Copper Flashings - A Manual - 1925 - Page 58
a vertical pipe than will flow in a horizontal trough of equal area. Therefore it might appear that the leader could well be much smaller than the gutter and take care of all the water flowing into a gutter. The problem would resolve itself into one of hydraulics were it not for practical considerations.
It is also good practice to make the leader the same size in its descending length as at the outlet, so that there may be no stoppage due to leaves or ice. These factors enter so acutely into the design that the problem becomes one more practical than hydraulic, although the principles of hydraulics enter into it.
Practice for leader sizes varies with different authorities from 75 to 250 square feet of roof surface to each square inch of leader cross section. This variation is due, in part, to varying conditions of rainfall in different parts of the country. The maximum rate varies from 4.5 to 8.7 inches per hour. In short periods during thunder showers even heavier falls have been recorded.
It seems reasonable to base computations on a rate of 8 inches per hour. At this rate of fall the water to be handled for 1,000 square feet of roof surface is 666.7 cubic feet per hour, or 0.185 cubic feet per second, or 83 gallons per minute.
Gutters and leaders large enough to carry away this amount of water will insure a satisfactory system.
The first step in designing such a system is location of the leaders. Seventy-five feet is the maximum spacing recommended. This done, the area drained per leader is computed and the area of the leaders determined. A safe rule is 150 sq. ft. of roof area to 1 sq. in. of leader area.
An application of this rule gives the following tabulation:
The above figures can be reduced or increased to meet local conditions where the intensity of rainfall is definitely known.
There are practical considerations to the problem. No leader should be less than 3 inches where there is a possibility of leaves, etc., passing into it. Two-inch leaders are often used for porches and decks, and are permissible if precaution is taken to safeguard the gutter outlet against stoppage.
The size of gutters depends upon
1. The number and spacing of the outlets.
The gutter acts as a reservoir or collecting channel which holds the water and carries it to the outlet. The slope of the gutter determines the flow toward the outlets.
2. The slope of the roof.
A steep roof carries the water to the gutter faster than a flat one does.
3. The style of gutter used.
Some gutters are not effective for their full depth and width. In proportioning gutters proper consideration of the available area is essential.
The best type of gutter has the minimum depth equal to half and the maximum depth not exceeding three-quarters the width. Thus the width becomes the deciding factor in proportioning its size. There is no reason for a gutter deeper than three-quarters of the width except for ornamental purposes.
Assuming that this proportion is observed the gutter may be referred to by its width only.
A gutter smaller than four inches wide is to be avoided. In common practice 4-inch gutters are seldom used for they are difficult to solder and increase the labor cost. The gutter may be the same size as the leader it serves, but, of course, can not be smaller.
Half-round gutters are most economical in material and insure a proper proportioning of width and depth.
Safe rules for determining the size of gutters are:
1. If spacing of leaders is 50 feet or less, use a gutter the same size as the leader, but not less than 4-inch.
2. If spacing of leaders is more than 50 feet, add 1 inch to the leader diameter for every 20 feet (or fraction) additional spacing on peaked roofs.
3. For flat roofs add 1 inch to the leader size for every 30 additional feet of gutter length.
1. A 40-foot gutter serves a 3-inch leader. The gutter should be 4-inch.
2. A 75-foot gutter serves a 4-inch leader on a steep roof. The gutter size is 6-inch.
3. A 75-foot gutter serves a 4-inch leader on a flat roof. The gutter size is 5-inch.
For ordinary residence construction 3 or 4-inch round and 2” x 3” or 2” x 4” rectangular leaders will generally suffice. Five-inch half round gutters meet practically every requirement.
- 58 -