16.4 Rainfall Rates, Roof Area, and the Flow-Rate Conversion

The design storm

IPC 1106.1:

The size of the vertical conductors and leaders, building storm drains, building storm sewers and any horizontal branches of such drains or sewers shall be based on the 100-year hourly rainfall rate indicated in Figures 1106.1(1) through 1106.1(5) or on other rainfall rates determined from approved local weather data.

Five figures cover the Western, Central and Eastern United States, Alaska and Hawaii. The rate is expressed in inches per hour for a 1-hour storm with a 100-year return period.

Appendix B tabulates the same data for major cities. Note its status: "This appendix is informative and is not part of the code," and its rates are derived from Figures 1106.1(1) through 1106.1(5). Colorado adopts Appendix B by reference in Board Rule 1.2(A)(1), which is useful in the field; on the exam, the figures are the primary source and the appendix is a convenient lookup.

Colorado rates from Table B101

Two things to notice. First, the range across a single state is twice over — a design that works in Grand Junction is badly undersized in Lamar. Second, Colorado's rates are modest by national standards: Miami is 4.7, New Orleans 4.8, Hilo 6.2.

Equation 11-1 — converting rainfall to flow

IPC 1106.2.1 Rainfall rate conversion method: the rainfall rate falling on a roof surface shall be converted to a gallon per minute flow rate in accordance with Equation 11-1, where:

• R = rainfall intensity in inches per hour
• A = roof area in square feet

The conversion constant follows directly from unit arithmetic:

1 inch of rain over 1 square foot
  = 1/12 cubic foot
  = 0.0833 cubic feet
  x 7.48 gallons per cubic foot
  = 0.623 gallons per hour
  / 60 minutes
  = 0.0104 gallons per minute

So:  Q (gpm) = 0.0104 x A (square feet) x R (inches per hour)

Carry 0.0104 and you can compute any storm flow without a table.

Worked conversions

Same roof, four Colorado cities, and the required flow varies by 76 percent.

Vertical walls — IPC 1106.4

In sizing roof drains and storm drainage piping, one-half of the area of any vertical wall that diverts rainwater to the roof shall be added to the projected roof area for inclusion in calculating the required size of vertical conductors, leaders and horizontal storm drainage piping.

Why one-half? Rain falls at an angle in wind. A vertical wall rising above a roof intercepts wind-driven rain and sheds it onto the roof below, but only from one side and only part of the time. The code's empirical allowance is 50 percent of the wall's area.

Worked vertical wall adjustment

A 60 ft x 40 ft lower roof (2,400 square feet) abuts a taller building whose wall rises 25 feet above the lower roof along the full 60-foot length.

• Wall area = 60 x 25 = 1,500 square feet.
• Add one-half: 1,500 / 2 = 750 square feet.
• Adjusted design area = 2,400 + 750 = 3,150 square feet.
• In Denver: Q = 0.0104 x 3,150 x 2.4 = 78.6 gpm.

Ignoring the wall would have given 0.0104 x 2,400 x 2.4 = 59.9 gpm — a 31 percent undersizing error.

Where two walls divert onto the same roof, add one-half of each.

Checking against the roof drain

IPC 1106.2:

Vertical and horizontal storm drain piping shall be sized based on the flow rate through the roof drain. The flow rate, as calculated in accordance with Section 1106.2.1, shall be checked against the roof drain manufacturer's published flow rate for the specific roof drain model and size to verify that the selected roof drain will handle the anticipated flow. The flow rate in storm drain piping shall not exceed that specified in Table 1106.2.

Three checks, in order:

1. Compute the flow from rainfall and area (Equation 11-1).
2. Verify the roof drain can pass that flow at the anticipated ponding head (manufacturer's published rate, 1105.2).
3. Verify the piping can pass it within the Table 1106.2 limits.

Distributing flow among multiple drains

A roof with several drains divides the total flow among them, but not necessarily equally. Each drain serves the roof area that actually slopes to it. The practical method:

1. Determine the drainage area tributary to each drain from the roof slopes and crickets.
2. Add one-half of any diverting vertical wall area that sheds into that tributary area (1106.4).
3. Compute Q for each drain with Equation 11-1.
4. Size each conductor from Table 1106.2 (vertical column) for its own Q.
5. Size the horizontal collector for the accumulated Q at each point, at its installed slope.

Worked distribution

A 12,000 square foot Denver roof has four drains, with tributary areas of 4,000, 3,500, 2,500 and 2,000 square feet.

Each vertical conductor is sized for its own flow; the horizontal main accumulates to 299.5 gpm at the building storm drain.

What Appendix B is and is not