10/21 188, is so made that it can shrink or swell without losing its flatness. Shingles when properly laid, can shrink or swell without the roof leaking. _Where feasible, undercut joined surfaces so as to give clearance on the inside and insure a tight appearance. But glued surfaces should be made to meet flat._ Illustrations of this principle are as follows: The inner end of the socket in a dovetail joint, Fig. |