This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
The proper size of leaders, of course, will vary with the climate, the amount of rainfall, and the manner in which the gutter catches the water; but a good safe rule for every-day practice is to allow 1 square inch of sectional area of conductor pipe for every 75 square feet of flat roof area. No leaders, however, should be less than 2 inches in diameter, and even this size should be used only for small roofs, such as those over bay windows, small porches, etc.
The underground pipe (often drain tile), into which the leaders discharge, should not be less than 4 inches, inside diameter, and they need not be more than 8 inches in diameter for any work where the roof area is less than 1 acre. In measuring the watershed of pitched roofs, calculate the projected or horizontal area - i. e., the area on the plan.
59. Rain-water heads, sometimes called conductor heads, are often employed on the top of a leader, and treated as an architectural feature by forming a graceful outline to the receptacle for the gutter discharge. Two or more leaders may deliver into one head, as the conditions may require.
Fig. 46 shows a conductor head in section. It is set on top of a rectangular corrugated sheet-metal leader a, which should be securely fastened to the wall by a band on top. The lower end of the head slips into the conductor, and the top is secured against the stone wall with expansion bolts or other fasteners. A brass angle should be fitted inside the rim. The roof gutter empties into the head through a large lead or copper pipe b, which runs up a chase in the back of the wall, thus concealing it from outward view. The top of the head should be covered to prevent birds from building their nests in it, or in the pipe b.
The proper position for a rain-water head should be carefully shown on the elevation, as mechanics are prone to set them at the handiest place, without reference to the architectural treatment of the facade. All rain-water heads should be lined inside with a tapering box to extend below all moldings and prevent possible leakage, and also to prevent noise during heavy rains.