C2b Leaders

Also called downspouts, leaders generally have round or rectangular sections with plain or corrugated profiles. The latter are more resistant to damage from freezing. Every gutter should have at least two leaders in case one is clogged, each should have a minimum section area of 7 in2 for the first 100 sf of horizontal roof area and 1 in2 for each added 100 sf, and leaders should not be more than 50 ft apart on any gutter. Any leader that is not vertical or nearly so should have its section area enlarged according to Formula 2.C.2.b.3. below. Other guidelines:

fl Wide downspouts clog less easily than narrow ones. fl If a gutter is connected to its downspouts with sheet metal screws, flowing debris can catch on them; but if blind rivets are used they must be drilled out if the assembly ever needs to be taken apart for cleaning or painting. Instead, rivet the leader to the bottom art of the gutter's inlet flange, and screw the inlet flange's upper part to the gutter's bottom so the screw tips protrude from the gutter's underside. Use short screws and paint their tips if they will be unsightly. fl Use as few elbows as possible, have a long vertical run above each, and avoid slanting leaders if possible. fl Secure each leader to the building with a U-shaped holddown at its top, bottom, and at each floor level. fl Install basket strainers at the top of every downspout; especially if it empties into any subsurface piping below it.

Regarding interior downspouts: the inlet of each should be fitted with a strainer, the downspout diameter should be 1 in. larger than the gutter outlet diameter, and no wood or masonry or metal should touch the downspout's outer surface other than at structural supports. Both inlets and downspouts should be wrapped in double vapor barrier or equal and enclosed with 4 in. batt insulation to keep surface condensation and any subfreezing temperatures from damaging adjacent construction.

Roof runoff management doesn't end at the bottom of the leaders but only when the runoff is well away from the building. Under the base of leaders that drain small roof areas locate a metal or concrete splash pan. If this is unsightly or impractical, lay subsurface drain piping that conveys the water to an outfall at least 20 ft from the building. This piping should be large enough, steep enough, and smooth enough (flexible corrugated piping is a poor choice here as it collects silt) to keep silt and leafy debris from potentially clogging it. The outfall should empty into a pebbled or cobbled retainage area, its opening should be covered with mesh or porous cap that bars entry by vermin and is easily removable for cleaning, and the outfall should be accessible for servicing in deep snow.

Gutter Leader Assembly

plain corrugated rectangular

plain corrugated




plain corrugated rectangular

plain corrugated




standard profiles


gutter-to-leader assembly

Fig. 2-11. Gutter leader details.

Example 1. A bank in Cripple Creek, CO, has a 62 x 46 ft flat roof that drains into a gutter with a round downspout at each end. What is the downspouts' diameter?

2 Rectangular sections: WR - 420 (w d)133

W = watershed area of roof drained by each leader, sf. W = area of horizontal projection + 0.5 area of vertical projection if roof slopes + 0.5 vertical area of any surface above roof that drains onto it. Roof is drained by 2 leaders 4 W = 62 x 46/2 = 1,430 sf. R = maximum rainfall intensity, based on 50-year-frequency storm.

From Fig. 2-9, R for central Colorado - 3 in/hr. d = minimum diameter of round leader section, ? in. d :> 2.5 in. w = width of rectangular section, in. b :> 2 in. Not applicable. b = depth of rectangular section, in. b :> 2 in. Not applicable.

Example 2. If part of the downspout in the above example slopes at a 6-in-12 pitch over a shed roof behind the bank's first floor, what is the downspout's minimum diameter above this area?

2C2b3 4P - (1 + cos2 A)

Ap required section area of nonvertical leader, ? in2. Ap ^ 2 Av.

Av = section area of vertical leader, in2. From Step 1, d = 2.61 in.

A Av = 0.785 d2 = 0.785 x 2.612 = 5.35 in2. â = angle from horizontal of leader, °. â = tan-1 6/i2 pitch = 26.6°.

Ap * 5.35 (1 + cos2 26.6°) ^ 9.62 in2 4 3.5 in. dia.

2.0.2.c. Footing Drains

Every foundation wall below grade should be as waterproof as the hull of a boat. This is done by covering the wall's outer surface below grade with a layer of thick gooey asphalt laid on with a trowel — not the liquid kind that is painted on with a brush— down to the wall's base, over the top of the footing protruding below, and down the side of the footing down to its bottom edge. Then a minimum 4 in. diameter perforated drain should be laid completely around the building with its crown 4 in. below the floor inside the wall. The drain should slope at least 1/16 in/ft both ways from its crown to a dry well or surface outfall some distance from the building, it should be covered with gravel 1 ft deep, and a wide strip of resin paper should be laid over the gravel to retard siltation around the drain. Failure to heed these strict guidelines is the chief cause of leaky basements. For these reasons, one of the most important occasions for an ^ designer to make a periodic inspection of the construction site is just before the foundation is backfilled. Otherwise, caveat emptor.

Gutter Drainage Northern Climate
Fig. 2-12. Footing drain & outfall design details.
2C2C1 5.6 nQ = k^ 62 67

n = roughness coefficient of footing drain. From Table 2-3, n for PVC

piping = 0.010. Q = water discharge rate into footing drain, 70 gpm k^ = footing drain slope coefficient, depending on pitch, from Fig. 2-10

on page 38. At 1/8 in/—, k^ = 0.102. 6 = minimum diameter of footing drain, ? in. 6 i4 in. dia.

5.6 x 0.010 x 70 = 0.102 x 62 67 ... 6 ^ 8.2 in. 4 use 9 in.

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