of sheet metal or other materials usedto protect the building from water seepage. Flashingmust be made watertight and be water shedding.Flashing materials used on roofs may beasphalt-saturated felt, metal, or plastic. Felt flashing isgenerally used at the ridges, hips, and valleys. However,metal flashing, made of aluminum, galvanized steel, orcopper, is considered superior to felt. Metal used forflashing must be corrosion resistant. It should begalvanized steel (at least 26 gauge), 0.019-inch-thickaluminum, or 16-ounce copper.Flashing is available in various shapes (fig. 3-16,view A), formed from 26-gauge galvanized steel. Itshould extend back approximately 3 inches from theroof edge and bend downward over the edge. Thiscauses the water to drip free of underlying corniceFigure 3-16.-Drip edges A. Basic shapes B. At the eave; C. Atthe rake.construction. At the eaves, the underpayment should belaid over the drip edge (view B). At the rake (view C),place the underpayment under the drip edge. Galvanizednails, spaced 8 to 10 inches apart, are recommended forfastening the drip edge to the sheathing.The shape and construction of different types ofroofs can create different types of water leakageproblems. Water leakage can be prevented by placingflashing materials in and around the vulnerable areas ofthe roof. These areas include the point of intersectionbetween roof and soil stack or ventilator, the valley of aroof, around chimneys, and at the point where a wallintersects a roof.3-15
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