EXTERIOR NEW WALL CONSTRUCTION Design and Application Details and Instructions Double Coursing To obtain an attractive wall characterized by wide weather exposures and deep shadow lines, shingles and shakes can be applied double coursed. This method offers economy because of the wide exposures of the outer course and the use of less expensive undercoursing shingles for the under layer (Fig. 3). The maximum weather exposures recommended for #1 grade shingles are 12" for 16" shingles, 14" for 18" shingles, and 16" for 24" shingles. With shakes, the #1 grade 18" lengths can be laid at weather exposures up to 14" and 24" shakes up to 18." In double coursing, the bottom or starter course is laid triple with two undercourse shingles or one undercourse shingle over a wood lath and then the outer course. This gives the bottom course the same slant as succeeding courses. Outer-courses are applied ½", lower than the undercourse (Fig. 3). A straight edge can be used to facilitate placement and nailing of both the undercourse and exposed course. Undercourse shingles are fastened at the top with one nail or staple in the center The exposed shingle or shake is face-nailed with two casing-type nails, driven about 2" above the butt line, ¾" from each edge. With shingles wider than 8" drive two additional nails approximately 1" apart near the center. Three remanufactured products are popular in double course application. The first is Heritage rebutted-rejointed shingles in 16" and 18" lengths, machine trimmed for parallel edges with butts sawn at right angles, for application where snugly fitting joints are desired to give a clapboard look. Also available with smooth-sanded face. The second is Heritage sidewall shakes, manufactured from standard shingles and having striated faces and parallel edges. They are available prime coated for finish treatment after application. The third is Heritage True-Edge Straight-Split shakes, 18" long, with parallel edges and rather than tapered have the same thickness throughout. All of these can be applied with closed joints to form a continuous appearance, and as double coursed have distinctive shadow line.